Viewing from Europe (EU)
Auto-detectedCable sizes are in mm² (metric), consistent with EU/UK/AU standards.
Electrical Units & Basics#
You don't need an electrical engineering degree to design a safe off-grid system — but you do need to be comfortable with a handful of core concepts.
Volt (V)
Electrical pressure — the force that pushes current through a circuit. Higher voltage = same power at lower current.
Ampere (A)
Current — the rate of electron flow. High current = thick cables, big fuses, and heat if undersized.
Watt (W)
Power — the rate of energy use or production. W = V × A. Every device has a wattage rating.
Watt-hour (Wh)
Energy — power over time. A 100W device running for 10 hours uses 1000Wh (1kWh). Batteries are sized in Wh or Ah.
Understanding Amp-hours (Ah)
Amp-hours measure how much charge a battery can store — but the number only makes sense when paired with voltage. A 100Ah battery at 12V stores 1200Wh. The same 100Ah at 24V stores 2400Wh — twice the energy, same Ah rating.
// Always convert to Wh when comparing batteries at different voltages
100Ah @ 12V: = 1,200 Wh = 1.2 kWh
100Ah @ 24V: = 2,400 Wh = 2.4 kWh
Resistance & Voltage Drop
Every cable has resistance. Resistance causes voltage drop and heat. The longer and thinner the cable, the higher the resistance.
// Resistance of copper cable (approximate)
4mm² × 10m run: R ≈ 0.044Ω (both ways)
At 50A: Vdrop = 50 × 0.044 = 2.2V
Power lost: 50² × 0.044 = 110W as heat
Still confused about Electrical Units?
Ask the community — real builders who've done it will answer.
Essential Formulas#
These seven formulas are the ones that come up on every single build. Master these and you can size any component in your system.
The Seven Essential Calculations
Ohm's Law — Voltage
V = I × R
Voltage (V) equals current (A) multiplied by resistance (Ω). The foundation of all DC circuit analysis.
Example
A 100A load through 0.01Ω of cable resistance drops 1V.
Ohm's Law — Current
I = V ÷ R
Current (A) equals voltage divided by resistance. Used to find fault current or check load draw.
Example
A 12V battery into a 0.006Ω short delivers 2000A — why Class T fuses are mandatory.
Power
P = V × I
Power in watts equals voltage multiplied by current. The most-used formula in system sizing.
Example
A 24V system drawing 50A delivers 1200W to the load.
Energy (Watt-hours)
Wh = W × hours
Energy consumed equals power multiplied by time. Used to calculate daily load and battery sizing.
Example
A 60W fridge running 24 hours uses 1440Wh (1.44kWh) per day.
Amp-hours to Watt-hours
Wh = Ah × V
Convert battery capacity from Ah to Wh by multiplying by system voltage.
Example
A 100Ah 12V battery holds 1200Wh. The same 100Ah at 24V holds 2400Wh.
Voltage Drop
Vdrop = I × R_cable
Resistive loss in a cable run. R_cable = (ρ × 2L) ÷ A, where ρ is resistivity, L is one-way length, A is cross-section.
Example
10m of 6mm² cable at 50A drops ~0.3V — within the 3% limit for a 12V system.
Power Loss (I²R)
P_loss = I² × R
Heat generated in a cable. Doubles with every doubling of current — the reason undersized cables overheat.
Example
50A through 0.01Ω loses 25W as heat. 100A through the same cable loses 100W.
Skip the manual maths
Our DC Wire Calculator applies all of these formulas automatically — enter your load, cable length, and installation method and it returns the correct gauge, voltage drop, and fuse size.
Apply these formulas to your circuits
Enter your load, cable length, and system voltage — the Wire Calculator does all the maths.
Still confused about Essential Formulas?
Ask the community — real builders who've done it will answer.
Equipment Overview
The physical hardware that connects your system — wire types, termination hardware, and distribution components.
Wire & Cable Types
Flexible DC Cable (V90 / TFX)
Battery interconnects, busbar feeds, inverter cables
Multi-strand, flexible, rated 90°C. Standard for all high-current DC runs inside the vehicle or cabin. Available in 4mm² to 120mm².
