How the battery calculation works
Every battery bank starts with one number: how many watt-hours (Wh) you use in a day. For each device, multiply its wattage by the hours it runs. A 50W fridge compressor that runs about 8 hours across a day uses 400Wh. Add every device together and you have your daily energy budget.
Devices that plug into an inverter cost a little extra, because inverters lose around 10% of the power they convert. The calculator divides AC loads by 0.9 to account for that.
The usable fraction is the part of the battery you can drain without damage. Lithium iron phosphate (LiFePO4) handles deep discharges well, so we plan around 80% to leave a buffer. Lead-acid batteries, including AGM, last far longer when you keep discharges to about 50%.
Worked example
Say you use 1,000Wh a day on a 12V system with lithium batteries and want one day of autonomy. 1,000 ÷ 0.8 ÷ 12 = about 104Ah. You'd round up to two 100Ah batteries, which also gives you a margin for cloudy days.
Typical daily loads
| Device | Typical watts | Typical Wh/day |
|---|---|---|
| 12V compressor fridge | 40–60 | 300–500 |
| LED light (each) | 3–10 | 10–40 |
| Phone charging | 10–20 | 20–40 |
| Laptop | 45–90 | 150–300 |
| Roof vent fan | 15–40 | 100–200 |
| Diesel heater (running) | 10–40 | 80–250 |
| Induction cooktop | 1,200–1,800 | 600–900 |
| Starlink (standard) | 50–75 | 500–1,500 |
The label on a device shows its maximum draw. Most things use less on average, so a cheap plug-in watt meter or a battery monitor gives you more accurate numbers.
Lithium vs. lead-acid
LiFePO4 costs more up front but gives you nearly twice the usable energy per amp-hour, weighs about a third as much, and typically lasts thousands of cycles. AGM is cheaper and handles freezing temperatures better while charging. For most vans and boats that are used often, lithium works out cheaper over its life. For a weekend trailer that sits most of the year, AGM can still make sense.
If your loads are small and you'd rather skip the wiring entirely, a portable power station bundles battery, inverter and solar charger in one box.