How to compute the heater power of a swimming pool?

Answer 1

multiply the mass of the water by the specific heat and the (temperature required minus the ambient temperature).that will give you the energy in joules.if you know in how much time you want this temperature to be achieved you can calculate the power as power is nothing but the rate at which the energy is dispensed

Answer 2

Do you mean the power to heat it up? or the power to keep it hot? To find the latter you need to know the rate of heat loss.