I bought a beer chiller, it helps but won't keep up with the evaporation rate of a 3Kw heater. You could try pre-chilling a barrel, but again cost is an issue, chillers aren't cheap to run.
I've settled on using mains supply which is pretty constant at around 10°C.
I'm not on a meter and I'll worry about wasting the planet's resources when the water company sort their leaks out ... currently my water company waste 24.5 million litres per day! One of the larger companies leak in excess of 600 million litres a day, so I don't think our usage will make a whole heap of difference.