Cost to heat DHW oil v electric?

[ericmark]

As yet not getting paid for export, so clearly electric is the cheapest, once set up I will get 15p per kWh for exported electric, and if I use electric over night it will cost me 8.95p per kWh so really no point in having the iboost+. However last year I see it showed around just over a kWh per day to keep water warm which seems a lot less than expected, as boiler would run for around 20 minutes 4 times a week, and it is around 20 kW. This points to some massive losses in the pipe work, however the immersion does not heat up as much as the boiler, but don't really need much only used to wash our hands.

I hear it said that electric costs 5 times what gas costs, and I know we pay less for oil than we did for gas, but it seems using around 80 minutes at 20 kW a week with oil, and 8 kWh with electric, so the numbers don't seem to add up.

I can't see how much electric is being used until we stop using central heating. The picture was from last year, but it seems electric may be cheaper than using the central heating boiler, has anyone actually measured the two? Or is it just an assumption that electric costs more than gas so it must be cheaper to heat DHW by gas?

It seems gas cost around 7.5p per kWh and oil around 8.7p per kWh so it seems not 5 times as expensive anyway.

 

[Adam_151]

It seems gas cost around 7.5p per kWh and oil around 8.7p per kWh so it seems not 5 times as expensive anyway.

What supplier are you with? If octopus then see if you can get on tracker for gas, aside from a slight peak in the autumn mine has been hovering around 4p/kwh for gas over the last few months and I don't think the peak went as high as 7.5p. Obviously thats no guarentee of future performance, but if you are happy with the risk that prices could in theory go quite high if theres some sort of crisis in the world (and if it goes on too long, you can always move back to standard tarriff, you just cant come back onto it for a good while after if you did that).

 

[ericmark]

I am aware there are cheaper, but £75 fee to leave, so not worth changing yet. However my point is domestic hot water does not cost anywhere near 5 times as much to heat with electric v gas and oil, considering we have a 40 gallon tank it can be heated at any time in the day, so we can heat over night or using solar.

There are also extra losses when heating with gas or oil due to losses in the pipe work, the shorter time the pipes are hot, and the shorter the pipe work the better of course, and added further is the fact often less water is stored when using electric, that point may not be good however.

Even with combi boilers, the question is how much to heat the boiler and how much to heat the water, I used an instant gas boiler for years, it was fitted to save space, so airing cupboard could be used as an office. But still had 32 mm pipe work, so the delay turning on tap to getting hot water was massive. When we had hot and cold fill washing machines, the pipe work was kept hot, so delay was shorter, but I use a cup boiler and often think quicker to put a cup of boiling water in the bowl to was hands. Often don't bother and use cold water.

But we see on these pages again and again people claiming gas is cheaper than electric to heat DHW, and I am questioning if that is in fact true?

 

[DetlefSchmitz]

You seem to be comparing apples and oranges. Gas comes in at about 7.3p / kWh and if you don't make any special arrangement electricity is typical over 29p / kWh. I'm sure you can do the sums.

 

[ericmark]

7.3 x 3 = 21.9p as experiment has shown to keep DHW warm enough to wash hands I need to run the oil boiler 4 times a week for ½ hour a time, and it actually runs for 20 minutes, it is 20 kW and does not modulate so uses 27 kWh approx, the iboost+ displays energy saved each day, so it is 1.2 kWh per day, so 8.4 kWh using electric, so 27/8.4 = approx 3, so the electric uses 1/3rd of the energy used by oil.

Gas would be harder to measure, as most gas boilers modulate, so some one would need to be only using gas for DHW and would need to read the gas meter. Can't simply use time running.

There are clearly many variables, length of pipes, size of pipes, if lagged or not, so any observations are approximate. I heat the same tank using an immersion as the boiler uses, but often hand wash units are placed close to where the water is required, so further reducing electric cost. I will admit the immersion only heats top of cylinder where oil fills it all, but only use the water for hand washing, so that is enough.

