WattWhat size generator do you need to charge an EV?
Short answer: more than you think, and the number on the box isn't the number you get.
Three things shrink the sticker number
1. Peak watts are marketing
A generator advertised at 10,500 watts is quoting the surge it can hold for a few seconds. The number that matters is running watts, often 25% lower. Mine is a 10,500W unit that runs at 7,800W.
2. EV charging is a continuous load
Continuous loads get derated to 80% — the same rule your electrician applies to breakers. That 7,800W generator has about 6,240W available for hours-long charging, not 7,800W.
3. Alternate fuels cost you output
Propane typically runs about 10% below gasoline, natural gas about 20% below. Convenient fuels, smaller numbers. Altitude takes another ~3% per 1,000 feet above 3,000.
Realistic charge rates by generator size
| Running watts | Set car to | Range/hr (mild) | Range/hr (freezing) |
|---|---|---|---|
| 3,500W | 12A | ~10 mi | ~6 mi |
| 5,000W | 16A | ~13 mi | ~8 mi |
| 7,800W | 24A | ~19 mi | ~12 mi |
| 9,500W | 28A | ~22 mi | ~14 mi |
| 12,000W | 40A | ~32 mi | ~20 mi |
Inverter or conventional?
Inverter. There is no second option, and this is the spec people most often try to economise on. The full argument, including what conventional power actually does to an onboard charger →
Inverter, without much debate. Conventional generators produce a rougher waveform — total harmonic distortion of 15–25% is normal. An EV's onboard charger is a sensitive electronic load, and some will simply refuse dirty power. Inverter units hold under 3%.
Open-frame inverter generators are the sweet spot for this job: inverter-clean output, higher wattage per dollar than enclosed models, at the cost of some noise. That's what I run.
What you actually need besides the generator
- A bonding plug if yours has a floating neutral — this stops most people cold
- NEMA 14-50 connection for 240V charging; the L14-30 works at lower amperage
- A short, heavy cord — voltage sag over thin 50-foot extensions causes phantom faults
- Fuel for the duration — at 24A you're burning roughly half a gallon per 10 miles of range
Run generators outdoors only, well away from windows, doors, and any enclosed space. Carbon monoxide from a portable generator kills people every year, and it does it quickly. A CO sensor with automatic shutoff is worth insisting on when you buy.