Hot Water: Immersion Heater v Oil ?

kah22

Some time back a chartered architect friend suggested it might be cheaper to heat the water from my immersion heater rather than use the oil to heat it.

As we have some of the highest energy costs in the uk has anyone sat down and done the sums?   How would you work it out?

macman

Very simple. Work out the cost per kWh of your oil (convert from the cost per litre using 10.35kWh per litre as the energy value) and then compare to the unit cost per kWh of your electricity. If you have an immersion heater then you may have cheap rate E7 electricity, so check your tariff-and bear in mind that the latter will rise on 1/10 approx 27% for SVT under the cap, unless you are on a fixed tariff.
If you are on single rate electricity then that is likely to be much more than oil, even allowing for the much lower efficiency of your oil boiler vs the 100% efficient immersion heater.

Mistral001

Your architect friend talks a lot of sense.  When my oil central heating was working, I always used the immersion heater in the summer.  It only takes about 8 to10 minutes to get hot water for doing the dishes which costs about 10p in electricity.  I cannot imagine how much it would cost to fire up the central heaing just to get hot water - probably a least a litre would be needed and this costs about 95p at today's prices.

kah22

Thanks for your replies.

I must be reading something wrong here. Oil is going to cost me 9.7 pence per kWh. On the other hand electric seems to be coming in at 28.11p per kWt  I don’t understand the difference and I'm speaking hot water from immersion heater

Restart https://www.rensmart.com/Calculators/HeatingOil 

Price Per Litre (in pence)

83

Number of Litres

500

Boiler efficiency (%)

80 (I’ll go with that, my boiler is pretty efficient)

Total cost £415

Cost 9.7 pence per kWh

Energy 4288.09 kWh

Now Electric. 

1.2 The new price for tariff customers on the Standard Home Energy tariff will increase to 28.11 pence per kWh (ex VAT) or 29.52 pence per kWh (incl. VAT) from 1 July 2022. This represents an increase of 27.5%.1 Jul 2022

https://www.uregni.gov.uk › ...PDF

Conclusion of the Utility Regulator's Review of the Power NI Ltd Maximum 

Have I missed something ?

macman

Your calculations are correct, the only possible variable being the efficiency of your boiler, but based on 80%, they are right.
Why don't you understand the difference? Single rate electricity costs about 350% more than mains gas. Traditionally, heating oil has been more expensive than mains gas, but with the recent increases, that gap has now narrowed. Yours is 9.7p (with 20% added for the lower efficiency than the immersion heater), but is that based on the price you last bought at, or the current price, which will be higher?
Electricity is, on your figures, about 300% more expensive. But mains gas will rise to 10.7p from 1/10 under the EPG, so, if you can still buy oil at 9.7p, it's cheaper than gas.
I'm using GB rates for the price cap and mains gas, but the principle remains the same. It's always cheaper to use any fuel other than single rate electricity for heating and hot water. 
The only problem with using heating oil is the price volatility by season/volume/supplier.

Ticked

Just to have hot water, the boiler has to be heated, also the water in the pipe work to the cylinder. This is then wasted to atmosphere. I’d never consider doing that, 10 minutes of immersion gives us enough hot water for dishes and washing.
This argument is like having the heat on reduced temperature between heating times, keeps a low temperature so the boiler doesn’t have to drag the system up from cold. I suspect there’s no definitive answer, there’s too many variables, every setup is different.