Watts to Heat Calculator
Use the watts to heat calculator to know the power you need to heat a particular substance.
The formula
Q = mcΔT for sensible heat, Q = mL for a phase change
Latent heat hides from the thermometer
Sensible heat changes a temperature and is what mcΔT computes. Latent heat changes a phase at constant temperature, so it does not register on a thermometer at all — which is why a pan of boiling water stays at 100 °C however hard you heat it, and why the water level falls instead.
The magnitudes are strikingly unequal. Heating water from 20 °C to 100 °C takes about 335 kJ per kilogram; boiling it takes 2,256 kJ per kilogram — nearly seven times as much. Melting ice takes 334 kJ/kg, almost exactly the same as heating the resulting water through the full 80 degrees, which is why ice is such an effective coolant.
Water's specific heat of 4,182 J/(kg·K) is anomalously high — several times most metals and about four times air. That single property moderates the planet's climate, makes water the default coolant in almost every engineering context, and is why coastal temperatures swing far less than inland ones.
Note that ΔT is a difference, so it is the same number in kelvin and Celsius. That is the one place in thermodynamics where Celsius is safe to use, and it is worth being deliberate about which kind of temperature a formula wants.