Do 12V Car Heaters Work?
This lesson draws on Mazda and Nissan owner’s manuals, RoadPro’s heater product page, Save on Energy, The Globe and Mail and the National Weather Service. How we research explains the method.
The power math
A 12V socket has a fuse, and the fuse sets the ceiling. Mazda’s CX-50 manual says to use only accessories of 120 W (12 V, 10 A) or below, and Nissan’s Juke manual gives the same 120 W (10 A) limit. Some vehicles differ, so check your own manual.
Watts are volts times amps, so 120 W at 12 V is 10 A. RoadPro rates its 12-Volt Heater Fan/Defroster (RPAT859) at 150 W, with a 15 A in-line fuse: 150 W divided by 12 V is 12.5 A, more than a 10 A socket is rated for.
| Heat source | Power | Source |
|---|---|---|
| 12V socket limit (Mazda, Nissan) | 120 W | Owner’s manuals |
| RoadPro 12V heater/defroster | 150 W | RoadPro |
| Electric car cabin heater | 1,000-5,000 W | The Globe and Mail |
| Typical “whole room” space heater | About 1,500 W | Save on Energy |
Why a 12V heater can’t warm the cabin
Save on Energy says many space heaters sold as “whole room” units are about 1,500 W, since that is about the most a home wall outlet can safely supply. A 120 W socket gives less than a tenth of that.
A car’s own heater plays in a different league. In The Globe and Mail, Plug In BC’s Michael Stanyer put an electric car’s cabin heat at about 1 kW near freezing, and more at -4°F (-20°C), and the paper puts a full-blast EV heater at 1,000-5,000 W. These are electric-car figures: we found no published watt figure for a gas car’s heater. The paper notes that for heat in a gas car, you have to be idling.
Put the numbers side by side and a 120 W socket supplies about an eighth of the 1 kW Stanyer gave for an electric car near freezing, and far less than a heater needs to warm a cold cabin from scratch.
What it can do
RoadPro sells its unit as a heater fan and defroster, and says to use it as a heater or to defrost vehicle windshields. Aimed at the glass, its small stream of warm air can clear a patch to see through, a better use of 150 W than trying to heat the cabin.
That is a modest job, and the car’s own defroster does it better once the engine is warm. Treat a 12V heater as a spot warmer for your hands or a patch of glass while the engine warms up.
Engine off: battery and CO risks
With the engine off, the heater runs on the car battery alone. Take a 60 Ah battery as an example: at 12.5 A, a 150 W heater would use half of it (30 Ah) in about 2.4 hours, before counting cold, which already leaves a battery weaker (why a weak battery shows up in cold weather). Nissan’s manual says to use the outlet with the engine running and not for long periods with the engine stopped or idling.
Running the engine to power it brings carbon monoxide risk. If you are stranded, the National Weather Service says to run the motor about 10 minutes each hour, crack a window and clear snow from the exhaust pipe. If the battery does go flat, a charged pack helps, within its cold-weather limits.
What works better
- Warm clothes. The National Weather Service suggests layers of loose-fitting, lightweight, warm clothing; keep a blanket in the car too, as in our emergency kit gear.
- The car’s own heater. It is designed to heat the cabin, so let the engine warm up and use it.
- A block heater. Warming the engine before you start means warm air sooner; see when to plug in a block heater.
- A battery warmer. For starting, not comfort: see do battery blankets work.
