Most people start with the wrong number. They look at a power station's inverter rating, see 1,800W or 2,400W, and assume bigger is better. The number that decides whether your fridge is still cold on day three is the battery capacity in watt-hours, measured against what the van uses in a day. Size for that first, then check the inverter can handle your biggest load.
1. Add up a day, in watt-hours
For each appliance, multiply its power draw by the hours it runs in a typical day. These are sensible starting figures for a UK conversion. Your own appliances' labels are better than any guide.
- 12V compressor fridge: it cycles on and off, so roughly 300 to 600Wh a day, more in a hot van in July.
- LED lighting: 10 to 20W for four or five hours, around 50 to 100Wh.
- Phones: 15 to 25Wh per full charge, each.
- Laptop: a 65W charger running three or four hours of work is around 150 to 250Wh.
- Diesel heater: a short high draw while it glows up, then a low running draw. Allow 100 to 200Wh for a night.
- Induction hob: 1,800 to 2,000W. Half an hour of cooking is close to 1,000Wh on its own.
A weekend setup of fridge, lights, phones and a laptop usually lands between 400 and 600Wh a day. Add daily cooking on induction and you are quickly past 1,500Wh.
2. Allow for losses
Anything that plugs into a 230V socket goes through the inverter, which loses energy as heat. Add around 15% to your mains loads. 12V loads such as the fridge lose far less.
3. Decide how many days you need without charging
Multiply your daily figure by the number of days you want to last with no solar, no driving and no hookup. For a summer weekend that might be one or two days. For full-time living through a British winter, when a grey week can give you almost nothing from a panel, plan for more, or plan for a charging source that doesn't depend on the weather.
4. Check the inverter against your biggest load
Only now does the inverter rating matter. It needs to cover the largest thing you run, plus anything running at the same time. An 1,800W unit will not run a 2,000W induction hob. If you want to cook on induction, you need 2,400W or more.
How that maps to real units
- Weekends, 400 to 600Wh a day: the AC180P has 1,440Wh and 1,800W, enough for two to three days of that load before it needs charging.
- Long trips, around 850Wh a day with a heater: the AC200PL has 2,304Wh and 2,400W, which also takes an induction hob.
- Full time, 1.5kWh a day and up: the Apex 300 has 2,764.8Wh and a 3,840W inverter, and can be expanded with extra batteries. At this level, charging matters as much as capacity.
Size for the worst day
The mistake we see most is sizing for a sunny day in June. Work out what your van needs on a wet week in February, when the heater runs longer and the panel gives you very little, and buy for that. Everything else is a bonus.
If you'd rather not do the arithmetic by hand, the Roam Wired electrical calculator does it from a plain list of appliances, and gives you cable and fuse sizes as well. Anything wired into 230V mains should be signed off by a qualified electrician.