Oil boiler or off peak electric to heat water?

[stu550]

Worcester Greenstar Camray 18/25 external oil boiler, 93.2% efficient from the website. I think it uses about 2.33 litres/ hour (from the manual).

Current oil price, say £0.60/litre.

Off peak electricity is current 6.465 pence/kw.

I have a 170litre unvented tank with a 3kw element that was installed earlier in the year. The direct heating time is quoted as being 163 minutes.
So... 3kw/ hour is £0.19395/ hour x 2.72hours (roughly) = £0.53.

The manual also says that the coil recovery time is 25 minutes for 75% of the water. Say 33 minutes for 100%. If the boiler is using 2.33 litres/ hour thats about £0.77 for 33 minutes.

I have always assumed it's cheaper to use night electric to heat the water but are my workings out correct? Is any other info needed?

I'd like to see the maths so I can work it out in future as fuel prices increase.

Many thanks

Stuart

 

[flameport]

What a boiler may use according to the manual is of no relevance. The actual usage depends on the heating load.

The only accurate way to determine what is cheaper is to start with the cylinder completely cold, then switch on the boiler to heat it, making a note of how much oil is used to do that.
The next time, use the immersion heater to heat it, and therefore identify how much electricity is used.

The much simpler and easier way is to know that 1 litre of oil is the equivalent of about 10kWh, making it 6p per kWh compared to the 6.45kWh for electricity.
Add in the fact that the boiler is not 100% efficient, and the end result is that both are probably very similar, so not worth wasting any time or effort over.

Something else to consider is why you have off peak electricity anyway - on the majority of tariffs, having off peak means you will be paying more for the daytime units than on a single rate.

 

[stu550]

Thanks. Yes, I appreciate the actual useage depends on the load but I was just trying to get the maths roughly right!

Interesting to know that 1 litre of oil is the equivalent to 10kwh. If thats the case then 21kwh is just over 2 litres of oil/ hour.

RE Off Peak - we have night storage heaters for the winter when the cottage is empty as background heat.

I hadn't realised the figures would be so close. I thought off peak electric would be much cheaper during the night...

 

[Corgigrouch]

I'd be inclined to avoid the Greenstar Camray, they may be the installers choice but they are a pain in the backside to service, removing the baffles is a nightmare.... Plenty of better oil boilers about for the same money.

 

[stu550]

Greenstar Camray installed 6 months ago - just working out the most efficient way to heat the water....