The arithmetic, in one table
Electric resistance heating converts watts to heat at essentially 100% efficiency, which sounds good and is exactly the problem: there is no cleverness available. A 1,500W heater draws 1,500W, continuously, for as long as it is on.
| Battery | Heater on high, 1,500 W | Heater on low, 750 W |
|---|
| 512 Wh | About 17 minutes | About 35 minutes |
|---|
| 1,056 Wh | About 36 minutes | About 72 minutes |
|---|
| 2,042 Wh | About 69 minutes | About 2.3 hours |
|---|
| 3,840 Wh | About 2.2 hours | About 4.4 hours |
|---|
Calculated as capacity x 0.85 / heater watts, using the 85% inverter efficiency figure documented on the methodology page.
The bottom row is the one that settles it. The largest portable battery on this site, costing several thousand dollars, runs one small space heater for a bit over two hours. A winter night is twelve.
Why battery-powered space heaters do not exist
People search for these, and the reason the search comes up empty is physics rather than a gap in the market.
Meaningful room heating starts around 750W. A battery that ran 750W for eight hours would need roughly 7 kilowatt-hours of usable capacity — larger and heavier than any portable power station sold, in a product that would cost more than a standby generator and last one night.
What does exist, and is worth knowing about, is battery-powered heated clothing — heated vests, jackets, socks and gloves. Those draw single-digit watts because they heat a person rather than a room, which is the only version of the idea where the arithmetic works.
What to do instead, in order
1. Shrink the space. Pick one room, preferably small with few exterior walls, close the doors and put a towel at the gap. Heating 150 square feet instead of 2,000 changes the problem by an order of magnitude before you spend anything.
2. Burn something, safely. A propane radiant heater with an oxygen depletion sensor is the practical answer for one room, and a wood stove or a fireplace is better still if you have one.
3. Power the furnace instead of a heater. If you have gas or oil heat, the furnace itself burns fuel and only needs electricity for its blower and controls — often under 1,600W. The furnace backup page covers that, and it is by far the most efficient use of a generator in winter.
4. Insulate the people, not the house. Sleeping bags rated below the temperature you expect, layers, hats indoors. The staying warm page works through the whole approach.
The safety part, which is not optional
Winter carbon monoxide deaths spike during outages, and the cause is almost always a heating improvisation rather than the generator itself.
- Never a grill, a camp stove or a generator indoors. Not in the garage with the door open, not in a breezeway, not in a fireplace.
- Never an oven as a heater. A gas oven is not vented for continuous operation and an electric one is not running anyway.
- An indoor-rated propane heater needs ventilation. An oxygen depletion sensor protects against oxygen depletion; it is not a substitute for a cracked window.
- A battery CO alarm in the room, every time. It is the cheapest item on this site and the only thing that will tell you.
- Do not run a combustion heater while asleep. The whole class of accidents happens overnight.
If you already own a big battery
Use it on the loads where electricity has no substitute: the refrigerator, the freezer, the network, lighting, and any medical equipment. Those are modest, continuous and genuinely electrical.
Then heat one room with propane and wear more clothes. That combination costs a fraction of what a heat-capable battery would and works for days rather than hours.
The short version
- A 2,000Wh battery gives about 69 minutes of a 1,500W heater.
- Battery-powered room heaters do not exist because the physics does not allow it.
- Propane radiant heat with an ODS is the practical one-room answer.
- Powering the furnace blower beats powering a heater if you have gas or oil heat.
