Start with watt-hours, not battery size
Battery size is the last step, not the first. For each thing you plan to run, multiply its watts by the hours it runs in a day. That's its daily watt-hours (Wh). Add them up.
Two things trip people up:
- Fridges cycle. A compressor fridge doesn't pull its rated watts all day. Use the manual's average or daily consumption figure if it has one.
- Peak and daily are different numbers. Daily watt-hours sizes the battery. The biggest load running at one moment sizes the inverter. A cooktop you use for 20 minutes barely moves the daily total, but it sets the inverter.
What it depends on
- What you run. Fridge, fan, lights and phones are small. Electric cooking, air conditioning, a hair dryer, an espresso machine or electric heat are big. One of those can double the system.
- How long between charges. One night at a trailhead is different from four days parked in the woods.
- How you charge. Driving most days lets the alternator do a lot of the work. Sitting still puts it on solar. Shore power changes the picture again.
- Season. Winter means shorter days, less solar, more lights, and heater fans running.
- Who's aboard. Two people working remotely use more than one person on a weekend trip.
Worked sample (planning estimates)
Two people, laptops during the day, compressor fridge, no electric cooking.
| Load | Planning assumption | Wh/day |
|---|---|---|
| Compressor fridge | 25 W average × 24 h | 600 |
| Roof fan | 15 W × 8 h | 120 |
| LED lights | 10 W × 5 h | 50 |
| Two laptops | 60 W combined × 4 h | 240 |
| Phones | 20 | |
| Water pump | 50 W × 0.3 h | 15 |
| Subtotal | 1,045 | |
| Losses and things you forgot, +15% | about 1,200 |
Now size the battery. Say you want two days with no charging at all: 1,200 × 2 = 2,400 Wh.
A 12 V lithium (LiFePO4) battery is nominally 12.8 V. Planning to use about 80% of it leaves headroom: 2,400 ÷ (12.8 × 0.8) ≈ 234 Ah. Round up to the next size you can actually buy, and the planning range is about 230 to 300 Ah.
Here's where "it depends" shows up. Add a 1,500 W induction cooktop for 30 minutes a day. That's 750 Wh, plus inverter losses, call it about 830 Wh. The day is now around 2,000 Wh, the same two-day buffer wants roughly 400 Ah, and the inverter has to carry 1,500 W plus whatever else is on. Same van, one decision, very different system.
All of these are planning estimates. Real draw comes from the exact products' manuals, and once the van is running, from a battery monitor.
When to bring in a professional
The daily number is yours to work out. The wiring that carries it is a different job. Battery-to-inverter cables, the main fuse, the alternator charging run and anything on 120 V AC or shore power carry high current. Have a qualified electrician or RV/marine electrical tech design or check that part before you connect it.