PV Solar Cable (PV1-F / USE-2)
Panel strings, roof-to-MPPT runs
Double-insulated, UV-resistant, rated 1000V DC. Mandatory for all outdoor panel wiring. Not interchangeable with standard DC cable.
Automotive TXL / GXL
Low-current branch circuits, signal wiring
Thin-wall insulation, rated 125°C. Good for 12V accessories and control wiring under 20A. Not suitable for high-current runs.
Marine Tinned Copper
Any installation near salt water or high humidity
Tinned conductors resist corrosion at terminations. Required by ABYC E-11 for marine DC systems. Costs more but lasts significantly longer in wet environments.
Welding Cable
Budget high-current runs (not recommended)
Very flexible, cheap, but not rated for fixed electrical installations. Insulation degrades faster than purpose-made DC cable. Use proper flexible DC cable instead.
Crimp, don't solder high-current lugs
Solder wicks into the strands of flexible cable, making them rigid. Under vibration, the cable flexes at the solder boundary and breaks — often inside the insulation where you can't see it.
Still confused about Equipment Overview?
Ask the community — real builders who've done it will answer.
Cable Sizing Reference#
Cable sizing is a two-variable problem: current (Amps) and run length (metres). Get either wrong and you get resistive heating, voltage drop, or fire.
Ampacity Table — Flexible DC Cable
75°C rated conductor · single cable · free air · 30°C base · AU/EU: mm² · USA/CA: AWG
| mm² (AU/EU/UK) | AWG (USA/CA) | Max Amps | Resistance (mΩ/m) | Typical application |
|---|---|---|---|---|
| 2.5mm² | AWG 14 | 25A | 7.41 | Branch circuits, lighting, USB hubs |
| 4mm² | AWG 12 | 32A | 4.61 | MPPT battery cable (small), fridge |
| 6mm² | AWG 10 | 40A | 3.08 | MPPT battery cable (mid), DC-DC charger |
| 10mm² | AWG 8 | 57A | 1.83 | MPPT battery cable (large), 600W inverter |
| 16mm² | AWG 6 | 76A | 1.15 | 1000W inverter @ 24V, large battery interconnects |
| 25mm² | AWG 4 | 101A | 0.727 | 1500W inverter @ 12V, battery to busbar |
| 35mm² | AWG 2 | 125A | 0.524 | 2000W inverter @ 12V, large battery banks |
| 50mm² | AWG 1/0 | 151A | 0.387 | 2000W+ inverter @ 12V, commercial/high-current runs |
Wire Size Calculator
Enter current, run length, and system voltage — get the correct cable size, voltage drop percentage, and fuse rating instantly.
Still confused about Cable Sizing?
Ask the community — real builders who've done it will answer.
Was this section helpful?
Required Tools
Cutting
Wire cutters / cable cutters (sized for your largest cable)
Conduit cutter or hacksaw for cable management
Utility knife for insulation trimming
Stripping
Automatic wire stripper (for branch wiring 0.5–6mm²)
Heavy-duty cable stripper (for 10–120mm² battery cable)
Rotary cable stripper for large lugs
Crimping
Ratchet lug crimper (hydraulic or mechanical) — for battery lugs 10–120mm²
Ratchet ferrule crimper — for bootlace ferrules on fine-strand wire
Insulated terminal crimper — for blade connectors and ring terminals
Measuring
Digital multimeter (voltage, current, resistance, continuity)
Clamp meter (non-contact current measurement — essential for commissioning)
Thermal camera or IR thermometer (post-install hot-spot check)
Fastening
Torque wrench or torque screwdriver (for terminal bolts — over-torquing cracks lugs)
Hex key / Allen key set (for busbar and fuse holder bolts)
Cable tie gun and stainless cable ties
Safety
Insulated gloves (rated for DC voltage)
Safety glasses
Voltage tester / non-contact tester before touching any terminal
System Voltage
System voltage is the first decision — and one of the hardest to change later. The core principle: higher voltage = lower current for the same power, which means smaller, cheaper cable throughout the system.
