I’ve built electrical systems on four vans now. The first one was a disaster—undersized cables, no fusing, random wire colours, and a battery that lasted about 8 months before dying from constant over-discharge. The second was overengineered—£3,000 spent on components I didn’t need, could barely fit under the seat.

The forth system? Perfect. Well, not perfect. But it’s been running flawlessly for 26 months, cost £1,200, powers everything I need, and I understand every component and why it’s there.

Here’s what nobody tells you: electrical systems aren’t complicated if you understand the fundamentals. Voltage, current, resistance—that’s literally it. Everything else is just application of those three concepts. But people skip the fundamentals, jump straight to “what battery should I buy,” and end up with systems that don’t make sense.

I’ve made every mistake: mixed cable sizes, forgotten fuses, undersized batteries, oversized inverters, poor crimping, no battery monitoring, inadequate ventilation. Learn from my expensive education.

This is a complete guide to campervan electrical systems: the theory you need to understand why things work, the practical application for actually building systems, the calculations everyone avoids, and the mistakes that cost me plenty so they don’t cost you anything.

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Table of Contents

1. Electrical Fundamentals
2. 12V vs 230V Systems
3. Battery Technology
4. Charging Sources
5. Power Consumption
6. System Design
7. Wiring and Cables
8. Fusing and Protection
9. Distribution and Switching
10. Monitoring
11. Safety
12. Common Mistakes
13. Example Systems

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Electrical Fundamentals

Right. If you don’t understand voltage, current, and resistance, you’ll struggle with everything else. Five minutes of theory saves hours of confusion.

Voltage (V)

What it is: Electrical pressure. Think of water pressure in pipes.

In vans:

• 12V nominal (actually 11-14.6V depending on charge state)
• 24V in some larger vehicles
• 230V AC from inverter or hookup

Why it matters: Your devices need specific voltage. Wrong voltage damages equipment. A 12V fridge won’t work on 6V. A 12V LED on 24V will burn out.

Analogy: Voltage is like water pressure. Higher pressure pushes more water. Higher voltage pushes more electrons.

Current (A – Amps)

What it is: Flow rate of electricity. Like litres per minute of water.

In vans:

• Small devices: 0.5-5A (LED lights, phone charging)
• Medium devices: 5-20A (water pump, laptop charging, TV)
• Large devices: 20-100A+ (inverter, diesel heater, fridge compressor)

Why it matters: Current determines cable size. High current needs thick cables. It also determines battery capacity needed.

Analogy: Current is flow rate. A trickle vs a fire hose. Both are water, but volume differs.

Resistance (Ω – Ohms)

What it is: Opposition to current flow. Like friction in pipes.

In vans:

• Good conductors (copper wire): Very low resistance
• Poor conductors: High resistance, generate heat
• Fuses: Designed to have specific resistance

Why it matters: Resistance causes voltage drop and heat. Long, thin cables have high resistance. This wastes power and creates fire risk.

Analogy: Resistance is like pipe friction. Narrow pipes have more resistance than wide pipes.

Ohm’s Law: The Only Formula You Need

V = I × R

Where:

• V = Voltage (volts)
• I = Current (amps)
• R = Resistance (ohms)

Rearranged:

• I = V ÷ R (current equals voltage divided by resistance)
• R = V ÷ I (resistance equals voltage divided by current)

Example: 12V LED drawing 1A

• Resistance = 12V ÷ 1A = 12Ω

Power (W – Watts)

The formula: P = V × I

Where:

• P = Power (watts)
• V = Voltage (volts)
• I = Current (amps)

Rearranged:

• I = P ÷ V (current equals power divided by voltage)
• V = P ÷ I (voltage equals power divided by current)

Example 1: 60W laptop charger at 12V

• Current = 60W ÷ 12V = 5A

Example 2: 900W kettle at 230V

• Current = 900W ÷ 230V = 3.9A

Example 3: 900W kettle through inverter at 12V

• Inverter draws from 12V battery
• Current = 900W ÷ 12V ÷ 0.9 (efficiency) = 83A
• That’s why you need thick cables for inverters

Energy (Wh or Ah)

Watt-hours (Wh): Total energy used

Formula: Energy (Wh) = Power (W) × Time (hours)

Example: 60W laptop for 4 hours = 240Wh

Amp-hours (Ah): Energy at specific voltage

Conversion: Ah = Wh ÷ Voltage

Example: 240Wh at 12V = 20Ah

Why both measurements?

• Watt-hours (Wh) is absolute energy
• Amp-hours (Ah) is energy at specific voltage
• Batteries are rated in Ah (at their voltage)
• Devices are rated in W (watts)

Practical Application

Calculate current draw:

Device: 12V fridge rated 45W

• Current = 45W ÷ 12V = 3.75A

Calculate daily energy:

Same fridge runs 8 hours per day

• Energy = 45W × 8h = 360Wh
• Or: 3.75A × 8h = 30Ah (at 12V)

Calculate battery size needed:

Daily consumption: 360Wh (30Ah at 12V) Want 3 days autonomy (no charging)

• Total needed = 360Wh × 3 = 1,080Wh
• At 12V = 90Ah minimum
• Account for discharge limits (50% for lead-acid, 80% for lithium)
• Lead-acid: 90Ah ÷ 0.5 = 180Ah battery
• Lithium: 90Ah ÷ 0.8 = 112Ah battery (call it 120Ah)

This is how you size systems. Everything stems from these calculations.

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12V vs 230V Systems

Understanding when to use 12V vs 230V saves money and improves efficiency.

12V DC System (Low Voltage)

What it is: Your van’s native electrical system. Battery voltage.

Voltage range:

• Fully charged: 12.7-14.6V (charging)
• Nominal: 12V
• Discharged: 10.5-11V (stop using here)

Advantages:

• Direct from battery (no conversion loss)
• Efficient for 12V devices
• Safe (low voltage won’t kill you)
• Simple wiring
• Standard automotive components

Disadvantages:

• High current for given power (thick cables needed)
• Limited device availability (not everything comes in 12V)
• Voltage drop issues over distance

Best for:

• LED lighting
• Fans
• Water pumps
• 12V fridges/coolers
• USB charging (via 12V adapters)
• Diesel heaters
• Anything designed for automotive use

230V AC System (Mains Voltage)

What it is: Household mains voltage. Requires inverter or hookup.

