ApplianceCalc

Air & Comfort

Space Heater Size Calculator

Estimate a room heating wattage range and electricity use from area, ceiling height, insulation and temperature rise.

Empirical heating estimate
1

Your appliance details

Units

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2

Your results

Estimated heating requirement400–700 W
Heating output1,365–2,388 BTU/h
Hourly energy (midpoint)0.58 kWh
Daily energy (midpoint)4.61 kWh
Daily cost (midpoint)$0.74

What this means

This estimates steady heat loss relative to the entered unheated temperature, not how quickly a cold room warms up.

Show calculation
  1. Power = room area × height × temperature difference × estimated heat-loss coefficient.
  2. Coefficient = 1.2 W/(m³·°C); display a ±20% range.

Important notes

Empirical estimate: actual requirements depend on conditions this simplified model cannot measure.

A current indoor temperature alone cannot establish outdoor heat loss. Treat it as your unheated baseline; drafts and outdoor weather can change demand considerably.

Energy assumes continuous output at the midpoint. Thermostat cycling reduces actual consumption.

Calculation basis: Empirical heating estimate

A portable heater must overcome ongoing heat loss as well as warm the room. This simplified room model helps you explore how area, ceiling height and insulation affect a rough power range. Current-to-target temperature rise is a screening input, not a substitute for outdoor design temperature or a full heat-loss calculation.

How to use

  1. Enter floor area and ceiling height.
  2. Select insulation quality and enter the current and target room temperatures.
  3. Review the power range and hourly energy, then check appliance and circuit limits before selecting a heater.

How the calculation works

Formula

Estimated watts = room volume (m³) × temperature rise (°C) × assumed loss coefficient. Coefficients of 0.8, 1.2 and 1.8 W/(m³·°C) represent good, average and poor insulation. These are site-selected empirical assumptions; the displayed range is ±20%. Energy (kWh) = watts ÷ 1,000 × hours.

Example

A heater drawing a constant 1,500 W consumes 1.5 kWh in one hour or 6 kWh in four hours. Thermostat cycling can reduce the actual use; this energy example is independent of the room-sizing estimate.

Factors affecting the result

  • Outdoor conditions, drafts and external wall area can dominate heat loss.
  • The estimate does not predict warm-up time for walls, furnishings or the building fabric.
  • Do not infer that a high calculated load is suitable for one portable heater or outlet.

Frequently asked questions

Does an oil-filled heater use less energy at the same wattage?

At the same electrical power and operating time, resistance heaters use the same electricity. Controls and heat distribution may change how long each runs.

Can I use an extension cord?

CPSC advises plugging portable electric heaters directly into a wall outlet. Follow the heater instructions, keep combustible items clear, and switch the heater off when unattended or sleeping.

Sources & methodology

Calculation basis: Empirical heating estimate. Methodology

Last updated: September 15, 2026