To estimate an electric heater’s energy cost, multiply its power in kilowatts by the hours it runs, then multiply by your electricity price per kilowatt-hour (kWh): cost = (watts ÷ 1,000) × hours × price per kWh. For example, a 1,500-watt heater running for three hours uses 4.5 kWh. At the 9.1 cents per kWh used in a U.S. Department of Energy example, that would cost about 40 cents. Your own estimate depends on your heater’s actual operating time and the rate on your bill or utility tariff.
Calculate the heater’s energy cost
Electricity use is measured in kilowatt-hours. A heater’s wattage tells you how much power it draws at a given moment; dividing watts by 1,000 converts that figure to kilowatts. Multiply kilowatts by the number of hours the heater operates to estimate its energy use. Multiplying that result by your price per kWh gives the energy charge for that use.
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Formula: (heater watts ÷ 1,000) × operating hours × electricity price per kWh = estimated energy cost.
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The calculation has three inputs: the heater’s rated wattage, its full-power operating hours, and the applicable price per kWh. Keep the units consistent. Convert watts to kilowatts by dividing by 1,000, and convert cents to dollars if you want the answer in dollars. For example, enter 20 cents as $0.20 per kWh. If you use 20 as the price in a calculation where the other figures are in dollars, the result will be off by a factor of 100.
Worked example: a 1,500-watt heater
A U.S. Department of Energy report published in 2016 illustrates the calculation with a 1,500-watt electric space heater operated for three hours. Its example uses an average residential rate of 9.1 cents per kWh for Minnesota Power customers in that report. The rate is historical and specific to the report’s example—not a current national price or a recommended default for your estimate. The calculation is:
- Convert watts to kilowatts: 1,500 ÷ 1,000 = 1.5 kW.
- Calculate energy use: 1.5 kW × 3 hours = 4.5 kWh.
- Multiply by the example rate: 4.5 kWh × $0.091 per kWh = $0.4095, or about 40 cents.
The report’s figures and worked example appear in the U.S. Department of Energy’s Minnesota Power evaluation report. The arithmetic shows how to estimate a cost; the rate does not tell you what the same heater costs today in another location.
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Estimate cost for your own hours of use
The table below extends the same example to other operating times. It assumes a 1,500-watt heater runs continuously at full power and uses the report’s illustrative rate of 9.1 cents per kWh. Daily cost assumes one day of use; the 30-day figure assumes the same daily schedule every day. These are calculations from that old example rate, not current prices or a forecast of an individual bill.
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| Full-power operating time | Energy used | Illustrative energy cost at 9.1¢/kWh | Illustrative cost for 30 days at that schedule |
|---|---|---|---|
| 1 hour | 1.5 kWh | About 14¢ | About $4.10 |
| 3 hours | 4.5 kWh | About 41¢ (about 40¢ in DOE’s rounded example) | About $12.29 |
| 8 hours | 12 kWh | About $1.09 | About $32.76 |
To adapt the estimate, replace 1,500 with the wattage on your heater’s label or in its manual, replace the hours with your expected full-power runtime, and replace 9.1 cents with your applicable price. For example, if your rate is stated in dollars, enter it as dollars per kWh: 20 cents is $0.20. The result estimates the energy charge associated with the heater’s use, not necessarily the total change in your bill.
For a monthly estimate, first work out the energy used during one day or one typical use period, then multiply by the number of days or periods you expect to use the heater. The table’s 30-day figures make that assumption explicit: the heater is used for the listed number of full-power hours every day for 30 days. If your schedule varies, estimate each schedule separately and add the energy use or costs. Use the rates applicable to those periods if your tariff changes with time or season.
Costs scale directly with each input in this formula. Holding the rate and runtime constant, a higher wattage uses more energy; holding wattage and rate constant, more full-power hours use more energy. A higher price per kWh increases the energy charge for the same amount of use. That is why estimates for two heaters—or for the same heater in two households—can differ even when both are described as electric space heaters.
Use operating time, not just the time the heater is switched on
The calculation is most accurate when “hours” means the time the heating element actually draws its rated power. A thermostat-controlled heater may switch on and off while the unit remains switched on. In that case, treating every switched-on hour as a full-power hour can overestimate energy use. The amount of cycling depends on actual conditions; there is no universal adjustment factor to apply.
If you cannot determine the heater’s full-power runtime, calculate a range rather than pretending to know an exact figure. One estimate can assume continuous full-power operation for all the hours you expect the heater to be on; that is a straightforward upper-use scenario for those hours. A lower-use estimate needs a defensible runtime or measured energy use. Do not invent a duty-cycle percentage: the basic wattage-and-hours method does not establish one for your room or heater.
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Be clear about what your estimate describes. “The heater was on for eight hours” may mean it was plugged in and thermostat-controlled for eight hours, not that it drew its rated power for all eight. The table uses full-power operating time, so it should not be read as an exact bill prediction for a unit that cycles. If you do know the actual full-power hours, use those; otherwise, state that the estimate assumes continuous operation rather than presenting it as a measured cost.
