How much an electric garage heater costs to run: watts × hours × your rate

The kWh formula

Electric resistance heat is a watt-hour problem. The nameplate tells you watts. Your clock (or the thermostat) tells you hours. Your utility bill tells you dollars per kilowatt-hour. That is the whole machine:

• kWh = (watts ÷ 1,000) × hours the element is on. A 5,000 W heater running one full hour is 5 kWh. A 1,500 W portable running one full hour is 1.5 kWh.
• Cost = kWh × $/kWh. At an example $0.20/kWh, that hour is $1.00 on the 5 kW unit and $0.30 on the 1,500 W portable.

Resistance heat is essentially 100% efficient at the point of use. The element turns nearly all of its watts into heat in the room. 240 V is not “cheaper electricity” than 120 V. Voltage does not change $/kWh. It only changes how many watts you can deliver without melting a 15 A receptacle. That circuit limit is 120 V vs 240 V, not a rate discount.

If a listing promises a monthly bill or a “saves $X versus propane” figure without your hours and your rate, it invented the inputs. We will not invent them either.

Nameplate examples at three example rates

These rows are common garage-heater nameplates, not a shopping list. The 3 / 4 / 5 kW steps match the jumper taps on Comfort Zone CZ220-class and Fahrenheat FUH54-class ceiling units. The 7.5 kW row is the next cabinet class — a different circuit, usually 8 AWG copper in that manual family. Costs are for one hour at full nameplate, which is the honest starting point before duty cycle.

Read the table left to right, then stop. A 5 kW unit at $0.30/kWh is $1.50 an hour while the element is on. It is not $1.50 for every clock hour you are in the garage. That is the next section. Wattage brackets for the bay itself live on electric garage heater size.

Duty cycle and the thermostat

Heaters do not run 100% once the bay is up to the setpoint. A thermostat (built-in or wall) cycles the element. Hours-on is a fraction of hours-occupied. That fraction is the duty cycle. Insulation and infiltration dominate it. A tight attached two-car with an insulated door and a 5 kW unit may sit at a modest duty cycle after warmup. The same 5 kW cabinet under a single-skin door with a crushed sill will stay closer to 100% and still feel like a carport.

Two loads hide inside “it ran all morning”:

• First-hour / warmup. The slab, the cars, and the air start cold. The element is often on continuously until the air (or the bulb) catches up. That hour is close to the full-nameplate row in the table.
• Hold. After that, the thermostat is fighting envelope loss: door leaks, uninsulated planes, stack effect, and whatever you left open. Hours-on collapse if you sealed the building. They do not collapse if you bought more watts and left the threshold crushed. See seal and insulate first.

A worked hold, not a promise: if a 5 kW unit is on 40% of the time after warmup, you are paying for 2 kWh per clock hour (5 × 0.4), not 5. At the example $0.20/kWh that is $0.40 per occupied hour to hold, plus whatever the warmup already used. Change the 40% and the dollar figure moves with it. We did not measure your garage. Weatherstrip and an insulated door change the 40% more reliably than a jumper tap.

Setpoint matters the same way. Shirt-sleeve shop temperature against a 10 °F night is a different hours-on problem than “take the edge off” at 50 °F. We will not convert that into a fake monthly bill.

1,500 W portable vs 5 kW 240 V

A 1,500 W milkhouse heater is cheaper per hour than a 5 kW ceiling unit because it is one-third the watts. At the example $0.20/kWh that is $0.30 versus $1.00 per full hour. That is not a bargain on bay heat. It is spot heat. The portable cannot deliver the two-car wattage bracket. It warms a person and a small radius of air. The 5 kW hardwired unit is the common whole-bay tool — and it costs more to run because it is doing a bigger job.

The 15 A / 120 V receptacle is the hard ceiling, not a pricing feature. Continuous-load practice puts that circuit near 1,440 W; 1,500 W is 12.5 A and wants to be the only load. Details: portable heaters on a 15 A circuit. The 5 kW class is typically 240 V, hardwired, 30 A two-pole, 10 AWG copper — see voltage and breakers and ceiling-mount units under $200.

