How do you calculate the watts needed to heat a substance?
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. See also our Enthalpy Calculator (Hess's Law).
What is specific heat capacity and where do I find it?
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.
What is the difference between cₚ and cᵥ for gases?
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.
How do I calculate heat from watts?
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.
How much does a 1500-watt heater cost to run?
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.
What is the difference between heat (energy) and power?
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.
Can this calculator be used for cooling as well?
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.
Why does heating time affect the watts required?
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.