Vehicles with existing 12V DC loads, simple weekend setups, small campers under 400Ah
Most van builds, camper trailers, small cabins — the most balanced option for solar systems above 600W
| Factor | 12V | 24V |
|---|---|---|
| Typical use case | Camper / small RV | Van / cabin / medium RV |
| Max practical solar | ~600W | ~2400W+ |
| Wire gauge benefit | None — thickest cable | Same power, half the amps |
| DC appliances | Wide availability | Good availability |
| Charge controller | Any MPPT | Any MPPT |
| System complexity | Lowest | Moderate |
Still confused about System Voltage?
Ask the community — real builders who've done it will answer.
Solar Panels
Nearly all modern off-grid panels use monocrystalline silicon. The panel label (Pmax) is measured under Standard Test Conditions (STC): 1000W/m² irradiance at exactly 25°C. Real-world output is typically 75–85% of STC.
The cold Voc problem
Solar cell voltage increases as temperature drops — typically by 0.3–0.4% per °C below 25°C. Always apply the cold-temperature correction using the panel's Voc temperature coefficient before selecting a controller.
Reading the Datasheet
Still confused about Solar Panels?
Ask the community — real builders who've done it will answer.
Batteries#
LiFePO4
Usable DoD
95–100%
Cycle life
3,000–6,000
Weight
Lightest
Self-discharge
~2% / month
Requires BMS. Low internal resistance — Class T fusing mandatory. No memory effect. Best choice for mobile.
Cost
Highest upfront
AGM
Usable DoD
50–60%
Cycle life
400–800
Weight
Heavy
Self-discharge
~3% / month
Sealed — no venting required. Tolerates overcharge better than flooded. Widely available. Good for budget builds.
Cost
Moderate
Lead Acid (Flooded)
Usable DoD
40–50%
Cycle life
200–500
Weight
Heaviest
Self-discharge
~5% / month
Requires venting. Regular watering needed. Temperature sensitive. Suitable for stationary, low-budget applications.
Cost
Lowest
Key Battery Concepts
Depth of Discharge
How much of the battery's capacity you can safely use. LiFePO4: 80–95%. AGM: 50%. Exceeding DoD shortens battery life.
State of Charge
How full the battery is, expressed as a percentage. A shunt-based monitor is the only accurate way to track SoC on LiFePO4.
C-Rate
The rate of charge or discharge relative to capacity. 1C = full charge in 1 hour. Exceeding the C-rate reduces capacity and damages cells.
Still confused about Batteries?
Ask the community — real builders who've done it will answer.
Charge Controllers#
A charge controller sits between your solar array and battery bank, preventing overcharging and managing charge stages.
| Aspect | MPPT | PWM |
|---|---|---|
| Efficiency | 93–98% | ~70% (at battery voltage) |
| High Voc arrays | Handles any Voc > V_battery | Panel Vmp must ≈ battery V |
| Cost | Higher | Lower |
| Small 12V systems | Slight overkill < 200W | Works fine if panel Vmp ≈ 14–15V |
| Winter performance | Better (converts voltage boost) | Poor — cold = higher Voc = waste |
| Cable cost | Lower — higher V, lower A | Higher — must match battery voltage |
Still confused about Charge Controllers?
Ask the community — real builders who've done it will answer.
Inverters & AC Systems
An inverter converts your DC battery bank into AC mains power.
Standalone Inverter
Simple buildsConverts DC to AC only. No built-in charger. Best for systems where you only need AC occasionally.
Inverter-Charger (Combi)
Full-time buildsCombines a pure sine wave inverter with a multi-stage battery charger and shore power passthrough.
Pure Sine Wave
Always choose thisProduces clean AC power identical to the grid. Required for sensitive electronics, motors, and all modern appliances.
Modified Sine Wave
AvoidProduces a stepped waveform that approximates AC. Can damage sensitive electronics.
Still confused about Inverters?
Ask the community — real builders who've done it will answer.
Was this section helpful?
Sizing Your System#
The most common and costly mistakes in off-grid builds come from skipping the sizing process — buying components first and calculating later.