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Find the right electricity price on your bill
Check the bill or your utility’s tariff for the charge tied to energy use, usually shown in cents or dollars per kWh. If the rate changes with the time of day, use the rate for the period when you plan to run the heater. If it changes seasonally, use the applicable seasonal rate. A single flat rate may be sufficient for a rough estimate only when it reasonably represents the hours and usage being estimated.
Your total bill may include items that the heater’s kWh calculation does not capture. A fixed monthly charge generally is not the incremental energy charge for using this appliance; demand charges or other billing components can also complicate attributing a precise bill change to one heater. For that reason, distinguish the calculated energy cost from the complete amount due on the account. DOE describes time-dependent and seasonal energy charges as well as fixed and demand charges in its utility rate options guidance.
There is no universal electricity price to use for every estimate. DOE’s 2016 report used 9.1 cents per kWh for its example involving Minnesota Power customers, but that historical example is not a current national average. Check your own bill or utility tariff instead. If the bill has multiple energy prices, match the price to when the heater will operate; multiplying all its use by a rate that does not apply to those hours can make the estimate misleading.
Compare heating technologies on the same basis
A portable resistance heater and a whole-home heat pump are not interchangeable comparisons unless they provide the same heating service. The wattage formula estimates electricity used by a particular heater; it does not by itself compare the cost of keeping an entire home warm using different systems.
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DOE states that resistance heating requires at least twice as much energy as a heat pump to keep a house warm. A separate DOE Home Upgrades page estimates that current heat pumps can reduce heating electricity use by approximately 65% compared with electric resistance heating. These are broad comparisons, not guaranteed savings for a particular home or a conversion factor to apply to every portable heater. Climate, equipment, building characteristics, usage and electricity rates affect an individual comparison. The statements are available from DOE’s heat-pump discussion and its Home Upgrades page.
When comparing options, use the same scope and identify what the figures represent. For a portable heater, compare rated input power, expected full-power hours, and the rate that applies to those hours. For whole-home heating, compare systems that provide the same heating service rather than comparing one room heater’s consumption with a whole-home system’s performance. The DOE heat-pump figures are useful context for technology differences, but they do not calculate a household’s bill savings or establish what a specific heater will cost.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.A practical checklist for a reliable estimate
- Read the heater’s rated input power. Use watts from the product label or manual, not a guess based on heater size.
- Estimate the hours it actually heats at full power. Account for thermostat cycling if you know the runtime; otherwise, label a continuous-operation calculation as an assumption.
- Use the rate that matches the operating period. Check for time-of-use or seasonal pricing, rather than treating an old illustrative rate as current.
- Keep the result in perspective. The formula estimates energy cost. Fixed charges, demand charges and other tariff details can mean the total bill will not rise by exactly that amount.
- For whole-home decisions, compare like with like. A room heater’s calculated use is not a direct cost comparison with a heat pump serving an entire home.
When all three main inputs—rated watts, operating hours and applicable price per kWh—are known, the calculation is simple. Most uncertainty comes from runtime and tariff structure, not from the multiplication.
Frequently Asked Questions
How much does a 1,500-watt electric heater cost to run for one hour?
A 1,500-watt heater uses 1.5 kWh for one full-power hour. Multiply 1.5 by your electricity price per kWh. At the 9.1-cent rate in the U.S. Department of Energy’s 2016 example, that works out to about 14 cents for one full-power hour; use your own tariff for a current estimate.
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How do I calculate the cost of running an electric heater?
Divide the heater’s watts by 1,000 to get kilowatts, multiply by its full-power operating hours to get kWh, then multiply by the applicable electricity price per kWh. For example: (watts ÷ 1,000) × hours × price per kWh.
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Does the estimate include the whole electricity bill?
No. The formula estimates the energy charge associated with the heater’s kWh use. A bill may also include fixed charges, demand charges, or other tariff components, and those can complicate attributing a precise bill change to one appliance.
Should I count all the hours the heater is switched on?
Only if it draws its rated power continuously during those hours. A thermostat may switch the heating element on and off while the heater remains switched on, so counting every switched-on hour as full-power operation can overestimate use. There is no universal cycling factor; use known full-power runtime or clearly label continuous operation as an assumption.
Is a heat pump always 65% cheaper to run than an electric heater?
No specific household savings are guaranteed by that figure. DOE says current heat pumps can reduce heating electricity use by approximately 65% compared with electric resistance heating, and separately says resistance heating requires at least twice as much energy as a heat pump to keep a house warm. These are broad comparisons; climate, equipment, building, usage and electricity rates affect an individual cost comparison.
The Bottom Line
Estimate an electric heater’s energy cost with (watts ÷ 1,000) × full-power hours × your price per kWh. Use the rate and runtime that actually apply, and treat the DOE’s 9.1-cent example as historical illustration—not a current universal price. Thermostat cycling, time-dependent rates, fixed charges and demand charges can all affect how closely the estimate matches your bill.
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