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.