List every load
Write down every device you plan to run — fridge, lighting, laptop, inverter loads, water pump, fans. For each, note the wattage (from the label or datasheet) and the realistic hours per day you'll use it.
Pro tip
Don't guess wattage — measure it. A cheap plug-in power meter (Kill-A-Watt or similar) on your actual appliances gives far more accurate numbers than label ratings, which are often peak not average.
Calculate daily Wh
Multiply each device's watts by its daily hours to get Wh/day. Sum all devices for your total daily energy requirement. Add a 20% buffer for inefficiencies (inverter losses, cable losses, charger losses).
Pro tip
Compressor fridges are the biggest variable — they cycle on and off. A 50L fridge in a hot van might run 60–70% of the time; in a cool cabin, 30–40%. Measure or use a conservative estimate.
Size the battery bank
Divide daily Wh by your chemistry's usable DoD, then multiply by autonomy days. For LiFePO4: ÷ 0.95. For AGM: ÷ 0.50. For flooded: ÷ 0.45.
Pro tip
For most van builds, 1–2 days autonomy is realistic. For remote cabins, 3–5 days. More autonomy = more battery cost and weight — balance against your solar array size.
Size the solar array
Divide your daily Wh requirement by your location's peak sun hours. Then multiply by 1.25 to account for real-world derating (heat, dust, wiring losses).
Pro tip
Peak sun hours are not daylight hours — they're the equivalent hours of full 1000W/m² irradiance. Always use your worst-case month (winter) for sizing if you use the system year-round.
Select the MPPT controller
MPPT output current = array watts ÷ battery voltage × 1.1. Round up to the next standard size. Check that the cold-corrected array Voc is below the controller's maximum input voltage.
Pro tip
It's fine to slightly oversize the MPPT — a 40A controller on a 30A array just means the controller never runs at full capacity. Undersizing clips your solar harvest on bright days.
Choose the inverter
Inverter continuous rating must exceed your peak simultaneous AC load. For motor loads (pumps, compressors, power tools), size for the startup surge — typically 3–6× the running wattage. Always choose pure sine wave.
Pro tip
If you only run one large AC load at a time, size for the largest single load plus a 20% margin — not the sum of all loads.
Still confused about System Sizing?
Ask the community — real builders who've done it will answer.
Fuse Types & Hierarchy#
The most common cause of DC electrical fires is inadequate fusing. Use a Class T fuse for LiFePO4 batteries and an ANL fuse for AGM. Size every fuse to the cable's ampacity.
The Fuse Hierarchy
Battery terminal
Class T / ANLMain protection — first defence against catastrophic fault. Within 150–300mm of battery positive.
At distribution panel feed
ANL / MIDIProtects the main cable run from battery to busbars. Sized to the cable ampacity.
Each branch circuit
Blade (ATO) / MIDIIndividual device protection. Sized to the branch cable, NOT the device draw.
Solar array strings
MC4 inline fuseString protection for parallel solar arrays (2+ strings). See MC4 Fuse Calculator.
Still confused about Fuse Types?
Ask the community — real builders who've done it will answer.
Monitoring & Safety
A well-monitored system is a safe system.
BMS Protection Functions
Over-voltage protection (HVC)
Disconnects charging when any cell exceeds the maximum voltage (typically 3.65V for LiFePO4). Prevents overcharging and thermal runaway.
Under-voltage protection (LVC)
Disconnects load when any cell drops below minimum voltage (typically 2.5V). Prevents deep discharge damage.
Over-current protection
Disconnects if current exceeds the BMS rating. Protects cells from excessive discharge rates.
Short circuit protection
Disconnects within microseconds of a short circuit. Essential for LiFePO4 which can deliver enormous fault currents.
Temperature protection
Blocks charging below 0°C (prevents lithium plating) and above 60°C.
Cell balancing
Equalises cell voltages to maximise usable capacity and extend pack life.
System Monitoring Options
Victron BMV-712
Shunt-based monitorThe gold standard for off-grid monitoring. Tracks SoC, voltage, current, power, and time-to-go. Bluetooth app.