Voltage: 230V AC (50Hz in UK)

Sources:

• Inverter (converts 12V DC to 230V AC)
• Hookup at campsites
• Generator (rare in vans)

Advantages:

• Powers all household devices
• Lower current for given power (thinner cables)
• Familiar to everyone

Disadvantages:

• Requires inverter (cost, efficiency loss 10-20%)
• Dangerous voltage (can kill)
• More complex wiring
• RCD protection required

Best for:

• Laptops (if no USB-C charging)
• Kitchen appliances (blenders, toasters, kettles)
• Power tools
• Hair dryers
• Anything that only comes in mains voltage

The Efficiency Argument

Example: Charging a laptop

Method 1: Inverter (12V → 230V → 19V)

• Battery (12V DC) → Inverter (230V AC) → Laptop charger (19V DC)
• Inverter efficiency: 85-92%
• Laptop charger efficiency: 85-90%
• Total efficiency: 72-83% (17-28% loss)

Method 2: 12V DC adapter (12V → 19V)

• Battery (12V DC) → DC-DC adapter (19V DC)
• Adapter efficiency: 88-94%
• Total efficiency: 88-94% (6-12% loss)

Difference: Method 1 wastes 2-3× more power

Real numbers: 60W laptop, 4 hours daily

• Inverter method: 288Wh from battery
• DC adapter: 256Wh from battery
• Savings: 32Wh daily = 960Wh monthly

On 200Ah battery (2,400Wh capacity), that’s 40% extra capacity recovered.

When to Use Each

Use 12V DC when:

• Device available in 12V version
• High-frequency usage (daily)
• Efficiency matters (off-grid living)
• Example: Fridge, lights, water pump, laptop (USB-C), phone charging

Use 230V AC when:

• No 12V alternative exists
• Occasional use only (efficiency loss acceptable)
• High power (paradoxically easier—e.g., 2000W kettle via inverter uses thick cables but for 3 minutes only)
• Example: Hair dryer, blender, power tools, toaster

My system: 90% is 12V DC. Inverter exists for occasional use (power tools, blender, guests’ laptop chargers). It’s off unless needed.

Hookup vs Off-Grid

Hookup (campsite mains):

• Connect to 230V mains supply
• Battery charger converts 230V → 12V (charges battery)
• Can run 230V devices directly
• Unlimited power (within campsite limits)

Off-grid (no hookup):

• Battery powers everything
• Solar/alternator recharge battery
• Must manage power carefully
• Inverter for 230V devices (if needed)

My reality: 95% off-grid. Hookup maybe 5 nights per year. System designed for off-grid, hookup is bonus.

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Battery Technology

Your battery is the heart of your system. Choose wrong and everything else suffers.

Lead-Acid Batteries

Types:

• Flooded: Traditional car batteries, require maintenance
• AGM (Absorbed Glass Mat): Sealed, maintenance-free
• Gel: Sealed, different chemistry

Advantages:

• Cheap (£100-200 for 100Ah)
• Available everywhere
• Proven technology
• Safe

Disadvantages:

• Heavy (25-30kg for 100Ah)
• Only 50% usable capacity (over-discharge damages them)
• Short lifespan (500-1000 cycles)
• Slow charging
• Temperature sensitive
• Require ventilation (hydrogen gas when charging)

Real capacity: 100Ah lead-acid battery

• Only use 50Ah (50% discharge maximum)
• Effective capacity: 50Ah

Lithium Batteries (LiFePO4)

Chemistry: Lithium Iron Phosphate (safest lithium variant)

Advantages:

• 80-90% usable capacity
• Lightweight (10-12kg for 100Ah)
• Long lifespan (2000-5000 cycles)
• Fast charging
• Temperature tolerant
• No maintenance
• No ventilation needed

Disadvantages:

• Expensive (£400-800 for 100Ah)
• Requires BMS (Battery Management System)
• Cannot charge below 0°C
• Initial cost barrier

Real capacity: 100Ah lithium battery

• Can use 80-90Ah safely
• Effective capacity: 80-90Ah

Capacity Comparison

100Ah lead-acid (AGM):

• Cost: £150-200
• Weight: 28kg
• Usable: 50Ah
• Lifespan: 500-800 cycles
• Cost per usable Ah per cycle: £0.0075/Ah/cycle

100Ah lithium (LiFePO4):

• Cost: £500-700
• Weight: 11kg
• Usable: 85Ah
• Lifespan: 3000-4000 cycles
• Cost per usable Ah per cycle: £0.0020/Ah/cycle

Lithium is actually cheaper long-term. Plus weighs 1/3 as much.

My Experience

Van 1: 110Ah AGM lead-acid (£160)

• Lasted 18 months (350-400 cycles)
• Weight was 30kg
• Usable capacity ~55Ah
• Died from over-discharge (my fault, no monitoring)

Van 2: 200Ah lithium LiFePO4 (£680)

• Still going after 26 months (600+ cycles)
• Weight is 22kg
• Usable capacity ~170Ah
• No degradation noticed

Lithium paid for itself through longevity and better capacity. Would never go back to lead-acid.