Stacking two 1,500 W portables on one run does not give you 3 kW of honest bay heat. It gives you a tripped breaker and a fire risk. If the job is the bay, the next adult step is a dedicated 240 V circuit (if the panel can take it), not a second milkhouse heater.

Forced-air vs infrared: same watts, different hours

A watt is a watt at the meter. Forced-air and infrared convert nearly all of their nameplate watts into heat. Same watts, same $/kWh, same cost while the element is on. The fork is whether you need those watts for fewer hours because you only occupy a beam.

Infrared can feel warmer with fewer hours-on if the job is one person at a bench, a detail bay with the door up, or a high leaky shop where heating the cubic feet is a losing war. You still pay for every watt the quartz or panel draws. You may pay for fewer clock hours because you stop trying to hold 60 °F air in an open volume. Forced-air is the better default when the door is closed and you want the space itself less miserable — and then duty cycle is an envelope problem again.

That split is the whole of forced-air vs infrared. Do not buy IR because a listing implied it “uses less electricity” at the same nameplate. Buy it if you are heating a person instead of a room.

Read the rate on your bill

“The” U.S. residential rate does not exist on this page. Bills bundle generation, transmission, distribution, riders, and sometimes a fixed customer charge that does not move with kWh. For this math you want the volumetric number: what one extra kWh costs you this month, all-in. Ways that go wrong:

• Using only the generation line and ignoring delivery. Delivery is often as large as supply.
• Using a summer bill to price a January heater, or the reverse, on a utility with seasonal rates.
• Ignoring time-of-use. A 5 kW unit on a 4–9 p.m. peak is a different dollar figure than the same unit at 10 a.m. Saturday.
• Treating the fixed monthly charge as part of $/kWh. The heater does not change that line.

Crude but usable: take the variable charges on the bill, divide by kWh used, and use that as your example rate. Then multiply by the kWh you actually attribute to the heater (nameplate × hours-on). If your utility publishes an “all-in” ¢/kWh on the statement, use that and skip the division.

A worked Saturday session

Assumptions, labeled as assumptions: 5 kW ceiling unit, example $0.20/kWh, one warmup hour at 100% on, then three occupied hours at 40% duty cycle, door closed. That is 5 + (5 × 3 × 0.4) = 11 kWh, or $2.20 for the session. Change any input and the dollar figure is yours, not ours.

The portable row is cheaper because it is 1.5 kW, not because it heated the bay. If you only needed the bench, $1.20 for four hours of spot heat may be the honest spend. If you needed the two-car to feel like a room, the $2.20 session is the 5 kW job — and only if the 40% hold is real. A drafty door can push the hold toward the full $1.00/hour row for all three hours. That is infiltration, not a defective heater. Fix the envelope before you treat $4–$5 as “what 5 kW costs.”

Scale that session across a month and you still do not have a national average. Twenty such Saturdays at $2.20 is $44 at this example rate and this example duty cycle. Ten evenings of warmup-only (one hour, 5 kWh) is $20. We will not pick one of those and call it “typical.” Multiply your hours by your rate.

What this page will not claim

• No “saves $X/month.” We do not know your hours, setpoint, climate, or rate. Anyone who states a monthly savings without those inputs is writing fiction.
• No coverage claims. Cost-per-hour is not “heats up to X sq ft.” Size the bay on the size guide. Do not repeat manufacturer square-foot copy as a bill estimate.
• The panel still gates the install. A cheap-to-run 1,500 W portable is still a 15 A problem. A 5 kW unit is still about 20.9 A at 240 V on a 30 A two-pole. A 7.5 kW unit is about 31 A. Operating cost does not create spare slots in a 100 A service that already feeds a range, dryer, HVAC, and an EV charger. Hire a licensed electrician for new 240 V work.
• No affiliate buy buttons on this page. Retailer links elsewhere on the site are still placeholders until programs are live. We are not tagging a heater SKU to “prove” a kWh number.

Next: if you do not know the circuit, start at 120 V vs 240 V. If the bay stays cold after you can do the kWh math, the hours-on problem is usually the envelope, not a missing review score. If you only occupy a bench, stay on 15 A portables or directed radiant and stop paying to heat cubic feet you are not standing in.