In article ,
Grunff writes:
No experience necessary, just basic physics.

Your pool contains ~ 5m^3 of water, or 5000 litres. This has a mass of
5000kg.

A 3kW heater provides 3000J/s.

The specific heat capacity of water is about 4200 J/(kg.K)

So to raise the temperature of your 5000kg by 1 Celcius we need 21MJ.
Your 3kW heater would take just under 2 hours to deliver this. So if
you're aiming for a 5 Celcius rise, it'd take just under 10 hours.

And this is before you factor in conductive and evaporative losses.

Not that I really approve of dumping large amounts of heat
outdoors, but this would be an ideal task for condensing boiler
with an extra zone, transfering the heat through a plate exchanger.
The temperatures involved will mean the condensing boiler will be
operating at peak efficiency (higher even than its published figures
for central heating use).

Cost wise, you should compare to running it overnight on cheap rate
electricity -- there may not be a lot in it. However, the electricity
option is nowhere near as environmentally clean, as electricity
generation efficiency is nowhere near as high as even the most
inefficient old gas boiler.

The other thing would be to ensure you have a very good pool cover
with regards to low thermal losses.

--
Andrew Gabriel