Oil or off peak electricty for domestic hot water?

Silvershadow12

I normally use off peak electricity to heat my hot water via the immersion heater. Now that electricity is so expensive, I wondered whther I should use my oil boiler to heat the water (I only use that for heating the house).

I've tried to calculate this: I've just bought some oil - price is 90.2 pence per litre. Off peak electricity is 26 pence per litre.

Apparently, 1 litre of oil provides 10.35 kwH of energy - i.e. 8.7 pence per litre.

If my calculations are right, it's a no brainer to use the oil to heat the hot water.

I can't believe that's right - where have I gone wrong in the calculations / logic?

Thanks

Silvershadow12

p.s. meant 26 pence pr kwH!!

lohr500

And I think you mean oil works out at 8.9 pence per kWh.

Your conclusions are more or less correct.

A couple of factors to consider. The oil boiler won't be 100% efficient. Depending on the age it could range between 70 and 95%. The immersion heater will be 100% efficient. Also there will be some heat loss in the pipework from the boiler to the hot water cylinder.

But with off Pak at 26p per kWh, it makes 100% sense financially to use oil 👍

Silvershadow12

Hi Lohr

Thank you for confirming what I thought - even if my numbers were a bit out.

I did think about heat loss through the pipes, but thought this wuld be fairly negligible as everything is well lagged. Also, I have quite a small house - not miles of pipes. Heater is 18 yo- but I have it serviced every year - so not the best but not the worst either.

I'll move over this evening - and hopefully see some savings soon!

Reed_Richards

Silvershadow12 said:

p.s. meant 26 pence pr kwH!!

The SVT rate for electricity is about 34p per kWh but presumably you pay more than this for your day rate in order to have off-peak electricity.  The cost of your off-peak electricity is so high, are you sure it is worthwhile to be on a dual rate tariff?

Fredw56

Silvershadow,
I experimented with this over a period over the summer. Like you we have an old boiler. It is unlikely that our installation will be directly equivalent. The boiler here is very old, 1977, but is a type and with a burner that was built to last indefinitely. It is serviced annually. Our hot water cylinder is almost new, a 150L unvented installed 2021. Very well lagged pipework. There just the two of us in a two bed bungalow.
over a reference period of a couple of months I found the oil boiler used 1.1L per day to heat our water in two periods per day using a timer control and cyl stat set at around 60C. Using the immersion heater (timer and cyl stat set for same periods) as an alternative increased the electricity consumption by between 1.5 and 2kWh per day. The latter is less precise because I can only measure by reading the meter frequently and many other variable things draw electricity whilst with oil, nothing else draws supply. 

My conclusion was that firing the boiler for such a short period was highly inefficient. It takes too much energy to raise the heat of the boiler casing to operating temperature to make effective output. Plus once it stops all those joules in the pipes, boiler water and boiler jacket get wasted. All that waste twice a day. I ought to have tried just one heating period, I know that works because I used to live in an all electric bungalow with economy 7.
oil price plays it part. Last autumn I topped up at 39p per litre, this autumn it was 84p. The price change makes a big difference.

That point about oil boilers running for short period is what I also found many years ago with oil combi boilers. They are hideously inefficient. They should have been banned long ago. 

QrizB

Fredw56 said:

The boiler here is very old, 1977, but is a type and with a burner that was built to last indefinitely.

A boiler of that age is likely to burn 40-50% more oil for a given heat output than one manufactured in the last 15 years will.

Put another way, replacing your boiler could save you a third on your oil costs.

Last autumn I topped up at 39p per litre, this autumn it was 84p. The price change makes a big difference.

Burned in your boiler, a litre of oil might only be giving you 6kWh of useful heat. At 84p/litre that's 14p/kWh, more expensive than many off-peak electricity tariffs (although not the OP's, who seems to have quite expensive off-peak electricity).

Burned in a more efficient condensing boiler, that same litre of oil might give 9kWh of useful heat. At 84p/litre that's 9.3p/kWh.

Fredw56

QRizB Even if I changed to a condensing boiler I know, I have done the research, I would only save about 15%. Once you break through the advertising and get it the facts the relative efficiency levels are nothing like those figures you quote. The two companies who have serviced my boiler for years have both told me not to be persuaded to change it until is beyond economic repair. As the last one explained. You may save 15% on your oil bill but the replacement will never pay for itself before it reaches the end of its service life. It would also never pay for itself in my life expectancy. 

The OP could test his installation in a similar way to how I tested mine.  It’s simple enough.  That way he will know how it performs. I did say from the outset that the two would likely be different.

lohr500

Everyone's installation and usage will differ, but I am so glad we replaced our ancient 1980's Myson oil boiler with a new Grant boiler at the start of last year.

It was getting harder and harder to get spares for the old boiler, it sat in the kitchen making a lot of noise and taking up space and drank oil for fun.

Our average consumption with the old boiler in Q1 last year was 16.2 litres per day.
This year in Q1 with the new boiler it has averaged out at 10.5 litres a day.
All consumption measured with an in-line fuel gauge/tally counter.

No changes in central heating times or temperatures AND we have boosted the hot water by oil on weekdays in Q1 this year at around 4pm for an hour vs electricity last year!
I don't have the external temperature data for Q1 this year and last to see if last year was particularly colder.

That's a 35% saving in oil, with the inclusion of a daily water heating boost for this year.

I have no doubt that some of the saving came from changing the system from a gravity circulation hot water setup to a valve/pump controlled one and through careful balancing of the radiators to achieve 60c flow & 40C return temperatures. 


For me, because I am fortunate enough to be on an EDF fixed tariff until August next year, I think it still makes sense to heat the water overnight on our off-peak rate of £0.100 inc VAT. 

Latest oil price in my area is £0.945 per litre inc VAT. That equates to £0.091 per kWh at 100% efficiency.
But with heat loss from the boiler pipework and losses through the boiler efficiency, I can't see it beating the £0.100 off peak electricity cost. However at our peak rate of £0.223 per kWh it makes 100% sense to use the oil boiler to boost the hot water as needed.

And in the OPs case, where he is paying £0.26 per kWh off peak, it must surely cost less to use oil.
Even if the OPs boiler and pipework efficiency losses were 50%, he would still be better off using oil.

Agree with Reed_Richards that the OP should review his Economy 7 rate and consider either moving to a single tariff or to a supplier with a more favourable night rate if he can maximise the off-peak usage. 

matelodave

If you've got oil central heating and hot water, why have you got a dual rate leccy tariff. If you off-peak rate is 26p, then I dread to think what the peak rate is. As R-R says, it's time to review how and when you use electricty.

QrizB

matelodave said:

If you off-peak rate is 26p, then I dread to think what the peak rate is.

If it''s capped by the EPG it should be something like 36p/kWh.