It does not put iboost+ in a good light, as loosing 15p used in day, but costs 8.95p per kWh over night, so would have been cheaper to use simple time switch.

However for years I have taken the "It cost 5 times more to heat DHW with electric compared with gas" as a fact, this is clearly not true, it seems it costs around 1.3 times more to heat with electric if not using solar or off peak.

Even using a shower, I know when using a gas boiler to directly supply shower I had to find a larger shower head as the one used with electric instant did not allow enough water to flow to ensure the boiler did not cut out, I would also have to allow the shower to run to hot, then cold, then hot again before stepping under it, so likely used double the water to using an electric shower.

We seem to consider 20 kW into a boiler means 20 kW out, this is of course not the case, not only the inefficacy of the boiler, but also heating up the heat exchanger and pipe work, heating house that heat will in the end enter the house, but having a shower in summer don't really want that heat in the house.

If I look at my consumption when using a shower it is very short lived, add all the data and the shower was only using 5.6 kW from the grid, so the cost is likely 60p for a 20 minute shower, to switch to a hot/cold supply shower will likely cost £200 plus, even DIY, so at 73 it will never pay to change to hot/cold supply type.

If I was to install from scratch another shower in the bathroom then likely it would be a power shower using the DHW supply, but not worth changing the two showers I already have, and do we really want 3 showers and two baths for two people? Down stairs bath has wife's e-bike stored in it.

I will agree in the main unless using a split tariff for electric gas is cheaper than electric, but the gain is marginal, and if one wants to save money, which is the cheapest options, fit a gas powered shower, or get a smart meter and split tariff? I think the maths is reasonably simple.

 

[DetlefSchmitz]

7.3 x 3 = 21.9p as experiment has shown to keep DHW warm enough to wash hands I need to run the oil boiler 4 times a week for ½ hour a time, and it actually runs for 20 minutes, it is 20 kW and does not modulate so uses 27 kWh approx, the iboost+ displays energy saved each day, so it is 1.2 kWh per day, so 8.4 kWh using electric, so 27/8.4 = approx 3, so the electric uses 1/3rd of the energy used by oil.

Gas would be harder to measure, as most gas boilers modulate, so some one would need to be only using gas for DHW and would need to read the gas meter. Can't simply use time running.

There are clearly many variables, length of pipes, size of pipes, if lagged or not, so any observations are approximate. I heat the same tank using an immersion as the boiler uses, but often hand wash units are placed close to where the water is required, so further reducing electric cost. I will admit the immersion only heats top of cylinder where oil fills it all, but only use the water for hand washing, so that is enough.

It does not put iboost+ in a good light, as loosing 15p used in day, but costs 8.95p per kWh over night, so would have been cheaper to use simple time switch.

However for years I have taken the "It cost 5 times more to heat DHW with electric compared with gas" as a fact, this is clearly not true, it seems it costs around 1.3 times more to heat with electric if not using solar or off peak.

Even using a shower, I know when using a gas boiler to directly supply shower I had to find a larger shower head as the one used with electric instant did not allow enough water to flow to ensure the boiler did not cut out, I would also have to allow the shower to run to hot, then cold, then hot again before stepping under it, so likely used double the water to using an electric shower.

We seem to consider 20 kW into a boiler means 20 kW out, this is of course not the case, not only the inefficacy of the boiler, but also heating up the heat exchanger and pipe work, heating house that heat will in the end enter the house, but having a shower in summer don't really want that heat in the house.

If I look at my consumption when using a shower View attachment 338589 it is very short lived, add all the data View attachment 338591 and the shower was only using 5.6 kW from the grid, so the cost is likely 60p for a 20 minute shower, to switch to a hot/cold supply shower will likely cost £200 plus, even DIY, so at 73 it will never pay to change to hot/cold supply type.

If I was to install from scratch another shower in the bathroom then likely it would be a power shower using the DHW supply, but not worth changing the two showers I already have, and do we really want 3 showers and two baths for two people? Down stairs bath has wife's e-bike stored in it.