Sizing Your Battery Bank

Formula: Daily consumption (Ah) × Days autonomy ÷ Usable capacity % = Battery size

Example 1: Weekend warrior

• Daily use: 30Ah
• Want 2 days autonomy
• Lead-acid (50% usable)
• Calculation: 30 × 2 ÷ 0.5 = 120Ah lead-acid

Example 2: Full-time living

• Daily use: 70Ah
• Want 3 days autonomy
• Lithium (85% usable)
• Calculation: 70 × 3 ÷ 0.85 = 247Ah lithium (call it 250Ah)

My system: 200Ah lithium

• Daily use: 65-70Ah
• Autonomy: 2-2.5 days (170Ah usable ÷ 70Ah daily)
• Plus solar recharges daily (usually)
• Perfect balance

Battery Location

Requirements:

• Low in van (weight distribution, safety)
• Ventilated (lead-acid) or sealed space okay (lithium)
• Protected from damage
• Accessible for connections
• Temperature controlled (cold affects performance)

Common locations:

• Under seating
• Under bed
• In front passenger footwell (single-seat conversions)
• Dedicated battery box

My setup: Under passenger seat

• Easy access
• Low center of gravity
• Protected by seat structure
• Ventilated naturally

Battery Safety

Lead-acid:

• Produces hydrogen when charging (explosive)
• Requires ventilation to outside
• Acid can leak if tipped (AGM less risk)
• Vent cap maintenance (flooded type)

Lithium:

• No gas production
• BMS prevents overcharge/over-discharge
• Fire risk if damaged (rare with LiFePO4)
• Cannot charge below 0°C (BMS should prevent)

Protection needed:

• Fusing on positive terminal
• Secure mounting (won’t move in crash)
• Ventilation (lead-acid)
• BMS (lithium)
• Temperature monitoring (optional but helpful)

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Charging Sources

Your battery needs recharging. Three main sources.

Solar Charging

How it works: Panels convert sunlight to electricity, charge battery via controller.

Components:

• Solar panels (£80-150 per 100W)
• MPPT controller (£80-200)
• Cables and mounting

Advantages:

• Free energy
• Silent
• No engine running required
• Enables off-grid living
• Low maintenance

Disadvantages:

• Weather dependent
• Initial cost (£400-1,000)
• Roof space required
• Winter output is poor (UK)

Typical output (UK, 200W panels):

• Summer: 60-80Ah daily
• Winter: 15-25Ah daily
• Overcast: 20-40Ah daily

When solar makes sense:

• Off-grid living
• Stationary camping (not driving daily)
• Have roof space
• Can afford initial investment

When solar doesn’t make sense:

• Drive daily (alternator charges anyway)
• Park in shade (trees, buildings)
• Very high consumption (solar can’t keep up)
• Tight budget (alternator charging cheaper)

Alternator Charging (DC-DC Charger)

How it works: Engine alternator charges starter battery. DC-DC charger takes power from starter battery, charges leisure battery safely.

Components:

• DC-DC charger (£150-300)
• Cables from starter to leisure battery
• Fusing

Advantages:

• Fast charging (20-60A typical)
• Works while driving
• Doesn’t drain starter battery
• No roof space needed

Disadvantages:

• Only charges while driving
• Engine must run (fuel cost)
• Initial cost (£200-400 installed)
• Noise/pollution

Typical output: 30A DC-DC charger

• 30 minutes driving = 15Ah
• 1 hour driving = 30Ah
• 2 hours driving = 60Ah

When DC-DC makes sense:

• Drive frequently
• Limited roof space
• High consumption (need fast charging)
• Cold climates (lithium heating)

When DC-DC doesn’t make sense:

• Stationary for weeks
• Trying to minimize driving
• Have adequate solar
• Tight budget (solar is better long-term)

My setup: 200W solar + 30A DC-DC

• Solar covers 90% of needs
• DC-DC is backup (drive 2-3× weekly)
• Perfect combination

Split Charge Relay vs DC-DC Charger

Old method: Split charge relay

• Simple switch connecting batteries when engine runs
• Cheap (£20-40)
• Works for lead-acid
• Doesn’t work well for lithium
• No charging optimization

Modern method: DC-DC charger

• Proper battery-to-battery charger
• Optimizes charging for battery type
• Protects starter battery
• Works with smart alternators
• Supports lithium batteries

Use DC-DC charger. Split charge is outdated and problematic with modern vehicles.

Hookup (Mains Charging)

How it works: Plug into campsite 230V supply. Battery charger converts 230V → 12V, charges battery.

Components:

• Battery charger (£50-200)
• Hookup cable and inlet
• RCD protection
• Consumer unit

Advantages:

• Unlimited power
• Fast charging
• Can run 230V devices directly
• No sun/driving needed

Disadvantages:

• Campsite cost (£10-30/night)
• Not off-grid
• Requires hookup facilities
• Additional components (£150-300)

When hookup makes sense:

• Mainly campsite camping
• High consumption
• Don’t want solar/DC-DC
• Winter camping (solar insufficient)

When hookup doesn’t make sense:

• Off-grid living
• Wild camping focus
• Already have solar/DC-DC

My usage: 5-10 nights per year

• Emergency backup only
• Mainly off-grid
• Charger cost £80, barely used
• Wish I’d skipped it

Combining Charging Sources

Best combination (what I run):

• Solar (primary): 200W panels, daily charging
• DC-DC (backup): 30A when driving
• Hookup (rarely): Emergency only

Budget combination:

• DC-DC only: 30-40A charger, drive 30+ mins daily
• Cheapest if you drive regularly
• No solar cost

Off-grid hardcore:

• Solar (oversized): 400-600W panels
• DC-DC (optional): Backup for winter
• No hookup at all

Sizing guidelines:

If daily consumption is 50Ah:

• Solar: 200-300W (summer coverage, winter struggles)
• DC-DC: 30A (1-2 hours driving replaces daily use)
• Or combination: 150W solar + 20A DC-DC

If daily consumption is 100Ah:

• Solar: 400-600W (winter still struggles)
• DC-DC: 40-60A (2-3 hours driving replaces daily use)
• Combination recommended

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Power Consumption

Understanding what uses power is essential for system sizing.

