This is a simple online calculator where you will not see a complicated formula for heating water. 
The heat capacity of the water is already built in. You simply enter your parameters and get a result, such as: how much energy it takes to heat 1m3 of water. Or in other words: how many kW are needed to heat 100 litres of water?
Or this heater power calculator will show you the time it takes to heat water based on the power of your electric heater.