What Size Generator for a Space Heater?
A 3,400W inverter per 1,500W heater, and no surge to plan for: resistance heat draws its full nameplate the whole time it's on. That's also why the space heater is the worst use of generator watts on this site; a gallon of gas through a generator buys about a third of the heat that the same gallon's worth of propane buys through a vented heater.
Wattage by type
| Space heater type | Running | Starting |
|---|---|---|
| Personal / desk, low | 400 to 750 W | no surge |
| Standard 1,500W (planning number) | 1,500 W | no surge |
| Oil-filled radiator, high | 1,500 W | no surge |
| 240V shop heater | 4,000 to 5,600 W | no surge |
What it takes with company
Same engine as the wattage picker: running total plus the single biggest start spike, then the smallest class that carries it with 20% headroom.
| Load list | Running | Peak | Buy |
|---|---|---|---|
| One 1,500W heater | 1,500 W | 1,500 W | 3,400W inverter |
| Heater + fridge + the basics | 2,400 W | 3,900 W | 3,500W portable |
| Two heaters + the basics | 3,200 W | 3,200 W | 5,000W portable |
The details that change the answer
Resistance heaters are refreshingly honest loads: 1,500 watts on the box means 1,500 watts on the cord, continuously, no start spike. The sizing catch is different. Heaters run for hours at that full draw, so they push the generator's continuous load up against the 80% ceiling in a way that a cycling fridge never does. One heater on a 2,200W inverter is an 83% continuous load: it works, but the engine runs full song all night, drinks accordingly, and has no room for the fridge to start. That's why the recommendation with headroom lands a class up.
Now the part the wattage chart won't tell you: making heat from gasoline via electricity is spectacularly inefficient. A generator turns maybe a fifth of the fuel's energy into electricity; a vented propane or kerosene heater delivers most of its fuel's energy as room heat. If the goal is keeping a house livable for days, the generator's job is the furnace blower (800W, not 1,500 per room), and the space heater is the tool of last resort for one small room.
Questions people ask
How many space heaters can a generator run?
Divide 80% of the rated (running) watts by 1,500. A 3,500W portable carries one heater plus the basics; a 5,000W unit carries two heaters and little else; a 7,500W unit carries three with the fridge. The generator will also burn roughly a gallon of gas every two to three hours doing it, which is why the furnace blower is the better plan when you have a furnace.
Do space heaters damage generators?
No, a heater is the easiest kind of load there is (steady, resistive, no spike). The damage pattern is indirect: hours at 90%+ continuous load on a small unit runs it hot and shortens its life. Keep continuous draw at or under 80% of the rating, which for one 1,500W heater means at least a 1,875W rating with nothing else attached.