All Night vs a Timer
A block heater draws its full wattage the whole time it's plugged in; it has no thermostat. Plugged in at bedtime, a 600-watt heater runs about 10 hours and uses 6 kWh, roughly $1.08 a night at 18¢ per kWh. On a timer that switches on 2 hours before you leave, it uses 1.2 kWh, about 22¢. Over 60 cold nights that's about $65 versus $13, and a $20 outdoor timer pays for itself in a few weeks.
The extra hours don't buy a warmer engine. In testing reported in the 1970s, engine coolant rose almost 20°C (36°F) in the first four hours of heating, and two more hours added only about 3°C (5°F). Most of the warm-up happens early, then the heater mostly replaces heat leaking from the block.
Cost Per Night by Heater Size
| Heater | 2 hours | 4 hours | 10 hours (all night) |
|---|---|---|---|
| 400 W | $0.14 | $0.29 | $0.72 |
| 600 W | $0.22 | $0.43 | $1.08 |
| 750 W | $0.27 | $0.54 | $1.35 |
| 1,000 W | $0.36 | $0.72 | $1.80 |
| 1,500 W | $0.54 | $1.08 | $2.70 |
At 18¢ per kWh. A North Dakota electric co-op puts most car and truck heaters at 300 to 1,000 watts; 1,500-watt heaters turn up on larger diesels and farm equipment.
How Long to Plug In by Temperature
| Overnight low | Small or midsize gas engine | Pickup, V8 or diesel |
|---|---|---|
| Above 20°F | Not needed | Usually not needed (check the manual for diesels) |
| 0 to 20°F | 1 hour | 2 hours |
| −20 to 0°F | 2 hours | 3 hours |
| Below −20°F | 3 hours | 4 hours |
Anchorage's air-quality program asks drivers to plug in at 20°F or colder, for at least an hour in small cars and up to two hours in the largest trucks. Natural Resources Canada recommends a timer set for two hours before you drive and puts the payoff at up to 10% better fuel economy at −20°C (−4°F), and around 20% for a single short trip. Electric co-ops in North Dakota and Minnesota also advise switching on a couple of hours before you need the vehicle; the North Dakota co-op says two to three hours does the job even in subzero weather.
What You Get for the Electricity
- Easier starts: warm oil flows to the bearings faster, and the starter and battery have less work.
- Less fuel and pollution: a cold engine runs rich until it warms up. Anchorage cites 50 to 100 times more pollution in the first minute after a cold start than after a minute of warm-up, which is why the city hands out cords and timers.
- Heat sooner: a warmer engine sends heat to the cabin earlier, so there's less temptation to idle in the driveway. Idling to warm up burns fuel for no mileage.
Picking a Timer
Use an outdoor-rated timer rated for 15 amps or more; a 1,000–1,500 W heater draws 8–13 amps at 120 volts. Plug-in timers with a 24-hour dial or a digital clock both work. Smart plugs rated for outdoor use let you start the heater from your phone when your schedule changes. Thermostatically controlled outlets that switch on only below a set temperature save more on nights that turn out milder than forecast.
If your car sits in a garage, it may not need a block heater at all; an attached garage often stays well above outdoor temperatures. For heating the garage itself, see the garage heater cost calculator.