Space Heater Electricity Cost

A 1,500-watt rating makes the arithmetic simple, not the outcome certain

Many portable electric resistance heaters have a high setting near 1,500 watts because of common branch-circuit limits, but actual products and settings vary. WattFigure’s preset uses 1,500 watts for six hours per use day with a 75% thermostat duty cycle. It is a scenario, not permission to use a heater on a particular circuit and not a statement that every heater draws the same power. Read the nameplate and follow manufacturer and CPSC safety instructions.

At 1,500 watts, a heater that draws full power for one hour uses about 1.5 kWh. With six entered hours and a 75% duty cycle, daily energy is 1.5 × 6 × 0.75, or 6.75 kWh. Monthly cost multiplies by the number of heating days and the electric rate. If the heater never reaches the thermostat setting and runs continuously, a lower assumed duty cycle understates use. If it cycles frequently or uses a lower power setting, 100% overstates it.

Identify power and cycling evidence

The nameplate or manual should list input watts for each heat setting. Do not infer power from marketing language such as “for large rooms.” A plug-in electricity meter rated for the full heater load may verify input and cycle behavior, but only if its instructions explicitly permit that load. Portable heaters are high-power appliances. Never use an undersized meter, extension cord, power strip, or improvised connection. Plugging and placement decisions must follow the heater manual and recognized safety guidance.

If a heater offers nominal low and high settings, create separate WattFigure rows or change watts when the setting changes. A fan may add a small load, but nearly all electrical input to a resistance heater ultimately becomes heat in the room. That does not mean all heaters provide the same comfort: controls, air movement, radiant direction, room losses, and placement affect the useful result.

Room heat loss determines duty cycle

Outdoor temperature, wind, insulation, windows, air leakage, ceiling height, open doors, and adjacent spaces determine how quickly a room loses heat. A heater in a small closed office can cycle differently from the same heater in an open basement. Thermostat location also matters. Heat near the unit may satisfy its sensor while distant areas remain cool, or cold drafts may keep the sensor calling for heat.

Do not invent a permanent duty-cycle percentage from one mild evening. Measure or observe across representative conditions. For seasonal planning, calculate a cold-month scenario and a shoulder-month scenario. WattFigure’s annual figure repeats one month twelve times, which is rarely appropriate for supplemental heat. Sum months with different use patterns or treat the annual display as a comparison only.

Portable heating can sometimes reduce central heating in a limited occupied zone, but the interaction is household-specific. If the central thermostat and heating system continue operating unchanged, the portable heater may simply add consumption. Fuel type, system efficiency, thermostat placement, and building airflow make a universal savings claim inappropriate.

Apply rates and bill structure carefully

The EIA state selector offers a residential average reference. It is based on reported revenue divided by sales, not an individual tariff. Your utility bill may include fixed customer charges, taxes, riders, tiers, or time-of-use rates. Fixed charges may remain after reducing heater use. High consumption may also reach a different pricing tier where applicable.

If a time-of-use plan has expensive evening periods, calculate those hours separately from lower-price hours. The same 6.75 kWh can have different costs depending on timing. Do not use an annual average price when the planning question specifically concerns a high-price interval unless you state that simplification.

For comparisons, normalize the amount of heat and occupied comfort delivered. Comparing a heater’s input cost with a whole-home system’s input without accounting for different heated areas is misleading. Cost alone also cannot answer electrical capacity or fire risk questions.

Safety and accuracy checklist

• Read and obey the exact product manual and current safety guidance.
• Use listed input watts for the selected setting.
• Never treat the calculator as circuit-capacity or extension-cord approval.
• Adjust duty cycle for representative room and weather conditions.
• Model heating months separately rather than annualizing one cold month.
• Separate variable electricity cost from fixed bill charges.

WattFigure provides general information only. It is not electrical, fire-safety, heating-system, medical, or purchasing advice, and it does not guarantee cost, comfort, or savings.