12V power

No, you cannot run a space heater off your inverter

Updated September 2026

Short answer: the inverter is not the problem — your batteries are. A 1,500-watt space heater pulls roughly 140 amps through an inverter, which empties a healthy dual-battery bank in about an hour and leaves you with a truck that will not start at 3 a.m. Here is the math, what actually happens if you try it anyway, and what keeps you warm instead.

Research-based. We did not test these hands-on. Scores come from published specs and driver reviews. Nothing is scored or sold on this page — the math is the point.

Watts divided by volts equals amps — that is the whole argument. 1,500 ÷ 12 = 125 amps. And the inverter is not free: at roughly 85–90% efficiency, the batteries actually supply something like 135–140 amps to keep that heater glowing.

Now the other side of the ledger. A pair of Group 31 starting batteries — the standard dual-bank setup in most tractors — holds roughly 200 amp-hours combined. In theory, 140 amps drains that in under 90 minutes. In practice you never get the theory, because:

• You need most of that bank to start a cold diesel at 6 a.m. Every amp-hour you burn as heat is an amp-hour missing from the crank.
• Voltage sags under a 140-amp load. The inverter's low-voltage alarm starts complaining around 11 volts and then shuts itself off to protect the batteries — long before the bank is "empty" on paper.
• Deep-discharging starting batteries kills them. They are built for short, violent cranking bursts, not slow overnight drains. A few deep cycles and you are buying batteries.

Even the heater's "low" 750-watt setting pulls roughly 70 amps — about five times what a 12V lighter socket is fused for, and still enough to put a serious dent in the bank over a couple of hours. The 150-watt socket ceiling is not a suggestion; it is the fuse doing its job.

Drivers who have tried this describe the same evening. First hour: toasty. The inverter's cooling fan screams the whole time. Second hour: the low-voltage alarm starts beeping as the bank sags, then the inverter shuts itself off. Middle of the night: silence, and the cab is cold again. Morning: the starter groans, clicks, or does nothing at all. You are now buying a service call instead of sleeping — in the cold, in the dark, on the shoulder of someone's schedule.

There is a fire footnote, too. Space heaters are built for a bedroom wall outlet, not a bouncing cab: extension cords coiled under the bunk, a heater tipping onto bedding at a hard brake, a glowing element inches from a curtain. Anything that glows should stay upright, clear, and attended — better yet, it should not be in the cab at all. If you are new to cab electrical, start with the buying guide before you plug anything in.

"But my inverter is 3,000 watts!" — it can absolutely deliver 1,500 watts. For about as long as the batteries let it, which is roughly an hour. The inverter was never the constraint; the amp-hours in the bank are. Sizing the inverter correctly still matters for everything else you run — it just does not change this math.

Running the engine to keep up does not fix it either. At idle, an alternator puts out well below its rated peak; you are burning diesel to make the most expensive heat per dollar of any option; and anti-idling rules make it illegal in much of Canada anyway. If winter idling is on your mind for other reasons, the new driver starter kit covers the cold-weather setup that actually works.

The principle is simple: heat the driver, not the cab. Warming a few cubic feet of sleeper air with battery power is a losing trade; warming your body takes a fraction of the energy:

• 12V heated blanket — about 45 watts, under 4 amps. All night on a healthy bank, with change to spare. The heated blanket guide compares the 12V-native picks, and the runtime calculator shows exactly what your bank can cover.
• 12V portable cab heater — 100 to 200 watts. Built for the socket ceiling, meant for defrosting the windshield and taking the edge off, not heating the room. The cab heater guide has the honest picks.
• Diesel bunk heater. The fleet answer: sips fuel from the tank, heats all night, barely touches the batteries. Cab heat without idling walks through the options.
• A winter sleeping bag rated for the cold you will actually see. Unpowered, unbreakable, and it works when everything electrical has given up.

Shore power. If you are parked at a stop with plug-ins and the heater is running off the pedestal — not your starting batteries — none of the above applies. Same story for an APU or a generator: the rule is about your starting batteries, not about electricity in general. The moment the power source stops being the bank that starts your truck, the math changes completely.