Measuring Consumption

Method 1: Nameplate ratings

• Check device label for watts or amps
• Calculate energy: Power × Hours used
• Works for planning

Method 2: Actual measurement

• Use DC clamp metre or power metre
• Measure real consumption
• More accurate (devices often use less than rated)

Typical Device Consumption

Lighting:

• LED strip (1m): 4-6W
• LED bulb: 3-8W
• Halogen bulb (don’t use these): 10-20W

12V Devices:

• Water pump: 30-60W (runs 5-15 mins daily)
• Diesel heater fan: 10-25W (runs hours in winter)
• MaxxFan vent: 5-40W (variable speed)
• USB charging: 10-20W

Fridge/Cooling:

• Compressor fridge: 40-60W (runs ~8h daily, cycles on/off)
• Thermoelectric cooler: 40-50W (runs constantly, inefficient)
• Coolbox: 30-40W

Computing:

• Laptop: 30-65W (4-8 hours daily for remote work)
• Tablet: 10-20W
• Phone charging: 5-15W

230V via Inverter:

• Hair dryer: 1000-2000W (5-10 mins)
• Kettle: 900-2000W (3-5 mins)
• Blender: 300-600W (2-5 mins)
• Toaster: 800-1200W (3-5 mins)

Add inverter loss: Multiply by 1.15 for inefficiency

• 1000W hair dryer = 1150W from battery

My Actual Daily Consumption

Summer day (no heating):

• LED lighting: 15W × 4h = 60Wh (5Ah)
• Fridge: 45W × 8h = 360Wh (30Ah)
• Laptop: 60W × 4h = 240Wh (20Ah)
• Phone charging: 15W × 2h = 30Wh (2.5Ah)
• Water pump: 40W × 0.25h = 10Wh (0.8Ah)
• Misc: 50Wh (4Ah)
• Total: 750Wh (62.5Ah)

Winter day (heating needed):

• Above items: 750Wh (62.5Ah)
• Diesel heater: 20W × 6h = 120Wh (10Ah)
• More lighting: 15W × 2h = 30Wh (2.5Ah)
• Total: 900Wh (75Ah)

Heavy use day (working + cooking):

• Above items: 750Wh
• Laptop: 60W × 8h = 480Wh (40Ah)
• Inverter for blender: 500W × 0.1h = 50Wh (4Ah)
• Extra lighting: 50Wh (4Ah)
• Total: 1,330Wh (110Ah)

Most days: 60-75Ah Heavy days: 90-110Ah (rare)

Calculating Your Consumption

Template:

Device	Power (W)	Hours/Day	Daily Wh	Daily Ah
Lights	15W	4h	60Wh	5Ah
Fridge	45W	8h	360Wh	30Ah
Laptop	60W	4h	240Wh	20Ah
Heating	20W	4h	80Wh	6.7Ah
Misc	–	–	50Wh	4.2Ah
Total			790Wh	65.9Ah

Then:

• Add 20% buffer: 65.9 × 1.2 = 79Ah
• This is your daily consumption target
• Size battery and charging for this

Reducing Consumption

Easy wins:

1. LED lighting (not halogen): Saves 50-80% on lighting
2. 12V fridge (not thermoelectric): 50% more efficient
3. USB-C laptop charging (not inverter): Saves 15-20% on laptop power
4. Good insulation (less heating needed): Saves 30-50% in winter

Behavioral changes:

1. Laptop sleep mode when not typing: Saves 50% laptop power
2. Switch off lights when leaving van: Obvious but forgotten
3. Fridge temperature: Set to 4°C not 1°C (saves 20% power)
4. Minimize inverter use: Only turn on when needed

My changes (reduced consumption 30%):

• Switched to 12V charging for laptop (USB-C)
• Better insulation (less heater runtime)
• More efficient fridge (40W instead of 60W)
• LED lights throughout

Went from 90Ah daily to 65Ah daily.

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System Design

Bringing everything together into a coherent system.

Design Process

Step 1: Calculate consumption

• List all devices
• Estimate usage hours
• Calculate daily Ah

Step 2: Size battery

• Daily Ah × Days autonomy ÷ Usable %
• Choose battery type (lithium recommended)

Step 3: Plan charging

• Solar? Size for typical weather
• DC-DC? Size for driving frequency
• Hookup? Maybe, maybe not

Step 4: Calculate cable sizes

• Maximum current per circuit
• Cable length
• Voltage drop calculation

Step 5: Plan distribution

• Fused circuits
• Switches for circuits
• Monitoring points

Step 6: Safety protection

• Fuses on all circuits
• RCD for 230V
• Battery protection

Example System 1: Weekend Warrior

Consumption: 30Ah daily Usage: Weekends, occasional week trips Driving: Yes, frequently

Battery: 100Ah lithium (£500)

• Usable: 85Ah
• Autonomy: 2.5 days

Charging: 30A DC-DC charger (£180)

• 1 hour driving = 30Ah
• No solar (saves £400)

Power distribution:

• Lights: 10A circuit
• Fridge: 10A circuit
• USB charging: 15A circuit
• Water pump: 15A circuit

Inverter: 600W (£100)

• Occasional use only

Total cost: ~£950 (battery, DC-DC, fuses, cables, inverter)

Example System 2: Full-Time Off-Grid

Consumption: 70Ah daily Usage: Full-time, stationary weeks at a time Driving: Occasionally

Battery: 200Ah lithium (£680)

• Usable: 170Ah
• Autonomy: 2.4 days

Charging:

• Solar: 300W (£380 with controller)
• DC-DC: 30A backup (£180)

Power distribution:

• Lights: 10A circuit
• Fridge: 15A circuit
• Laptop/USB: 20A circuit
• Heater: 10A circuit
• Water pump: 10A circuit
• Misc: 10A circuit

Inverter: 1000W (£150)

• Occasional use

Monitoring: Battery monitor (£120)

• Essential for off-grid

Total cost: ~£1,850 (battery, solar, DC-DC, distribution, inverter, monitoring)

My System (Real-World)

Consumption: 65Ah daily average, 90Ah heavy days

Battery: 200Ah LiFePO4 (£680)

• Usable: 170Ah
• Autonomy: 2-2.5 days

Charging:

• Solar: 200W Renogy (£220 panels, £100 controller)
• DC-DC: 30A Renogy (£180)
• Hookup: 20A charger (£80) – rarely used

Distribution:

• Main bus bar with 6 fused circuits
• Individual switches for lights, fridge, heater
• Always-on circuit for battery monitor

Inverter: 1000W Renogy (£180)

• Off unless needed
• Switched

Monitoring: Victron SmartShunt (£130)

• Tracks everything
• Bluetooth to phone

Total cost: £1,570 (excluding labor)

Performance: Flawless for 26 months

• Never run out of power (came close once in winter)
• Solar covers 90% of charging
• DC-DC used 2-3× weekly
• Hookup maybe 5 nights in 2 years

Perfect balance for my usage.

