Space heater on a battery: the honest math

This is the appliance-specific version of a question our winter outage guide answers at house scale, and it deserves its own page because the search is so specific and the answer is so blunt. No portable battery sold today will run a space heater for a length of time that solves a winter outage. Not the biggest one. Not on the low setting. The arithmetic is one division and it is not close, so the useful part of this page is the second half: what the published numbers say you should be running instead. We have not tested any of these products, and this page carries no product picks, because the honest recommendation is not a product.

The load, as the maker publishes it

Lasko's product page for the 754200 ceramic heater publishes the whole specification in three lines: Energy Output 900 W to 1500 W, Current 7.5 A to 12.5 A, Voltage 120 V. The description states the two settings plainly: "two heat settings, 1500 watts on high and 900 watts on low, plus a fan-only mode." Lasko also publishes a maximum coverage area of 100 square feet.

Those numbers are typical of the entire plug-in resistive category in the United States, and the reason is the circuit rather than the product. A 120 V 15 A branch circuit tops out at 1,800 W, so manufacturers design to sit comfortably under it. That is why shopping for a different heater does not change the answer below.

Note the coverage figure while you are here: 100 square feet. This is a product designed to warm a person at a desk, not a house.

The arithmetic, unit by unit

Rated capacity divided by 1,500 W. These are the five largest portable units we track, with capacities read from each maker's own spec table on August 19, 2026.

| Unit | Rated capacity | At 1,500 W (high) | At 900 W (low) |

|---|---|---|---|

| BLUETTI Apex 300 | 2,764.8 Wh | 1.8 h | 3.1 h |

| Anker SOLIX F3800 Plus | 3,840 Wh | 2.6 h | 4.3 h |

| EcoFlow DELTA Pro 3 | 4,096 Wh | 2.7 h | 4.6 h |

| Jackery Explorer 5000 Plus | 5,040 Wh | 3.4 h | 5.6 h |

| EcoFlow DELTA Pro Ultra | 6,144 Wh | 4.1 h | 6.8 h |

Every one of those is a ceiling. They use rated rather than usable capacity, they ignore inverter conversion losses, they ignore the low voltage cutoff, and they ignore the station's own standby consumption, which is not trivial at these timescales: BLUETTI publishes a 20 W idle loss for the Apex 300. Shade the whole table down.

The output column is never the problem, which is part of why people are surprised. Every unit in that table publishes 3,840 W or more of continuous AC output, so all of them start and run a 1,500 W heater without complaint. They simply run out. A heater is the rare load where capacity is the binding constraint and surge headroom is irrelevant.

Why no product fixes this

A resistive heating element converts electricity to heat at one to one. Every watt in becomes a watt of heat, and there is no multiplication available anywhere in the physics. That is the opposite of every other appliance in a house, where a motor or a compressor does work that is worth more than the electricity it consumes, and it is the reason a battery that carries a refrigerator for two days carries a space heater for two hours.

Scaling up does not rescue the plan either. The largest single-battery capacity among the portable units on this page is the EcoFlow DELTA Pro Ultra's published 6,144 Wh, and one heater on high empties it in an evening. Heating a house with stored electricity is an installed-system-plus-solar problem, not a suitcase problem, and our winter outage guide works through why at whole-house scale.

The published number that shows how big the gap is

Heat pumps do the multiplication that resistive elements cannot, and the specification that captures it is published.

Mitsubishi Electric's submittal for the MSZ-FS06NA indoor unit paired with the MUZ-FS06NAH outdoor unit publishes, for heating at the AHRI 47 degree F rating condition:

• Rated Capacity: 8,700 BTU/H
• Rated Power Input: 545 W
• Maximum Power Input: 1,270 W
• COP at 47 degrees F: 4.68
• HSPF [IV]: 12.5, ENERGY STAR certified

Convert the heat output into the same units as the heater: 8,700 BTU per hour is about 2,550 W of heat. So at its rated condition that system produces roughly 1.7 times the heat of a 1,500 W space heater while drawing about a third of the watts. Against a battery, that is the difference between 4.1 hours and something in the region of 11 hours from the same 6,144 Wh, while the room gets warmer rather than cooler.

Mitsubishi also publishes what happens as it gets colder, and the honest version includes this: COP at 17 degrees F at maximum capacity is 2.46, at 5 degrees F it is 2.02, and at minus 13 degrees F it is 1.73. The advantage narrows on the worst night of the year. It never disappears, but a spec sheet COP of 4.68 is a 47 degree F number, not a blizzard number.

Two constraints sit in front of all of that, and they are the reason this section is context rather than a recommendation:

It is not a plug. Mitsubishi publishes the electrical supply as 208/230 V, single phase, 60 Hz, with an outdoor unit MCA of 10.0 A and maximum overcurrent protection of 15 A. A hardwired 240 V appliance needs a 240 V capable power station and an electrician-installed transfer switch or backup panel before the efficiency matters at all. That is the same gate covered in our critical loads panel guide and our 120 V vs 240 V guide.

Not every heat pump behaves like this one. This is a small, inverter-driven, twin rotary system, and Mitsubishi publishes compressor locked rotor amps of 7.4 A for it. A central air source system is a different animal: our winter outage guide records 148.0 compressor locked rotor amps from Goodman's GLXS3B specification sheet for a 5 ton model, and finds that no published surge rating among the portable units on this page clears it. Read the nameplate on the equipment you actually have. And if your system carries electric resistance backup strips, the load on the coldest night is resistive again, at several times a space heater.

The warm room strategy

If you are not going to make heat with the battery, the alternative that costs nothing is to stop trying to heat the house and heat one room and the people in it.

• Run the furnace instead, if you have one. Gas, oil and propane furnaces burn fuel for the heat and use electricity only for the blower, the inducer and the ignition. Our winter outage guide works the arithmetic from Trane's published electrical data for its 80 percent AFUE gas furnaces, where the largest published blower figure is 9.2 full load amps at 120 V, and shows what duty cycle does to it. The furnace is the single highest-value winter backup load in most houses, and it is hardwired, so arrange the connection before the storm rather than during it.
• Shrink the volume. Close doors to unused rooms, close blinds at night, and put everyone in one room. This is free and it changes the denominator in every calculation above.
• Warm the person, not the air. Heated blankets and mattress pads are the one heating product whose draw is small enough to matter, and here we have to flag a gap rather than fill it: of the brands we have checked, none publishes a wattage figure on its own product pages, so the number to trust is the one printed on the blanket you own. Our winter guide covers what is and is not published there.
• Check what the battery itself will tolerate. A unit stored in an unheated garage may refuse to charge at all. Every maker publishes a charging temperature range that is narrower than its discharging range, and those figures are collected in home batteries in cold weather.

The safety rules live elsewhere

The federal safety guidance for outage heating, gas ovens, generator placement and carbon monoxide is quoted in full in our winter outage guide; for any heating appliance you use, the manufacturer's own instructions govern.

Every figure above is a manufacturer datasheet claim or a federal publication recorded with its stated basis: heater wattage, current and coverage area from Lasko's own product page for the 754200, heat pump capacity, power input and COP from Mitsubishi Electric's MSZ-FS06NA and MUZ-FS06NAH submittal, power station capacities from each maker's own spec table, and the safety guidance quoted from Ready.gov. The runtime numbers are arithmetic on those claims under the assumptions stated, not measurements we made.