Oil vs Electricity to heat domestic water - where's the tipping point?

mikes01666

Apologies if this has been asked before.

Having just coughed up over £500 for 1000 litres of domestic heating oil, has anyone done the calculation as to where it become cheaper to heat domestic hot water via an immersion heater rather than an oil-fired boiler?

Without going into complex thermal efficiency calculations, age of boiler etc., has anyone got a rough rule of thumb? e.g. > 45p per litre use the immersion heater?

This will also be useful when the energy price cap is revised (upwards!) next April.

Oil prices appear to be going only one way and at some point it becomes cheaper to use electricity.

Thanks

Ebe_Scrooge

The generally-accepted rule of thumb is that a litre of oil gives 10.35 kwh of energy.  Call it 10 kwh to make things simple. It's a fairly simple comparison to see what electricity costs.  What's the cost of electricity these days - 20p per kwh, something like that?  So, being very approximate, 10 kwh is going to cost you £2 in electricity (again, ignore any standing charge for the sake of simplicity).  So, very roughly, oil needs to hit about £2 a litre before leccy becomes cheaper.

Of course, by then, leccy will be more expensive, since a lot of it is generated by oil or gas powered stations, so oil will still probably cheaper.

QrizB

Ebe_Scrooge said:

The generally-accepted rule of thumb is that a litre of oil gives 10.35 kwh of energy.  Call it 10 kwh to make things simple. It's a fairly simple comparison to see what electricity costs.  What's the cost of electricity these days - 20p per kwh, something like that?  So, being very approximate, 10 kwh is going to cost you £2 in electricity (again, ignore any standing charge for the sake of simplicity).  So, very roughly, oil needs to hit about £2 a litre before leccy becomes cheaper.

I was going to say exactly this.

With current electricity prices, heating water with duty-paid diesel at £1.40/litre would still be cheaper than electricity. Heating oil at 50p/litre is a bargain in comparison.

Reed_Richards

The price of heating oil has shot up in the last month.  The last time I looked it had reached around 60p per litre.  Are there any power stations that run on heating oil?  

canaldumidi

Reed_Richards said:

The price of heating oil has shot up in the last month.  The last time I looked it had reached around 60p per litre.  Are there any power stations that run on heating oil?  

I just paid 44.7p, but yes, it is rising.

The generally-accepted rule of thumb is that a litre of oil gives 10.35 kwh of energy.  Call it 10 kwh to make things simple. It's a fairly simple comparison to see what electricity costs.  What's the cost of electricity these days - 20p per kwh, something like that?  So, being very approximate, 10 kwh is going to cost you £2 in electricity (again, ignore any standing charge for the sake of simplicity).  So, very roughly, oil needs to hit about £2 a litre before leccy becomes cheaper.

In my case the calculation is more complex (impossible?). I have an extra large cylinder, with the boiler heating the whole tank whilst the shortish immersion just heats the top 1/3 or so. Given that my usage is low, heating the whole tank with oil only to not use the water so heated is KW down the drain (literally).

Indeed, with extra insulation, the immersion gives me enough hot water for 2 days - 3 in summer when I'm happy with a merely warm shower. A kettle for washing up if I need to top up boiling water.

Cardew

Whilst I understand your point, I think many people overestimate the losses from a well insulated hot water tank.

All HW tanks are tested to a British Standard and the rating(loss) should be stamped on the tank, unfortunately usually difficult to read. My large tank is rated to lose 1.3kWh over 24 hours with the water heated at 65C. In practice it is not heated at 65C so the losses will be less.

Even at the stupid current prices, gas & oil(allowing for boiler efficiency) are under half the cost of heating by electricity.

Then of course that leaked heat from the HW tank isn't lost as it warms the fabric of the house; which is why many tanks are situated in an airing cupboard.

Effician

Cardew said:

Whilst I understand your point, I think many people overestimate the losses from a well insulated hot water tank.

Agreed , but few think about well insulated pipework from tank to tap.  For infrequent users ( think 2 pot washes a day plus hand washing ) with uncladded pipework the energy wasted can be quite high.

Reed_Richards

I strongly suspect that hot water tanks are tested to a British Standard with no pipes connected to them.  Copper is a very good conductor of heat so even well-insulated pipes will carry a lot of heat away from the cylinder and there are typically several valves close by that cannot be fully insulated.  Also plumbers typically put the pipes too close together and too close to the wall to enable a decent thickness of insulation around them.  So the tank may be well insulated but the pipework connected to it lets it down.  

canaldumidi

My tank has the required BS insulation PLUS additional insulation I added. Pipework is also insulated though not ideally due to lack of space. I can only comment on heat loss in terms of long the water stays hot enough to shower under!

Plus as explained, there must be a significant cost saving in only heating 1/3rd of the tank.

QrizB

canaldumidi said:

Plus as explained, there must be a significant cost saving in only heating 1/3rd of the tank.

There's an energy saving, but whether there's a cost saving will depend on how much the energy costs. Using electricity at 21p/kWh to heat a third of a tank will cost more than using oil at 4.5p/kWh to heat the whole tank unless the boiler efficiency is less than 65%.