Van battery bank size calculator.

The method, step by step

1. Daily watt-hours. For each load, watts × hours used per day. Add them up, plus any fixed daily energy you enter separately. The U.S. EIA defines a watt-hour as one watt used for one hour; its example is a 40-watt bulb for five hours, 200 Wh.
2. Days of autonomy. Multiply by how many days the battery must last without recharging.
3. Usable depth of discharge. Divide by the fraction you are willing to use. Battery University notes that manufacturers often rate a battery at 80 percent depth of discharge, and that shallower cycling lengthens service life.
4. Conversion efficiency. Divide by the efficiency of any inverter or converter in the path.
5. Convert to amp-hours. Ah = Wh ÷ battery volts.

Worked examples

Default loads. The default list totals 875 Wh a day. For 2 days at 80% usable and 90% efficiency the bank is 875 × 2 ÷ 0.80 ÷ 0.90 = 2,431 Wh, about 203 Ah at 12 V. A steady 200 W load would run for 8.8 hours.

A more cautious bank. If only 50% is treated as usable (a common rule of thumb for lead-acid, not a published standard) and you plan just one day, the same loads need 875 ÷ 0.50 ÷ 0.90 = 1,944 Wh, about 162 Ah. Planning 1 day on a 50% limit needs fewer amp-hours than 2 days at 80%, so the choice of days of autonomy matters as much as chemistry.

12 V versus 24 V. A bank for 1,200 Wh a day, 3 days, 90% usable and 90% efficient needs 1,200 × 3 ÷ 0.9 ÷ 0.9 = 4,444 Wh. That is 370 Ah at 12 V but 185 Ah at 24 V. The energy is the same; the amp-hour number halves when the voltage doubles. This is why you should compare batteries in watt-hours, not amp-hours alone.

How to calculate the watt-hours of a power bank

Watt-hours = milliamp-hours × voltage ÷ 1,000. A power bank labelled 20,000 mAh at a 3.7 V cell voltage holds 20,000 × 3.7 ÷ 1,000 = 74 Wh. Its capacity is quoted at the cell voltage, not the 5 V a phone charges at, and conversion losses mean you get less than that to your devices. If the case prints a Wh figure, use it. The power bank phone charges calculator does this for a phone.

Choosing the numbers honestly

• Read the watts off your own appliances. Fridges and fans cycle, so use the average draw and actual hours.
• Add margin for the loads you forget and for cold weather, which reduces available capacity.
• Use the manufacturer's usable-capacity or depth-of-discharge figure for your chemistry rather than a rule of thumb.

What the calculator does not cover

It sizes storage only. Charging, cables, fusing and the inverter are separate decisions: see the van solar panel size calculator, the van inverter size calculator and the solar charge time calculator. Aged batteries, very high discharge rates and low temperatures reduce real capacity.