Oversizing vs Undersizing

Oversizing (my first van):

• 400W solar on van using 50Ah daily
• Overkill, wasted £300
• Battery fully charged by 11am, wasted rest
• Heavy (extra panel weight)

Undersizing (mate’s van):

• 100W solar, 100Ah battery, 80Ah daily use
• Constantly struggling
• Had to use hookup frequently
• Frustrating

Right-sizing (current van):

• 200W solar, 200Ah battery, 65Ah daily
• Balanced
• Occasional struggle in winter (expected)
• Comfortable summer
• Budget friendly

Guideline: Size for 80-90% coverage. Accept occasional shortage (drive, hookup, reduce usage). Don’t chase 100% coverage—expensive and wasteful.

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Wiring and Cables

Get this wrong and you have fires. Get it right and you never think about it.

Cable Sizing Fundamentals

Why size matters:

• Too thin: Overheats, fire risk, voltage drop
• Too thick: Expensive, difficult to work with, unnecessary

Factors affecting sizing:

1. Current: Higher current needs thicker cable
2. Length: Longer runs need thicker cable
3. Acceptable voltage drop: Usually 3% maximum
4. Temperature: Hot environments need derating

Voltage Drop Calculation

Formula: Voltage drop (V) = (Current × Length × 2 × Resistance per m) ÷ 1000

Where:

• Current in amps
• Length in metres (one way)
• ×2 for positive and negative
• Resistance per m depends on cable size

Resistance per metre (copper cable at 20°C):

Cable Size	Resistance (mΩ/m)
1.5mm²	13.3
2.5mm²	8.0
4mm²	5.0
6mm²	3.3
10mm²	1.95
16mm²	1.21
25mm²	0.78
35mm²	0.55

Example: 10A load, 5m cable run, 2.5mm² cable

• Voltage drop = (10 × 5 × 2 × 8.0) ÷ 1000 = 0.8V
• Percentage = 0.8V ÷ 12V = 6.7%
• Too high! (target <3%)

Better: Same load, 4mm² cable

• Voltage drop = (10 × 5 × 2 × 5.0) ÷ 1000 = 0.5V
• Percentage = 0.5V ÷ 12V = 4.2%
• Still high, but acceptable

Best: Same load, 6mm² cable

• Voltage drop = (10 × 5 × 2 × 3.3) ÷ 1000 = 0.33V
• Percentage = 0.33V ÷ 12V = 2.75%
• Good!

Practical Cable Sizing Guide

For 12V systems:

Current	Length <2m	Length 2-5m	Length >5m
Up to 5A	1.5mm²	2.5mm²	4mm²
5-10A	2.5mm²	4mm²	6mm²
10-20A	4mm²	6mm²	10mm²
20-40A	6mm²	10mm²	16mm²
40-60A	10mm²	16mm²	25mm²
60-100A	16mm²	25mm²	35mm²
100-150A	25mm²	35mm²	50mm²

My system:

• Lights (5A, 4m): 2.5mm²
• Fridge (6A, 3m): 4mm²
• Heater (3A, 2m): 2.5mm²
• USB (10A, 2m): 4mm²
• Inverter (100A, 0.5m): 25mm²
• Battery to bus (60A, 1m): 16mm²

Cable Types

Automotive cable:

• Stranded copper (flexible)
• PVC insulation
• Temp rated to 70-90°C
• Use this for everything

NOT household wire:

• Solid core (inflexible, breaks from vibration)
• Lower temperature rating
• Not suitable for vehicles

Marine/boat cable:

• Tinned copper (corrosion resistant)
• Expensive
• Overkill for vans unless very humid environment

Solar cable:

• UV resistant insulation
• Double-insulated
• Rated for outdoor use
• Use for roof panel connections

Crimping and Connections

Methods:

1. Crimped connections (my preference)

• Use proper crimping tool (£20-80)
• Cable lug/terminal
• Heat shrink over connection
• Permanent, reliable

2. Soldered connections

• Solder wire to terminal
• Heat shrink over
• Strong but more work
• Can fail if joint flexes (vibration)

3. Screw terminals (temporary only)

• Okay for testing
• Not suitable for permanent installation
• Can vibrate loose

4. Wago connectors

• Push-in spring connectors
• Quick and easy
• Okay for low-current household
• NOT suitable for vehicles (vibration)

Use crimping. It’s reliable, permanent, and vibration-resistant.

Crimping tips:

1. Strip correct length (15mm typical)
2. Insert fully into lug
3. Crimp in correct tool position
4. Tug test (should not pull out)
5. Heat shrink over connection
6. Label cable

Cable Routing

Best practices:

1. Secure every 30cm with cable ties or clamps
2. Protect through metal panels with grommets
3. Keep away from heat sources (exhausts, heaters)
4. Avoid water sources (sinks, roof leaks)
5. Bundle cables in loom or conduit
6. Label at both ends
7. Leave slack for service (not guitar-string tight)
8. Route positive and negative together (reduces electrical noise)

Where to route:

• Along van ribs/structure
• Behind panels/insulation
• Under floor (if protected)
• Through walls/bulkheads with grommets

My routing:

• Main positive from battery through bulkhead to rear
• Bus bar in rear electrical cabinet
• Individual circuits from bus bar to devices
• All cables in split-loom conduit
• Secured every 30cm
• Labeled at origin and destination

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Fusing and Protection

This is safety. Don’t skip fuses.

Why Fuses Matter

Without fuse: Cable short → high current → heat → fire → van burns

With fuse: Cable short → high current → fuse blows → circuit disconnected → no fire

Fuses sacrifice themselves to protect cables and equipment.

Fuse Sizing

Formula: Fuse rating = Maximum circuit current × 1.25

Example: 10A load

• Fuse = 10 × 1.25 = 12.5A
• Use 15A fuse (next standard size up)

Cable must be rated for fuse current, not load current.

