Electric Space Heater Generator Calculator

Data reviewed 2026-07-05 ·how we calculate

An electric space heater is the simplest load on this site — a 1,500W resistance element with no motor, no surge, no subtlety — and simultaneously the worst way to spend generator capacity ever popularized. Converting gasoline to electricity to heat throws away two-thirds of the fuel’s energy before it warms anything. This page gives the honest numbers for when a heater is still the right call (one room, medical need, no gas heat) and points to the furnace-blower alternative that heats the whole house on half the watts.

Size a generator for this load

Computed on typical values; ranges shown below. 25% headroom applied.

Generator safety — read every time

Never run a generator indoors, in a garage, or near windows, doors or vents.Carbon monoxide kills quickly and without warning — place the generator 20+ feet from the house with the exhaust pointed away, and use battery CO detectors inside.
Never backfeed your house through a wall outlet or dryer outlet. Backfeeding can electrocute utility line workers and destroy equipment when power returns.
Connecting a generator to home circuits requires a transfer switch or interlock installed by a licensed electrician. There is no safe DIY shortcut, and this site does not provide wiring instructions.
Use heavy, outdoor-rated extension cords (12-gauge or thicker), fully uncoiled, sized to the load and the distance.

How to size it step by step

Take the nameplate at face value: US space heaters are almost universally 1,500W on high, 750W on low. No surge math needed — this is the one load type where the label is the whole story.
Apply 25% headroom: 1,500W × 1.25 = 1,875W. A 2,000W inverter runs one heater and nothing else; each additional heater needs another ~1,900W of generator.
Do the alternatives check before committing watts: if you have gas heat, the furnace blower (700W) heats every room; if not, an indoor-rated propane heater (Mr. Heater Buddy class) delivers far more heat per fuel dollar than generator-fed resistance.
If the heater still wins (medical need, one small room, no alternatives): low setting, one room, door closed, and let the rest of the house go cool.

Pro tips

Check your nameplate first — every figure on this page is a planning estimate, and the label on your specific unit beats any chart.
Heat the person before the room: an electric blanket draws 50–100W — 15–30 nights of sleeping warmth for the generator fuel one space-heater evening costs.
Oil-filled radiator heaters draw the same 1,500W but hold heat through the thermostat’s off-cycles, wringing slightly more comfort from each generator-hour than fan heaters.

The data behind this calculator

Space heater load figures used by this calculator
Figure	Value	Source
High setting	1,500W continuous (the 120V design maximum)	Lasko/Honeywell/DeLonghi spec sheets; NEC 80% circuit-loading convention
Low setting	~750W continuous	Standard two-setting heater ratings
Surge allowance	None — resistive load, starting = running	Load physics: heating elements have no inrush of consequence
Heat delivered per gallon of generator gas (electric resistance)	roughly 1/3 of the gallon’s heat content	Small-engine thermal efficiency ~20–30% — estimate, see /methodology

Duty cycle: With the thermostat satisfied the element cycles off entirely, so average draw over a night is below nameplate — but size for the full 1,500W, since the element runs continuously in a cold room.

Portable electric heater generator questions, answered

Can a 2,000-watt generator run a space heater?

Yes — exactly one, on high, with almost nothing else. A 1,500W heater with 25% headroom wants 1,875W of the generator’s typical 1,600–1,800W continuous rating, so the fridge or the microwave has to wait its turn. On the 750W low setting the picture relaxes considerably. There is no starting surge to worry about; this is purely a continuous-capacity question.

Why is an electric heater a poor use of generator power?

Thermodynamics does it no favors: a portable generator turns roughly 20–30% of gasoline’s energy into electricity, and the heater turns that electricity back into heat — so two-thirds of every gallon warms the outdoors around the generator instead of your room. Burning fuel directly for heat (the gas furnace via its blower, or an indoor-rated propane heater) delivers 3–4× the warmth per gallon. Electric resistance is the fallback, not the plan.

What size generator for two space heaters?

Two heaters on high draw a flat 3,000W continuously — with headroom, 3,750W, which means the 3,500–4,500W portable class working hard for as long as they run. That is also roughly two gallons of gasoline every three-to-four hours just for heat. Before buying that generator, price an indoor-safe propane heater: 9,000 BTU (a Buddy heater on high) is the heat equivalent of about 2,600W of electric heaters on a fraction of the fuel cost.

Is it safe to run a space heater on an extension cord from the generator?

Only on a heavy one. A 1,500W heater pulls 12.5A continuously, which cooks undersized cords: use a 12-gauge (or heavier) outdoor-rated cord, fully uncoiled — a coiled cord under continuous load builds heat inside the coil. Plug the heater directly into the cord (no power strips, which their instructions prohibit), and as with any heater: three feet of clearance, hard level surface, and never unattended while sleeping unless it has tip-over and overheat protection.

Browse allHeating & Cooling calculators, combine appliances in themulti-appliance builder, or start from anoutage scenario.

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