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Van Electrical System Sizer
Add up what your fridge, lights, water pump, laptop and fan actually draw each day, then size a 12V battery bank and solar array around it.
How the sizing works
This tool multiplies each device's wattage by its daily hours of use to get watt-hours (Wh) per day, then sums every device for total daily energy use. The battery bank is sized in amp-hours (Ah) at 12V, scaled up by your chosen days of autonomy and divided by a realistic 80% usable depth of discharge for lithium batteries. Solar wattage is sized so your average sun hours per day can replace what you used, with an allowance for real-world charging losses.
Lithium vs. lead-acid batteries
These defaults assume lithium (LiFePO4) batteries, now the standard choice for van builds because of their higher usable capacity, longer lifespan and lighter weight. If you're using lead-acid or AGM batteries instead, size the bank for roughly half the usable depth of discharge — meaning a noticeably larger battery bank for the same daily usage.
Frequently asked questions
What counts as a device's “daily hours of use”?
The number of hours per day the device actually draws power — for a compressor fridge this is usually its average daily run time (often 8–12 effective hours from cycling on and off), not the full 24 hours it's switched on for.
Why does days of autonomy matter for battery sizing?
Days of autonomy is a buffer for cloudy days or high-use days when solar can't fully recharge the bank. Sizing the battery for more than a single day's usage means you're not left without power the first time the weather doesn't cooperate.
Why is the recommended depth of discharge only 80%?
This tool assumes lithium (LiFePO4) batteries, which can typically be discharged to about 80% of rated capacity without shortening their lifespan — so the usable Ah is smaller than the battery's printed capacity, and the sizing here already builds that in.
What if I don't have enough roof space for the solar wattage shown?
You can reduce total load (swap the highest-draw devices for lower-power alternatives), rely more on shore power or a DC-DC charger from the alternator when driving, or accept a smaller solar array and a larger battery bank you top up less frequently.