Estimate how much an electric space heater may add to your bill for a day, month, or heating season.
Table of contents
How to use our Space Heater Running Cost Calculator
- Enter Heater power (watts) from the label, manual, or heat setting, such as 750, 1000, or 1500.
- Enter Hours used per day (hours), Days used per month (days), and Electricity rate ($ per kWh) from your electric bill when possible.
- Open Advanced options if you run more than one heater, expect thermostat cycling, or only use the heater for part of the year.
- Click Calculate and read Estimated monthly cost first, then compare Electricity used per month with the kWh amount on your bill.
- Sanity-check the result: cutting Hours used per day (hours) in half, or setting Heater is on during scheduled time (%) to 50, should cut the cost about in half.
Definitions
Heater power (watts): The electric power the heater uses while it is actively heating. A 1500-watt heater uses 1.5 kilowatts while on.
kWh: Kilowatt-hour. This is an energy unit on electric bills. Using 1 kilowatt for 1 hour equals 1 kWh.
Electricity rate ($ per kWh): The price you pay for each kWh of electricity. Your bill may split this into supply and delivery charges.
Heater is on during scheduled time (%): The share of scheduled time when the heater is actually heating. A thermostat may turn heating off after the room warms up.
Cost per scheduled hour: The average cost for each hour the heater is scheduled to run, after the on-time percent is applied.
Estimated cost for selected months: The monthly cost multiplied by Months used per year (months), useful for a heating-season estimate.
Common mistakes and quick fixes
Mistake: Using the room size or heater model name for Heater power (watts).
Fix: Enter the watt number printed on the heater label, manual, or setting.
Mistake: Entering the whole electric bill total for Electricity rate ($ per kWh).
Fix: Use the price per kWh; if supply and delivery are both charged per kWh, add those rates together.
Mistake: Counting time the heater is plugged in instead of Hours used per day (hours).
Fix: Enter the hours it is switched on or scheduled to run during a typical day.
Mistake: Leaving Heater is on during scheduled time (%) at 100 when a thermostat turns the heater off part of the time.
Fix: Use a lower percent, such as 50, if the heater cycles on and off during the scheduled time.
Mistake: Using Number of heaters (heaters) for heaters that run on different schedules.
Fix: Use this field only for heaters that run the same hours, days, and on-time percent; otherwise calculate each group separately.
Mistake: Treating Estimated cost for selected months as a full-year cost when Months used per year (months) is less than 12.
Fix: Set Months used per year (months) to 12 only if the same use pattern happens all year.
Limitations & Key Assumptions / Boundary Conditions
- This estimates operating cost only. It does not check whether a heater, outlet, extension cord, or room setup is safe.
- The calculation uses the wattage you enter. Some heaters have multiple settings, so the result changes if the heater is not actually running at that wattage.
- The thermostat adjustment is an estimate. Real on-time percent changes with room temperature, insulation, drafts, heater placement, and thermostat setting.
- Electricity rate ($ per kWh) may vary by season, time of day, taxes, fees, or tiered pricing. Use the full per-kWh rate from your bill when you can.
- Estimated monthly cost uses the Days used per month (days) you enter, not an average calendar month.
- Estimated cost for selected months repeats the same monthly pattern for each selected month. It will be too high or too low if your use changes during the season.
- All outputs are before any rebates, bill credits, minimum charges, demand charges, or non-kWh fees that may appear on an electric bill.
Methodology
How the calculator works
The calculator first adjusts heater wattage for the number of heaters and for the percent of scheduled time the heater is actually on. It then converts watts to kilowatts, multiplies by time to get kWh, and multiplies kWh by the entered electric rate.
effective_watts = heater_watts * heater_count * (on_percent / 100)
kwh_per_scheduled_hour = effective_watts / 1000
daily_kwh = kwh_per_scheduled_hour * hours_per_day
monthly_kwh = daily_kwh * days_per_month
cost_per_scheduled_hour = kwh_per_scheduled_hour * electricity_rate
daily_cost = daily_kwh * electricity_rate
monthly_cost = monthly_kwh * electricity_rate
season_cost = monthly_cost * months_per_year
Worked example
For one 1500 W heater used 8 hours per day for 30 days at $0.17 per kWh, with 100% on-time, effective power is 1500 W. Scheduled hourly use is 1500 / 1000 = 1.5 kWh. Daily use is 1.5 * 8 = 12 kWh. Monthly use is 12 * 30 = 360 kWh. Monthly cost is 360 * $0.17 = $61.20. If that same pattern happens for 3 months, the selected-month cost is $183.60.
Calculation choices
Money outputs are displayed to the nearest cent. Electricity used per month is displayed in kWh. If Hours used per day (hours), Days used per month (days), Heater is on during scheduled time (%), or Months used per year (months) is 0, the related energy or cost can be $0.00.