My thermodynamics thesis was lost in the mail - hoping someone can fill in the gaps here... So I've read it takes 1kcal to heat 1 litre of water, 1°C. My Spa is approx. 1.8m x 1.8m x 0.8m V = 2.5 cubic meters V = 2500 litres We're not going to fill it all the way, and taking into account the curves and inward moulded plastics seats etc... we'll assume the working volume of water is around 2000 Litres. Therefore, if I have a Spa with 2,000L of water, which sits at an ambient of around 15°C - to get this up to a nice hot spa (~39°C) I'm going to need to heat it up 24 degrees. So far so good? I'll continue. Therefore, the energy required to heat 2,000L of water 24 degrees: E = Litres x Δ°C x Energy E = 2000L x 24°C x 1kcal E = 48,000,000 kcal Now to get us in to the electrical domain - since I'm considering an electric element, 1 kcal = 4.18 kJ, therefore: E = 48.0Gcal x 4.18 E = 200,640,000 kJ E = ~201 GJ And since 1 Joule = 1 Watt for 1 Second, E = 201 Gigajoules E = 201 Gigawatts for 1 second E = 55.8 MW for 1 hour Or, using a 3KW element, roughly 2 years. Hang on... Now I've obviously gone wrong somewhere, as by that number the amount of energy to heat my spa up in an hour is roughly equivalent to the needs of a small city.... Help?