Example: 10A load, 15A fuse

• Cable must handle 15A (the fuse won’t blow until 15A)
• Use 2.5mm² minimum (rated 20A+)

Fuse Types

Blade fuses (automotive):

• Mini: 2-30A
• Standard: 3-40A
• Maxi: 20-100A
• Common, cheap, available everywhere
• Use for loads <80A

ANL fuses (high current):

• 30-500A range
• Large format
• Excellent for battery protection
• Use for 80A+ circuits

MIDI fuses:

• 30-150A
• Compact
• Good for medium-high current

My system:

• Main battery: 100A ANL fuse
• Inverter: 125A MIDI fuse
• Individual circuits: 10-20A blade fuses

Fuse Locations

Critical rule: Fuse within 300mm of battery positive terminal

Why: If cable shorts before fuse, no protection. Cable from battery to fuse is vulnerable.

Every circuit needs fusing:

• Battery to bus bar: One large fuse
• Each circuit from bus bar: Individual fuse

My fusing:

• Battery positive: 100A ANL fuse (30cm from terminal)
• Bus bar to inverter: 125A fuse
• Bus bar circuits: 6× blade fuses (10-20A each)

Circuit Breakers vs Fuses

Circuit breakers:

• Resettable (flip switch)
• More expensive
• Bulky
• Useful as switches AND protection

Fuses:

• One-time use
• Cheap
• Compact
• Pure protection

My choice: Fuses for protection, switches for switching. Clearer separation of functions.

RCD (Residual Current Device)

For 230V systems only (inverter or hookup):

What it does: Detects current imbalance (leakage to earth), trips instantly

Why essential: 230V can kill. RCD trips in milliseconds if you touch live wire.

Rating: 30mA trip current (standard for human protection)

Installation: After hookup inlet, before consumer unit

Cost: £30-60 for quality RCD

Do not skip RCD if you have 230V in your van. It saves lives.

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Distribution and Switching

How you organize and control circuits.

Bus Bar System

What it is: Central point where circuits connect

Components:

• Positive bus bar (fused circuits)
• Negative bus bar (common ground)

Advantages:

• Organized wiring
• Easy to add circuits
• Clear fusing
• Professional appearance

My setup:

• 12-way positive bus bar with blade fuse holders
• Negative bus bar (unfused)
• Mounted in electrical cabinet
• All circuits radiate from here

Switches

Types:

Rocker switches:

• Panel-mount
• Illuminated or not
• 10-20A typical
• Good for permanent installations

Toggle switches:

• Smaller footprint
• Less intuitive
• Cheaper
• Good for secondary circuits

Push-button momentary:

• For water pumps (press = run)
• Not suitable for sustained loads

My switches:

• Main circuits: Illuminated rocker switches on control panel
• Inverter: Large rocker (accidentally left on is wasteful)
• Water pump: Momentary push-button

Control Panel

Centralized switching:

Benefits:

• All switches in one location
• Know what’s on/off at a glance
• Professional appearance
• Easy to use

My control panel (3D printed + switches):

• Row 1: Lights (3 zones), USB, Heater
• Row 2: Fridge (always on), Inverter, Aux
• Bottom: Battery monitor display

Takes 2 seconds to check what’s on.

Always-On vs Switched Circuits

Always-on (direct from battery):

• Battery monitor
• CO/smoke alarms
• Fridge (if desired)
• Emergency lighting

Switched (through control panel):

• Main lighting
• Heater
• Water pump
• Inverter
• USB charging (maybe—I leave mine always-on)

Balance: Essential devices always-on, everything else switched.

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Monitoring

You’re flying blind without monitoring. I learned this expensively.

Why Monitor?

Without monitoring:

• Guess at battery state
• Surprise dead battery
• Over-discharge (damages battery)
• No idea what’s using power
• Can’t diagnose issues

With monitoring:

• Know exact state of charge
• See current draw in real-time
• Track charging sources
• Diagnose power hogs
• Optimize usage

My first van: No monitoring

• Over-discharged battery multiple times
• Battery died after 18 months
• Constant anxiety about power

Current van: Victron SmartShunt

• Know exactly what’s happening
• Never stressed about power
• Battery health excellent after 26 months

Battery Monitors (Shunt-Based)

How they work:

• Shunt measures every amp in/out
• Counts amp-hours (coulomb counting)
• Calculates state of charge

Accuracy: ±1-3% with quality monitors

Best monitors:

• Victron SmartShunt: £120-140 (my choice)
• Renogy 500A: £80-100 (good value)
• BMV-712: £200-240 (display + Bluetooth)

Installation: Shunt on negative cable from battery. ALL negatives route through shunt.

Voltage Monitoring (Basic)

Cheaper option: Voltage display (£8-15)

Problems:

• Voltage varies by load
• Can’t determine accurate SOC
• 12.4V could be 60% or 80% depending on conditions

Only useful: Very rough indicator

Don’t rely on voltage alone. Get proper battery monitor if you can afford it.

What to Monitor

Essential:

• State of charge (%)
• Voltage (V)
• Current (A)
• Remaining capacity (Ah)

Useful:

• Power (W)
• Time remaining (hours)
• Historical data (Ah in/out)
• Daily min/max values

Nice-to-have:

• Temperature
• Battery health
• Charge cycles
• Per-device consumption (requires multiple shunts)

My Victron app shows everything essential in real-time. I check it daily.

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Safety

This section might save your life or van.

Fire Risks

Causes:

1. Undersized cables (overheating)
2. Poor connections (resistance → heat)
3. No fusing (short circuits)
4. Damaged insulation
5. Overloading circuits

Prevention:

• Proper cable sizing
• Quality connections (crimped)
• Fuse everything
• Regular inspections
• Avoid cheap components

Fire extinguisher: Mount one. ABC rated, 1-2kg. Near door. Check annually.

