Every other electrical sizing decision in a van build — battery, solar, wire, inverter — depends on getting this one calculation right first. Here's the actual method.
The Core Formula
Daily Watt-Hours = Appliance Watts × Hours Used Per Day
Do this for every 12V and 120V (via inverter) appliance you plan to run, then add them all together for your total daily load.
Step 1: List Every Appliance and Its Real Watts
Don't guess wattage — check the actual spec sheet or nameplate for each device. Common ranges as a starting reference:
| Appliance | Typical Watts |
|---|---|
| Compressor fridge (running, not startup) | 45–90W |
| LED lighting (per fixture) | 3–10W |
| Water pump | 40–60W |
| Vent fan | 10–30W |
| Laptop charging | 60–100W |
| Phone charging | 5–10W |
| Induction cooktop (per burner) | 1,200–1,800W |
Step 2: Estimate Realistic Daily Hours of Use
This is where most people either wildly overestimate or underestimate. Be honest about actual use patterns, not theoretical maximums — a fridge compressor cycles on and off rather than running continuously, so use a realistic duty-cycle estimate (often 30-40% of the day) rather than 24 hours of continuous full draw.
Step 3: Add It All Up
| Appliance | Watts | Hours/Day | Daily Wh |
|---|---|---|---|
| Fridge (cycling ~35%) | 60W | 8.4 | 504 |
| Lighting (4 fixtures) | 24W total | 4 | 96 |
| Water pump | 50W | 0.25 | 12.5 |
| Vent fan | 20W | 6 | 120 |
| Laptop | 80W | 3 | 240 |
| Phone | 8W | 2 | 16 |
| Total | ~989 Wh/day |
This total daily Wh figure is the single number that then drives your battery sizing (see our battery bank sizing guide), solar sizing (see our solar wattage guide), and — combined with your highest simultaneous appliance combination — your inverter sizing.
Don't Forget Simultaneous Peak Load
Total daily watt-hours tells you your battery and solar needs, but it doesn't tell you your inverter size — that's a separate calculation based on what you might run at the same time, not spread across a day. A fridge running quietly in the background plus a laptop charging plus an induction cooktop briefly firing up is a simultaneous peak load calculation, covered in our inverter sizing guide.
Common Mistakes in This Calculation
- Using an appliance's maximum rated wattage instead of typical running wattage — many devices have a lower steady-state draw than their peak/startup rating.
- Forgetting inverter conversion losses — running 120V appliances through an inverter typically loses 10-15% of the energy as heat; padding your calculation by this margin for inverter-run appliances is a reasonable safety margin.
- Underestimating "small" loads — phone chargers, LED strips, and USB devices seem trivial individually but add up meaningfully across a full day when there are several of them.
Frequently Asked Questions
What's a normal daily watt-hour usage for van life? Varies enormously by setup, but a moderate build with a fridge, lighting, laptop, and basic electronics commonly lands between 800-1,200 Wh/day — light use can be under 500 Wh, heavy use with cooking and remote work can exceed 2,000 Wh.
Do I need to calculate load before buying solar and batteries? Yes — this calculation is the foundation for every other electrical sizing decision. Buying components first and hoping they're "enough" is how builds end up either underpowered or with wasted budget on oversized systems.
How accurate does my load calculation need to be? Reasonably accurate, with a margin of safety (many builders add 15-20% buffer to their final number) — perfect precision isn't necessary, but working from real appliance specs rather than rough guesses meaningfully improves sizing accuracy.