how much energy needed to heat water?

james2012_2

How much energy, in kWhs, will it take to heat an insulated water cylinder of 115 litres from 15 deg C to 60 deg C, using either a 3 kWh immersion heater or a gas water heater with an (input or ouput ?) of 4.5 kWh?

The answer does not need to be overcomplicated, an approximation, in kWhs will be sufficient.

Pincher

It takes 1 kCal to raise 1kg (~1 litre) of water by 1 degree C.


1000 kCal = 1.163 KWh


115 x (60-15) = 115 x 45 = 5,175 kCal = 6.019kWh

With a gas boiler, it is able to condense initially (efficiency 90%), but as the cylinder water reaches 50+ degrees, the efficiency will drop to non-condensing levels, like 70%. Therefore, the worst case scenario is 6.019 / 0.7 = 8.599kWh.

Also, the electric immersion element is only able to heat the top half of the cylinder, unless you have a direct cylinder.

At 12p per kWh, 12 x 6.019 = 72.2p by electricity

At 3p per kWh, 3 x 8.599 = 25.8p by gas