Calculate the power (watts) needed to heat a substance by entering the mass, specific heat capacity, temperature change (ΔT), and heating time. Choose a preset substance or enter a custom specific heat value. The calculator returns the required power in watts, total energy in joules, and estimated electricity cost.
Power Required
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Total Energy Required
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Energy in kWh
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Estimated Cost
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Power in Kilowatts
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Use the formula: Power (W) = (c × m × ΔT) / t, where c is the specific heat capacity in J/(kg·°C), m is the mass in kilograms, ΔT is the temperature change in °C, and t is the heating time in seconds. This gives the average power in watts required to achieve the desired temperature increase.
Specific heat capacity (c) is the amount of energy required to raise 1 kg of a substance by 1°C. Common values are: water = 4186 J/(kg·°C), aluminum = 897 J/(kg·°C), air = 1005 J/(kg·°C). This calculator includes presets for many common substances, or you can enter a custom value.
For gases, cₚ is the specific heat at constant pressure and cᵥ is at constant volume. cₚ is larger because heating at constant pressure also does work expanding the gas. For solids and liquids, these values are essentially the same since volume change is negligible.
To find total heat (energy) from watts, multiply power by time: Q = P × t. For example, a 1500 W heater running for 600 seconds produces Q = 1500 × 600 = 900,000 J of energy. This is the reverse of finding power — here you know power and time, and want total heat delivered.
A 1500 W heater uses 1.5 kWh of electricity per hour. At an average US rate of $0.13/kWh, that's about $0.195 per hour, $4.68 per day (24 hours), and roughly $140 per month. Use the electricity cost field in this calculator to estimate costs at your local rate.
Heat (Q) is the total energy transferred, measured in joules (J) or kilowatt-hours (kWh). Power (P) is the rate at which that energy is delivered, measured in watts (W). The relationship is P = Q / t. A 2000 W heater and a 500 W heater can both deliver the same total heat — the 2000 W heater just does it four times faster.
Yes. The formula is symmetric — simply enter the magnitude of the temperature difference (ΔT) as a positive number. If you're removing heat to cool a substance, the required power is calculated the same way; the direction of heat flow changes, but the magnitude of watts or joules needed is identical.
The total energy needed to heat a substance is fixed by its mass, specific heat, and temperature change. However, if you want to achieve that heating in less time, you need to deliver that same energy faster — which means higher power (watts). Halving the heating time requires doubling the power.