How to use this calculator
Watts To Heat Calculator — fast, accurate results online. Enter your values and get instant answers. Enter your values in the fields above and click Calculate to see your results instantly. Click Clear to reset all fields and start a new calculation.
Understanding the inputs
Each field is labeled with the specific value it expects. For best results, use consistent units throughout — mixing unit systems (e.g. metric and imperial) will produce incorrect results. The hint icons (?) provide additional guidance on what each field means.
Interpreting the results
The highlighted result cards show the primary output — the values most people need first. Additional result cards provide supporting calculations that help verify the primary result makes sense. If results appear unexpected, double-check your inputs for typos or unit mismatches and recalculate.
What the watts to heat calculator does and when to use it
Every watt of electrical power that a resistive device draws ends up as heat in the room, so converting watts to heat units is mostly a matter of changing units. This calculator takes a device's power in watts and a run time in hours and returns the heat output rate in BTU per hour and kilocalories per hour, along with the energy used in kilowatt-hours and the total heat released in BTU over the run.
Use it to size a space heater against a room's heating needs, to estimate how much heat a computer rack, server closet or kiln adds to a space that has to be cooled, or to compare an electric heater's rating with a furnace or air conditioner rated in BTU per hour. It is a unit conversion and energy tally, so it assumes all electrical input becomes heat, which is close to true for resistive heaters and a fair approximation for most electronics.
The formulas and their variables
- P is the power in watts and t is the run time in hours.
- Heat rate (BTU/hr) = P × 3.412142.
- Heat rate (kcal/hr) = P × 0.859845, using the International Table calorie.
- Energy (kWh) = P × t / 1,000.
- Total heat (BTU) = heat rate in BTU/hr × t.
The conversion factors come from the definitions of the units: one watt is one joule per second, and one BTU is about 1,055 joules. That gives 3,600 joules per watt-hour divided by roughly 1,055 joules per BTU, or about 3.412 BTU per watt-hour.
Worked example
A 1,500 W space heater runs for 2 hours. The heat rate is 1,500 × 3.412142 = 5,118.2 BTU/hr. In kilocalories, 1,500 × 0.859845 = 1,289.8 kcal/hr. Energy used is 1,500 × 2 / 1,000 = 3.000 kWh. Total heat is 5,118.213 × 2 = 10,236.4 BTU. The four result cards read 5118.2 BTU/hr, 1289.8 kcal/hr, 3.000 kWh and 10236.4 BTU.
The familiar 1,500 W household heater therefore produces about 5,100 BTU/hr, which is why a portable unit is described as suitable for only a small room.
Common mistakes and how to interpret the result
- Confusing heat delivered with heat produced. Electric resistance converts essentially all input to heat, but a heat pump can deliver several times more heat than the electrical energy it uses, so this calculator understates a heat pump's output.
- Mixing watts and volt-amperes. For devices with a poor power factor, use real power in watts, not apparent power in VA.
- Treating nameplate power as constant. Thermostats cycle heaters on and off, so the average power over an hour is often lower than the rating. Use measured or duty-cycle-adjusted watts for energy totals.
- Forgetting the BTU-versus-BTU/hr distinction. BTU/hr is a rate, while BTU is an amount of energy. A 5,118 BTU/hr heater releases 5,118 BTU in one hour.
Related tools: Joule Heating Calculator, Ohm's Law Calculator for voltage, current and resistance, Watts to Amps Calculator and Heat Calculator.