Heater power calculator
Work out the heating power needed to raise a given mass of air, water or oil through a temperature rise in a target time, and read the nearest standard electric heater rating.
Heater power calculator
Medium
Formula: Q = m·c·ΔT / t where m is the mass in kg, c the specific heat (air 1.005 kJ/(kg·K), water 4.182, oil ~2.0), ΔT the rise in K and t the heating time in seconds. The heat-loss margin adds a percentage on top for uninsulated tanks, ducts or cold starts.
Pair the calculated rating with a stocked industrial heater, thermostat or contactor for the control side.
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AO Ctrl provides this tool for reference only.
Frequently asked questions
How do I calculate the kW needed to heat water or air?
The sensible-heat equation is Q = m·c·ΔT / t, where m is the mass in kg, c the specific heat (air 1.005 kJ/(kg·K), water 4.182, thermal oil ~2.0), ΔT the temperature rise in K and t the heating time in seconds. The tool multiplies by a heat-loss margin you set, then rounds up to the next standard heater rating.
What heater rating do I need to heat 10 kg of water by 50 K in an hour?
10 kg × 4.182 kJ/(kg·K) × 50 K = 2091 kJ, and 2091 kJ / 3600 s = 0.58 kW before losses. With the default 15% margin that is about 0.67 kW, so the recommended standard size is 1 kW. Add more margin for uninsulated tanks or cold-start from below freezing.
Why does air need so much less energy per kg than water?
Air has a lower specific heat capacity (1.005 vs 4.182 kJ/(kg·K)) and a very low density (~1.2 kg/m³), so heating a kilogram of air needs a quarter of the energy of water. In practice fan heaters move cubic metres, so the mass is small and the power often lands below 10 kW even for large rooms.
AO Ctrl provides this tool for reference only.