£700 sounds a bit steep. Its a 3kw water heater. If you were completly emptying the cylinders hot water it takes about an hour to reheat the cylinder which would be 3kw in energy. The average kw Price of electric today is about 15p . 15p per kw times 3 is 45 pence. So if you completly empty the cylinder three times a day it would cost about £1.35 a day. So for two 31 day months that would be £83.7. The water heater in the picture looks very small maybe 120 litre. An average 8 minute shower uses about 56 litres.