At What Temperature Should You Plug In a Block Heater?
4 lessons in this series, 13 min in total
- Do Car Battery Blankets Work? 4 min read
- Battery Blanket vs Battery Maintainer: Which Do You Need? 3 min read
- Block Heater vs Battery Warmer: Which Helps a Cold Start More? 3 min read
- At What Temperature Should You Plug In a Block Heater? 3 min read
This lesson draws on Ford’s 2020 Explorer and 2018 Super Duty owner’s manuals, Natural Resources Canada and an owner thread on OrangeTractorTalks. How we research explains the method.
The temperature: what makers say
Ford’s manuals for the 2020 Explorer and the 2018 Super Duty give the same line: the heater is most effective when outdoor temperatures are below 0°F (-18°C). That’s a “works best” point, not a rule that you must wait for it.
Natural Resources Canada (NRCan) gives no cutoff either, but its figures show the payoff in deep cold. It says that on a single short trip at -13°F (-25°C), a block heater’s fuel savings could be around 25 percent; its overall figure for -4°F (-20°C) is in block heater vs battery warmer.
Some owners plug in at milder temperatures. On OrangeTractorTalks, a tractor owner in British Columbia said he plugs in only when it drops below about 5-14°F (-15°C to -10°C), with a timer that runs it for about three hours.
If your manual gives its own figure, use that one. If your car has no block heater at all, a battery warmer or an aftermarket heater is the fallback.
How long before you leave
Ford says the system has no thermostat and reaches its maximum temperature after about three hours. Using it longer than three hours, the manual adds, does not improve performance and wastes electricity.
NRCan goes a little shorter. It says you don’t need to leave a block heater plugged in overnight, because two hours is more than enough, and suggests an automatic timer that switches it on two hours before you leave. Its AutoSmart driver training gives the same two-hour timer advice.
So set an outdoor-rated timer for 2-3 hours before your usual departure. In the same thread, a South Dakota owner said he aims for 1.5-2 hours on a plug-in timer. Another member suggested adding a thermostatic plug that passes power only below a set temperature; the South Dakota owner’s tip is to plug the timer into the wall and the thermostatic plug into the timer, since the reverse can switch on at unpredictable times.
What it costs to run
Ford’s manuals say the heater uses 0.4 to 1.0 kilowatt-hours (kWh) of energy per hour of use. Multiply by your run time, then by the price per kWh on your power bill.
| Run time | Energy used |
|---|---|
| 2 hours | 0.8-2.0 kWh |
| 3 hours | 1.2-3.0 kWh |
| 10 hours (overnight) | 4-10 kWh |
The figures come from Ford’s per-hour range, multiplied out. An overnight plug-in uses more than three times the electricity of a three-hour timer run, for no extra warmth by Ford’s account. A plug-in battery warmer is far smaller, at 60-80 W (do battery blankets work).
Plugging in safely
Ford’s 2020 Explorer manual lists these precautions for its factory heater:
- Outlet: a grounded 120 V AC outlet; never an ungrounded system or a two-prong adapter.
- Extension cord: an outdoor-rated, 16-gauge cord certified by UL or CSA, as short as possible, and never more than one.
- Hood: don’t fully close it or let it drop under its own weight while the heater is in use, and never close it on the extension cord.
- Before you drive: unplug the system and stow the cord before you start and drive the vehicle.
Other brands’ heaters may differ, so check your manual before you trust a hand-me-down cord or timer. If you also run a battery warmer, battery blanket vs battery maintainer covers what else the battery may need.
