To calculate the cost, what you need to do is find out the wattage of your electric immersion heating. A typical one is 3kW. That means it use 3 units or kW per hour of electricity to heat water. Have this on 2 hours a day it would use 6 units.

Once you work out how long the immersion heater takes to heat up the water you require, you can them make a comparison with your oil heating.

I had read that a properly working and timed immersion heater, can work out cheaper than using oil to heat water during the summer. But this was based on energy prices a few years back, so may no longer apply.

The other point, is whether there is any benefit to keeping the oil boiler working during the summer. Does it help keep it in working order ? I can remember having problems with boilers that have been switched off for a period, when you come to turn them back on for the winter.

The Energy Saving Trust helpline may be able to answer your query.

I have an oil fired central heating system which I run on hot water only during the warmer months.

I also have an electric immersion heater fitted to the hot water tank from when the heating was supplied by a solid fuel range and now replaced by the oil fired boiler (no gas)

Oil is now over 70p per ltr I pay 7.8p unit + 18p a day for electricity.

The timers are set for two hours in the morning and five hours in the evening

The tank is well insulated.

I use a dish washer every other day for most of the washing up and an electric shower daily.

I do five loads of washing @ 40 deg per week and like at least one long soak in the bath a week.

Would it be cheaper to use the immersion heater in the summer rather than the water only function on the oil boiler?