When I wired the electrical system in my campervan, I picked a 1000W pure-sine inverter. My loads summed to about 700W, so 1000W felt like comfortable headroom. Then I plugged in a 800W coffee maker and the inverter shut off instantly. Every time.
Turns out I'd sized for the wrong number.
There are two wattage figures for almost every device with a motor or compressor: running watts (what it draws steadily) and surge watts (what it yanks for a fraction of a second at startup). A coffee maker, a fridge compressor, a power tool, a vacuum — they all spike on startup, often 2–4× the running draw. My 800W coffee maker was surging close to 1600W the moment it switched on, and my 1000W inverter, rated for maybe 2000W surge for a few milliseconds, was not having it under a cold start.
The fix isn't just "buy a bigger inverter," either. Oversizing has its own cost: inverters are most efficient at mid-load, and a 3000W unit running a 50W fan wastes an ugly chunk of power just staying awake. You want headroom over the combined surge of everything that could run at once — not the running sum, and not a blind max.
So I listed every device I'd ever run off inverter power, with both numbers:
| Device | Running | Surge |
|---|---|---|
| Fridge compressor | 45W | 120W |
| Coffee maker | 800W | 1600W |
| Laptop charger | 90W | 90W |
| Phone | 18W | 18W |
Coffee + fridge starting together = ~1720W surge. That's the number the inverter has to survive for a second, not the 953W running total. I landed on a 2000W pure-sine unit and the trips stopped.
One more thing people skip: pure-sine vs modified-sine matters for anything with electronics in it. Cheap modified-sine inverters can fry sensitive gear or make weird noises in speakers. For a van where you're charging laptops and running a fridge controller, pure-sine is not optional.
I worked the whole thing through CampPowerCalc and their campervan inverter sizing guide — it walks through exactly this running-vs-surge step instead of just telling you to add up watts. The power inverter page on Wikipedia is a decent reference if you want the electrical background.
If your inverter trips on one specific thing, don't assume the device is broken. Check its surge draw first.
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