How to Calculate Your Van's Daily Power Usage
By Frank Zale · August 7, 2026 · 2 min read
Sizing a van electrical system starts with one number: your daily power usage in watt-hours. Get it right and everything else — battery, solar, inverter — falls into place. Get it wrong and you either overspend or run out of power. Here's how to calculate it in five minutes.
The one formula
For every appliance:
Watt-hours per day = watts × hours used per day
Add them all up, and that daily total drives the entire system. That's it — the rest is bookkeeping.
Step 1: List every appliance
Write down everything electrical and its power draw (on a label, in the manual, or online):
| Appliance | Watts | Hours/day | Wh/day |
|---|---|---|---|
| 12V fridge | 45 (cycling ≈ avg 22) | 24 | ~520 |
| Roof fan | 25 | 8 | 200 |
| LED lights | 15 | 4 | 60 |
| Laptop | 65 | 4 | 260 |
| Starlink | 50 | 8 | 400 |
| Phone charging | 10 | 3 | 30 |
| Total | ~1,470 Wh/day |
Tip: the fridge cycles on and off, so use about half its rated watts as the average — see how a fridge really draws.
Step 2: Turn watt-hours into a battery size
Multiply your daily total by the days of backup you want (2 is a sensible default), then divide by usable capacity per battery (~1,024 Wh per 100Ah LiFePO4):
- 1,470 Wh × 2 days = 2,940 Wh ÷ 1,024 ≈ 300Ah of LiFePO4
That matches the 200–300Ah range most full-timers land on.
Step 3: Turn watt-hours into solar
Divide daily watt-hours by (peak sun hours × 0.75 real-world factor):
- 1,470 ÷ (5 × 0.75) ≈ 390W of solar in good sun
Winter roughly doubles that requirement — output halves in low sun.
Step 4: Check the inverter
Your inverter only needs to cover your largest AC appliance running at once, not the daily total. Induction cooktop? ~2,000W. Just a laptop? A small inverter or none.
Skip the spreadsheet
The free calculator does all four steps from your appliance list — daily watt-hours, battery, solar, controller, inverter, wiring, and cost — in about two minutes. Use this article to understand the numbers; use the tool to get them fast.
FAQ
How do I calculate my van's daily power usage?
Multiply each appliance's watts by hours used per day, then add them up. That daily watt-hour total sizes your battery and solar.
How much battery do I need for my watt-hours?
Multiply daily watt-hours by your desired days of backup (usually 2), then divide by ~1,024 Wh per 100Ah LiFePO4. So 1,000 Wh/day × 2 ≈ 200Ah.
How much solar for my daily usage?
Divide daily watt-hours by (peak sun hours × 0.75). About 400W covers a typical 1,500 Wh/day remote-work van in good sun — more in winter.