What size generator do I need for a space heater?

When the power goes out in winter, a plug-in space heater is often the first thing people want to run. It's simple to size for, because it has no motor. It's also one of the hungriest things you can plug in, so it decides the generator size almost on its own.

How many watts does a space heater use?

Most plug-in heaters have two or three settings:

Setting	Watts	Amps at 120 V
Low	about 750	about 6
High	1,500	12.5

Champion lists space heaters at 750–1,500 W. LCEC, a Florida power co-op, lists 1,800 W, which fits a few larger models. Check the label on yours: it usually says "1500W" or "12.5A".

No starting surge. A heater is just a hot wire and maybe a small fan. Champion says heaters need no extra starting watts. So size by running watts only.

What size generator for one, two or three space heaters?

Add 20% headroom so the generator isn't running flat out all night.

Heaters on high	Running W	Generator running watts to look for
1	1,500	1,800+
2	3,000	3,600+
3	4,500	5,400+
1 on low	750	900+

These are running watts, not the peak figure in the model name. A "2,000 W" inverter may be rated about 1,600–1,900 running. For example, Westinghouse rates its iGen2550DFc at 1,900 running watts. One heater on high fits. Anything else big won't.

A worked example: a heater, the fridge and the basics

The calculator above is set up for this:

• Running: 1,500 (heater) + 400 (fridge) + 100 (lights) + 15 (Wi-Fi) + 80 (chargers) = 2,095 W
• Biggest surge: the fridge, 1,200 − 400 = 800 W
• Starting: 2,095 + 800 = 2,895 W
• With 20% headroom: about 2,550 running and 3,500 starting watts

That's a 3,500 watt class generator. On a 2,000 W unit, turn the heater to low when the fridge is running, or switch the heater off for a minute when the fridge starts.

Will a small generator run a space heater?

Generator	One heater on high?	Notes
1,000–1,200 W	No	Low setting only (about 750 W), and nothing else
2,000–2,500 W	Yes, on its own	Little room for a fridge at the same time
3,500 W	Yes	Plus a fridge, lights and Wi-Fi
5,000 W+	Yes, two	Or one heater with the fridge, a freezer and a sump pump

Outlets matter too

A 1,500 W heater pulls 12.5 A. Most portable generators have 20 A household outlets (5-20R). So:

• One heater per outlet. Two heaters on one duplex outlet can trip it.
• Use a heavy-duty outdoor cord rated for the load. A thin indoor cord can overheat. See 30 amp vs 50 amp generator cords.
• Don't chain power strips. Plug the heater straight into the cord.

How long will a generator run a space heater?

Not as long as you might hope. A heater on high is a big steady load, so the engine works hard the whole time.

As an example, Westinghouse says its iGen2550DFc runs about 7 hours at 50% load (950 W) on its 1.1-gallon tank. A 1,500 W heater is about 79% of that generator's 1,900 running watts, so expect noticeably less than 7 hours per tank. Our 5 gallons of gas guide shows how to estimate your own.

Is a space heater the best use of generator power?

Often not. Here's why:

• A gas furnace heats the whole house for far fewer watts. It only needs power for its blower and controls, about 700 W running for a 1/3 HP fan. See what size generator for a gas furnace.
• Heat one room. If you do use a heater, close the door to one room and heat just that.
• Use low. 750 W on low uses half the power of high, so the fuel lasts longer.

What about a power station?

A power station can run a space heater, but not for long. The math: usable Wh ÷ heater watts.

• Example: a 1,000 Wh station at about 85% efficiency gives about 850 Wh. 850 ÷ 1,500 = about 34 minutes on high.

A heater drains a battery faster than almost anything. See power stations for home backup.

Space heater safety on generator power

• Keep the heater indoors and the generator outdoors. Never bring a generator inside to keep it out of the cold.
• Give the heater space. Keep it clear of beds, curtains and paper, and follow its manual.
• Fit CO alarms. Ready.gov advises carbon monoxide detectors on every level of the home.

Safety: Run it outdoors only, 20+ feet from the house, away from windows and doors. Read our carbon monoxide guide.