Off-grid system designer

Design your van’s electrical system.

Pick what you run and watch the whole build size itself: battery, solar, controller, inverter, and the fused wiring between them.

Daily energy at these inputs

885Wh / day

73.8 Ah at 12V

Your electrical loads

Start with a build, then fine-tune. AC items run through the inverter; DC items run straight off the battery.

Battery

Chemistry?LiFePO4 (lithium) safely uses about 85% of its capacity. Flooded and AGM batteries should only use about 50%, so they need a bigger bank for the same usable energy.

System voltage?12V is simplest and most common for vans. 24V and 48V cut wire size and current for big inverters and large arrays.

Days of reserve?How many days the bank should cover with no charging at all (cloudy, parked, or driving without solar). Two is a common starting point.

2 days

Solar

Where will you camp most?

When??Year-round sizes for the short days of winter, so the system holds up when you need it most. Spring to fall is for fair-weather travel only.

Peak sun hours?The full-strength sun hours your panels see on an average day. Auto-filled from your region and season; drag to override if you know your number.

4.0 hrs · typical for this region and season

Battery bank180 Ah2.16 kWh, LiFePO4

Solar array300 W

Charge controller40 AMPPT

Inverter150 W120W surge headroom

Wiring and fusing

Solar array to charge controller
8 AWG, 40A fuse (31.3A design)
Battery to inverter
14 AWG, 20A fuse (17.4A design)
Battery to DC fuse block
14 AWG, 20A fuse (16.7A design)

Gauges use the 125% continuous factor on short runs. Upsize for long runs to limit voltage drop. Confirm every run with a qualified electrician.

Shopping list

~$1,465

Battery bank180 Ah at 12V (2.16 kWh), LiFePO4~$755
Built from one or more batteries wired to your system voltage. Round up to the nearest size you can buy.
Solar panels300W total~$360
Combine panels to reach the total. Rigid panels are cheaper per watt; flexible panels suit curved roofs.
Charge controller40A MPPT~$140
MPPT harvests more than PWM, especially in cold or low light. Match the controller to your panel voltage.
Inverter150W pure sine (continuous), 120W surge headroom~$60
Sized to run every AC device at once. If you never do, a smaller inverter is fine. Pure sine protects sensitive electronics.
Fuses and wiringSolar array to charge controller: 8 AWG, 40A fuse; Battery to inverter: 14 AWG, 20A fuse; Battery to DC fuse block: 14 AWG, 20A fuse~$150
Fuse every positive run close to the source. Upsize the gauge for long runs to limit voltage drop.

Rough DIY component estimate, before tax and shipping. Prices vary widely by brand and source; use it for ballpark planning, not a quote.

Field guide

Start with energy, not with battery size

Most van electrical builds start at the wrong end: someone picks a battery because it was on sale, then finds out it cannot run the fridge through a cloudy weekend. A system that holds up starts from the energy you actually use in a day, then works outward to the battery, the solar, and the wiring that carries it all.

The number that drives everything is daily watt-hours: each appliance multiplied by the hours it runs. A 45W fridge that cycles for the equivalent of eight hours uses about 360 watt-hours a day. Add a roof fan, lights, and device charging and a simple van often lands near 800 to 1,000 watt-hours. AC appliances cost a little extra, because the inverter that turns battery DC into 120V AC loses some energy in the conversion.

How the battery bank is sized

A battery's rated capacity is not all usable. Lithium (LiFePO4) batteries can be drawn down to roughly 15% without harm, so about 85% is usable. Flooded and AGM batteries last far longer if you stop near 50%. That difference is why an AGM bank has to be about two-thirds larger than a lithium one to deliver the same usable energy.

The designer multiplies your daily energy by the days of reserve you choose, then divides by the usable share for your chemistry. Two days of reserve is a common starting point: enough to ride out a cloudy stretch or a few days parked in the shade without driving or shore power.

Sizing solar from peak sun hours

Solar panels are rated at full sun, but a roof rarely sees full sun all day. Peak sun hours capture that: the number of full-strength hours an average day delivers in your area and season. Winter in the Pacific Northwest can be near two; a summer day in Arizona can top six.

To replace a day's energy, the array has to make your daily watt-hours within those peak hours, after a derate for heat, wiring loss, controller loss, and dust. The tool uses 75% as a planning derate. The result is the watts of panel that, on an average day for your inputs, keep the battery even.

Wire and fuses are the safety system

Low voltage does not mean low risk. A 12V battery can push hundreds of amps into a short, and undersized wire is how van fires start. Every positive run needs a fuse close to its source, rated for the load, with wire heavy enough to carry that fuse rating.

The designer sizes each main run with the 125% continuous-load factor used in the electrical codes, picks the next standard fuse, and recommends a gauge that can carry it. These are planning figures for the short runs typical in a van. A long cable run needs an even larger gauge to keep voltage drop low, and a qualified electrician should sign off on the final build.

Frequently asked questions

How big a battery bank does a campervan need?

It depends on how much energy you use per day and how many days of reserve you want. The bank is sized as daily watt-hours times days of reserve, divided by the share of the battery you can safely use (about 85% for LiFePO4, about 50% for AGM or flooded). A van running a fridge, fan, lights, and device charging often lands somewhere between a single 100Ah lithium battery and a 300Ah bank. Enter your own loads above to see the figure for your build.

How many solar watts do I need on my van?

Enough to replace the energy you use within the hours of strong sun your panels see. The tool divides your daily watt-hours by peak sun hours and a real-world derate (heat, wiring, and controller losses). Short winter days in the north push the number up; long summer days in the Sunbelt pull it down.

Do I need an inverter?

Only if you run 120V AC appliances like an induction cooktop, microwave, or some laptops. Anything that runs on 12V DC (most fridges, fans, lights, water pumps, and many CPAP machines) draws straight from the battery and needs no inverter. The designer leaves the inverter out when every load you pick is DC.

What size wire and fuse do I need?

The tool sizes each main run using the 125% continuous-load factor and copper ampacity from NEC 310.16, then picks the next standard fuse and a gauge that can carry it. These are planning figures for short runs. Long runs need a larger gauge to limit voltage drop, and a qualified electrician should confirm the final design.

Should I build a 12V or 24V system?

12V is the simplest and most common choice for a van, and most DC appliances are made for it. As the inverter and array grow, the current at 12V gets large, which means thick, expensive cable and big fuses. At higher power, a 24V or 48V system carries the same power at lower current, so the wiring gets smaller and cheaper. The designer shows the wiring at your chosen voltage so you can compare.

Sizing conventions: usable depth of discharge from battery manufacturer guidance; conductor and overcurrent sizing follow the 125% continuous-load factor (NEC 210.20, ABYC E-11) with copper ampacity from NEC 310.16 (75C column). Component cost figures are rough DIY market estimates that vary widely. Appliance wattages are typical figures you can edit.

For educational and illustrative purposes only. Not financial, tax, or investment advice. Results depend on the accuracy of your inputs and on assumptions that may not reflect your actual situation. ForestMatters, LLC is not a registered investment advisor. Full disclaimer.