Electrical Shock

12V is safe: Can’t feel it, won’t hurt you

230V is dangerous: Can kill

Protection:

• RCD on all 230V circuits
• Double insulation on cables
• Proper earth/ground
• Professional installation (if unsure)

Procedure:

• Never work on 230V live
• Disconnect before maintenance
• Verify dead with tester
• Treat all 230V as live until proven otherwise

Gas Detection (Indirect Electrical Issue)

Why in electrical guide? Because electrical fires produce CO, and lithium batteries can produce toxic gases if damaged.

Install:

• CO detector (carbon monoxide)
• Smoke detector
• Both mains/battery powered with backup
• Test monthly

My setup:

• Combined CO/smoke detector (hardwired to always-on circuit)
• Battery backup
• Located centrally, ceiling height

Hydrogen Gas (Lead-Acid)

Lead-acid batteries produce hydrogen when charging (explosive).

Requirements:

• Ventilate to outside
• No ignition sources near battery
• Sealed battery box with vent
• Never charge in sealed space

Lithium batteries: No hydrogen production, no venting needed.

Short Circuit Protection

Every circuit needs:

1. Fuse appropriately sized
2. Fuse close to battery (within 300mm)
3. Cable sized for fuse rating
4. Secure connections (won’t vibrate loose)

Check regularly:

• Tighten connections (vibration loosens)
• Inspect for damage
• Look for heat discoloration
• Verify fuses correct rating

Battery Safety

Lithium specific:

• BMS prevents overcharge/over-discharge
• Cannot charge below 0°C (BMS should prevent)
• Fire risk if physically damaged (punctured)
• Keep away from metal objects (short risk)

Lead-acid specific:

• Acid can leak (corrosive)
• Hydrogen gas (explosive)
• Heavy (secure mounting essential)

General:

• Fuse on positive terminal
• Secure mounting (won’t move in accident)
• Accessible for maintenance
• Protected from physical damage

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Common Mistakes

I’ve made all of these. Learn from my failures.

Mistake 1: Undersizing Battery

What I did: 110Ah AGM for 70Ah daily usage

• Usable capacity: 55Ah (50% limit)
• Couldn’t make it through one day
• Constant over-discharge
• Battery died in 18 months

Lesson: Size battery for 2-3 days autonomy, not just one day.

Mistake 2: No Fusing

What I did: First van had random circuits without fuses

• “It’ll be fine”
• Had a short circuit in LED strip
• Cable overheated
• Melted insulation
• Caught it before fire (lucky)

Lesson: Fuse everything, without exception.

Mistake 3: Thin Cables

What I did: Used 2.5mm² for inverter (should be 25mm²)

• 80A draw through 2.5mm² cable
• Cable got hot enough to burn skin
• Voltage drop was 2.8V (massive)
• Inverter shut down from low voltage

Lesson: Calculate voltage drop, don’t guess cable sizes.

Mistake 4: Mixed Cable Colours

What I did: Used whatever cable I had

• Black for positive, red for negative sometimes
• Blue for some things
• Caused confusion
• Nearly wired things backwards

Lesson: Red = positive, black = negative, always. Buy proper colours.

Mistake 5: Poor Crimping

What I did: Used pliers instead of crimping tool

• Connections looked okay
• Vibration loosened them
• Intermittent faults
• High resistance = heat

Lesson: Buy proper crimping tool (£20-80). Worth every penny.

Mistake 6: No Monitoring

What I did: First van had voltage display only

• Couldn’t tell real state of charge
• Over-discharged battery multiple times
• Battery sulfated (lead-acid damage)
• Died after 18 months

Lesson: Invest in proper battery monitor. £100-150 saves £400+ battery.

Mistake 7: Oversized Inverter

What I did: Bought 2000W inverter for occasional laptop charging

• Cost £250
• Used maximum 100W
• Wasted £150 vs 1000W inverter
• Higher idle draw (waste power)

Lesson: Size inverter for actual needs + 25%, not “what if” scenarios.

Mistake 8: Cheap Components

What I did: Bought generic fuse holders (£3 vs £8)

• Corroded within months
• High resistance
• Heat damage
• Had to replace with quality units

Lesson: Buy quality for critical safety components. Fuse holders, cable lugs, terminals—don’t cheap out.

Mistake 9: No Labeling

What I did: Didn’t label cables

• Six months later, needed to trace circuit
• No idea which cable was which
• Spent 2 hours tracing
• Eventually used multimeter on every cable

Lesson: Label everything at both ends. Future you will thank present you.

Mistake 10: Ignoring Voltage Drop

What I did: Long thin cables to fridge (10m of 2.5mm², 6A load)

• Voltage drop: 1.2V
• Fridge saw 11.2V instead of 12.4V
• Ran inefficiently
• Compressor struggled

Lesson: Calculate voltage drop for every circuit. Keep drops under 3%.

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Example Systems

Real-world system designs for different use cases.

System 1: Budget Weekend Warrior

Profile:

• Weekend camping
• Drives to sites (30+ mins)
• Low power use
• Budget: £800 max

Consumption: 30Ah daily

• Lights: 10Ah
• Phone charging: 5Ah
• Water pump: 2Ah
• Misc: 13Ah

Battery: 100Ah AGM (£150)

• Usable: 50Ah
• 1.5 days autonomy

Charging: 30A DC-DC charger (£180)

• Driving charges battery
• No solar (saves £300-400)

Distribution:

• 4-way fuse box (£25)
• 4 circuits: lights, pump, USB, aux

Inverter: 300W (£50)

• Occasional use only

Monitoring: Voltage display (£12)

• Basic but functional

Total: ~£650 (battery, DC-DC, distribution, inverter, cables)

Performance: Adequate for weekend use. Driving charges battery. No off-grid capability.

