Immersion heaters are like big kettles, the thermostat on your tank is set to a particular temp. With the immersion heater on it heats it to this level and keeps it at this level so it just continuously boils the water.

It costs a lot to run it on electric. If you have your oil heating on in winter then it should also be heating your water. You usually run both heating and hot water on a timer system.

So you should be able to choose when your heating comes on and goes off and separately you should be able to control the hot water too as you may only need a tank in the morning for showers/face washing/breakfast pots. And again in the afternoon for the evening stuff.

The time you use the immersion heater is in the summer when you don't have the heating on. BUT you still need to remember to turn the thing off. You don't want it on all day.

I have oil heating and an immersion heater. Also a multi fuel stove with back boiler which gives hot water.
A plumber told me to use the immersion in summer and leave it on all the time. The year I tried it my electricity consumption rocketed and my oil consumption didn't move down that much. Although oil is now much more expensive than it was , it's still cheaper than electricity.

We have oil heating/hot water and use it during the winter to heat hot water as it's on anyway. In the summer we use the immersion on economy 7.

Wow that's seriously clever. I'm impressed. Thank you!

OK back...

You have bought 2000l of oil for £800. This works out at 93250kJ per £.

Assuming electricity costs 10p a unit, you would get 36000kJ per £. Adjust up or down for your actual electricity tariff, but it would have to be less than 4p a unit to challenge the oil price.

The actual transfer of energy from the oil and electricity into your water is subject to inefficiencies - there are energy losses in the process, with the burning of oil having more losses. Still, the difference in costs still favours the oil.

Just bookmarking 37.3 MJ/l (ignore me)

OK, a gallon of heating oil is 139000 Btu.

I have to pick up kids from school now, but I can work it out when I get back.

oh blimey - no idea. just paid 800 for 2000 litres of oil but other than that i havent a clue

You can do the math...

Do you know what you are being charged for electricity per kwh compared to the cost of the energy in the oil?

Just moved into a new house with Oil fired cenral heating and also what looks like an immersion heater. DH thinks we should turn off the immersion heater since he says it will cost us a fortune but is this right? Will it not be more expensive to use the oil to heta the water?