Victron Cerbo GX
System hub + monitorConnects all Victron components into one system. Touchscreen display, VRM cloud portal, remote monitoring.
Victron SmartShunt
Shunt-only (no display)Same accuracy as BMV-712 but without the physical display. Pairs with the Victron Connect app.
Still confused about Monitoring & Safety?
Ask the community — real builders who've done it will answer.
Vehicle Integration
Charging from a vehicle alternator is one of the most misunderstood topics in van and RV builds. A DC-DC charger (B2B) is the correct solution.
Why VSRs Fail with Smart Alternators
Modern vehicles use smart alternators that vary output voltage (11.5–15V) to improve fuel economy. A VSR triggers at a fixed voltage — but a smart alternator may never reach this threshold.
Unreliable triggering with variable alternator voltage
No charge profile — dumps raw alternator voltage into battery
Can overload the alternator by drawing too much current
DC-DC Charger Advantages
A DC-DC charger uses a proper multi-stage charge profile, works with any alternator voltage, and limits current draw to protect the alternator.
Works with smart alternators — triggers on D+ signal or voltage threshold
Proper multi-stage charge profile for LiFePO4 or AGM
Galvanic isolation — protects both batteries from each other
Fixed output current — won't overload the alternator
Still confused about Vehicle Integration?
Ask the community — real builders who've done it will answer.
Step-by-Step Build Guide
Three complete worked examples — from a simple weekend camper through to a full off-grid cabin.
Weekend Camper Van
| Device | Watts | Hrs/day | Wh/day |
|---|---|---|---|
| 12V Compressor Fridge (50L) | 45W | 24h | 1,080 |
| LED Lighting | 20W | 5h | 100 |
| Phone / Laptop Charging | 60W | 3h | 180 |
| Induction Cooktop (via inverter) | 1800W | 0.5h | 900 |
| Fan / Ventilation | 15W | 8h | 120 |
| Total daily load | 2,380 Wh | ||
Component Selection
12V · 200Ah LiFePO4 · 400W Solar · 2000W Inverter
2× Renogy Core 12V 100Ah LiFePO4 (parallel)
200Ah · 2.4kWh · 1 day autonomy at 80% DoD
2× 200W Renogy Mono panels (400W total)
Roof-mounted, 4–5 peak sun hours → ~1.6–2kWh/day
Victron SmartSolar 100/30
400W ÷ 12V × 1.1 = 36A → 30A controller adequate
Renogy 2000W Pure Sine Inverter
Handles 1800W induction cooktop with 200W headroom
Victron Orion-Tr Smart 12/12-30A
Charges from van alternator while driving
Class T 200A at battery terminal
Mandatory for LiFePO4 — clears in microseconds
Key Design Decisions
The induction cooktop is the biggest single load at 1800W — this drives the inverter size.
200Ah LiFePO4 at 1C can deliver 200A (2400W at 12V) — just enough for the cooktop with margin.
400W solar covers the daily load on a good day. DC-DC charger from driving makes up the difference.
Still confused about Build Guide?
Ask the community — real builders who've done it will answer.
Was this section helpful?
Common Technical Mistakes
These are the mistakes that experienced builders still make — and that beginners make constantly. Several of them are fire hazards. Read every one.
Still confused about Common Mistakes?
Ask the community — real builders who've done it will answer.
Was this section helpful?
Ready to design your system?
Use our guided Setup Wizard to size your complete system — panels, battery, controller, and cables.
Something missing from this module?
Related Troubleshooting Issues
Common issues related to what you just read
15 more issues on the troubleshooting page
Open Troubleshooting GuideYou might also like
Electrical Units & Basics
Voltage, current, watts, watt-hours, amp-hours explained
Electrical 101Batteries — Beginner Guide
LiFePO4 vs AGM, series vs parallel, price vs lifespan
Electrical 101Charge Controllers — Basics
MPPT vs PWM, sizing formula, cold Voc warning
Electrical 101Common Beginner Mistakes
Undersized cable, wrong fuse, LiFePO4 with AGM settings
Electrical 101FAQ — Getting Started
Where to start, 12V vs 24V, how much solar, DIY permissibility
FAQ