System 2: Full-Time Off-Grid

Profile:

• Full-time living
• Stationary weeks at a time
• Moderate-high power use
• Budget: £1,800

Consumption: 70Ah daily

• Lights: 15Ah
• Fridge: 30Ah
• Laptop: 20Ah
• Heater: 8Ah (winter)
• Misc: 7Ah

Battery: 200Ah lithium (£680)

• Usable: 170Ah
• 2.4 days autonomy

Charging:

• Solar: 300W (£380 total)
• DC-DC: 30A (£180)
• Hookup charger: 20A (£80)

Distribution:

• 8-way bus bar (£40)
• Switches for all circuits (£60)
• Proper control panel

Inverter: 1000W (£150)

• Regular use

Monitoring: Victron SmartShunt (£130)

• Essential for off-grid

Total: ~£1,700 (battery, charging, distribution, inverter, monitoring)

Performance: Comfortable off-grid. Solar covers 85-90% of needs. DC-DC backup. Hookup emergency only.

System 3: Remote Worker

Profile:

• Full-time living
• High laptop usage
• Moderate movement
• Budget: £2,000

Consumption: 90Ah daily

• Lights: 12Ah
• Laptop: 45Ah (8 hours)
• Fridge: 28Ah
• Phone/tablet: 8Ah
• Misc: 7Ah

Battery: 300Ah lithium (£980)

• Usable: 255Ah
• 2.8 days autonomy

Charging:

• Solar: 400W (£520)
• DC-DC: 40A (£220)

Distribution:

• 10-way bus bar with monitoring (£60)
• Control panel with switches (£80)

Inverter: 1000W (£150)

• Daily use for laptop

Monitoring: Victron BMV-712 (£220)

• Display + app

Extras:

• USB-C PD outlets (£60) – efficient laptop charging

Total: ~£2,290 (over budget but worth it)

Performance: Handles high consumption. Large battery for cloudy days. Fast DC-DC for driving days. USB-C reduces inverter use.

My System (Reference)

Profile:

• Full-time living
• Remote work 3-4 days/week
• Stationary with occasional driving
• Built over time: £1,570 total

Consumption: 65Ah average, 90Ah heavy days

Battery: 200Ah lithium (£680)

Charging:

• Solar: 200W (£320)
• DC-DC: 30A (£180)
• Hookup: 20A (£80)

Distribution:

• 8-way bus bar (£35)
• Control panel (£70)
• All circuits switched

Inverter: 1000W (£180)

• Switched off when not in use

Monitoring: Victron SmartShunt (£130)

Extras:

• USB-C PD (£50)
• Water pump (£40)
• Switches/cables (£85)

Total: £1,570

Performance: Perfect for my use. Occasional winter struggles (expected). Solar covers 90% of charging. DC-DC used 2-3× weekly. Never run out of power (came close once in December).

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Final Thoughts

I’ve built four electrical systems now. The first cost £450 and lasted 14 months before major issues (dead battery, unsafe wiring, no monitoring). The current system cost £1,570 and has been flawless for 26 months.

The difference wasn’t spending more money. It was understanding the fundamentals and making informed decisions. The first system was guesswork: “I need a battery… this one’s cheap… that’ll do.” The current system was calculated: “I use 70Ah daily… I need 200Ah lithium with 300W solar and 30A DC-DC backup.”

Here’s what I’ve learned: electrical systems aren’t complicated if you understand the basics. Calculate consumption honestly. Size battery for 2-3 days autonomy. Match charging to consumption. Use proper cables. Fuse everything. Monitor everything. It’s not difficult—it’s methodical.

The most common mistake isn’t technical—it’s skipping the planning phase. People buy components before understanding their needs. They end up with random incompatible parts that sort-of work but aren’t optimal. Spend a week planning before spending a pound on components.

And please, don’t skimp on safety. Proper fusing costs £30. A van fire costs everything. Proper cables cost £100 extra. Melted cables cost a rebuild. Quality crimping tool costs £50. Poor connections cost intermittent faults and frustration. The savings aren’t worth it.

My £1,570 system powers laptop work, fridge, heating, lighting, cooking, and charging for unlimited off-grid living (9-10 months yearly in UK). That’s £60/month if I keep the van 2 years, £31/month over 4 years. Considering I’d spend £10-20/night on campsite hookup, the system paid for itself in months.

Now go calculate your actual consumption, size your system properly, and build something that works instead of cobbling together random components and hoping for the best.

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Further Resources

Books:

• “The 12 Volt Bible for Boats” (Miner/Maloney) – ignore boat-specific bits, fundamentals apply
• “RV Electrical Systems” (Bill Moeller) – comprehensive but American-focused

Websites:

• 12V Planet guides (www.12vplanet.co.uk/guides)
• Victron community forums (community.victronenergy.com)
• HandyBob’s Blog (handybobsolar.wordpress.com) – technical, American, but excellent

YouTube:

• DIY Solar Power with Will Prowse (American but good fundamentals)
• Victron Energy Official (product-specific but educational)
• Natures Generator (various builds and troubleshooting)

Calculators:

• Voltage drop: www.calculator.net/voltage-drop-calculator.html
• Battery sizing: Various van conversion sites
• Cable sizing: automotive wiring charts

Where to learn:

• Online electrical courses (not specific to vans but teach fundamentals)
• Ask experienced van builders (forums, Facebook groups)
• Experiment on test bench before installing in van

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Where to Buy (UK Sources)

Batteries:

• Lithium: Fogstar, Alpha Battery, Roamer, Power Queen (all on Amazon/direct)
• Lead-acid: Tayna, local automotive suppliers

Charging:

• Solar: Renogy UK, 12V Planet, Bimble Solar, Amazon UK
• DC-DC: Renogy, Victron, Sterling (12V Planet, Amazon)
• Chargers: Victron, CTEK, Ring (Amazon, specialist suppliers)

Components:

• Cable: 12V Planet, Vehicle Wiring Products, Auto Marine Electrical
• Fuses/distribution: 12V Planet, Blue Sea Systems (marine suppliers)
• Terminals/lugs: Vehicle Wiring Products, RS Components

Monitoring:

• Victron: 12V Planet, Amazon UK, Victron dealers
• Renogy: Renogy UK, Amazon UK
• Generic: Amazon UK (quality varies)

Tools:

• Crimping tools: Engineer PA-09 (Amazon), generic hydraulic crimpers
• Multimeters: Fluke (expensive but worth it), UNI-T (budget but decent)
• Cable strippers: Klein, Knipex, Weidmüller