I will agree in the main unless using a split tariff for electric gas is cheaper than electric, but the gain is marginal, and if one wants to save money, which is the cheapest options, fit a gas powered shower, or get a smart meter and split tariff? I think the maths is reasonably simple.

-sigh-

Are you taking your situation, with endless green but not well understood add-ons, to be representative of the typical household? Or are you just trying to demonstrate how, by spending enough on an installation, you can reduce the cost of electricity?

 

[ericmark]

It is really the reverse, I am guilty of telling people don't use electric, I looked at cost per kWh, without making any allowance for 100% efficiency with electric and well under that with gas or oil.

I had simply accepted electric is more expensive and failed to do the measuring and maths.

There are clearly other reasons for not using electric, I lived through the Winter of discontent and know how easy the goverment can turn off the power.

However I now realise my maths for many years has been wrong. I never tried using an immersion heater as had been told how expensive it was, I now realise my error.

 

[Rodders53]

My oil costs me 8.70 pence per kWh inc VAT (based on last Septembers delivery rate and 90% efficiency, Grant condenser).

My electric costs 29.22p / kWh.

That 3.34 times more expensive for me. Allowing for some other heat losses (welcome in winter, and even summer: the en-suite shower tray is warmed by losses from pipes to/from everywhere else) lets say 3 x more expensive?

The sum will change from Monday and when I refill the oil tank. (new rate for electric will be 25.26 electric, oil might be circa 7.41 to 7.58 p depending on supplier. So still of the order of 3.3n times more.)

My cheapest ever oil purchase was 3.53 and 3.60 p /kWh in 2020... when electric for me was around 12.2 p / kWh with a now gone bust supplier (3.38 times electric vs oil). IIRC oil was then cheaper than gas. So I'm not sure where the "five times the cost" has come from? MSE forums mostly suggest between 3 and 4 times (and I'd go nearer three).

You need to do your own sums; making your own efficiency/heat loss assumptions etc.,. and then choose the best (or simplest) way to operate. You really should not compare your unique (and imho eccentric) energy installation, tariff and use with anyone other than those with similar setups.

Many solar panel users can be reliably advised to divert solar generated power to an immersion to save money (especially with an immersion element that heats all the cylinder water rather than just a fraction at the top) ... but you seem to be able to obtain very cheap energy at off peak and be paid better money to export solar during the day... So you will have a different regime to achieve the most savings and a solar immersion diverter may never pay for itself in your installation.

 

[ericmark]

I do agree with you @Rodders53 it will change home to home as to what tariff and what losses one has, however I have seen it claimed that electric is 5 times more expensive to gas so many times, and I have realised yes more expensive, but in my case around 1.5 times rather than 5 times.

I am considering changing the immersion for a longer one, mine does only heat a small amount at top, however since showers are instant type, and washing machine cold fill, the water is only used for hand washing, and in the main if central heating not running then there is enough sun light for that. And there is no option to turn off DHW if central heating running, it is C Plan.

So you will have a different regime to achieve the most savings and a solar immersion diverter may never pay for itself in your installation.

Don't rub it in, I realise now that a simple time switch would have been better, my electric did cost 29.57p per kWh this went up to 31.31p per kWh for peak supply, but odd the standing charge dropped from 62.21 a day to 59.14 per day, and now off peak at 8.95p per kWh. So to gain one must use most of ones electric midnight to 8 am.

There are problems doing that, but three items have the delayed start option, the washing machine, tumble drier and dish washer. So even without solar one can gain from the split tariff. My tumble drier is the heat pump type, this is a small problem as one has to consider what the weather will be like 2.5 hours after it has started. And the washing machine we use to fill a ball with the soap, however with the delayed start it still turns the drum a few times to work out the load, so the soap now much go in the dispenser.

This

is not much help deciding what the day will be like. Even the Met Office the little pictures are useless to work out likely sunlight, it gives all sorts, temperature, rain fall, humidity, this may help, shows 11 am to 2 pm UV is 3 to 4, does this relate to sun light?

This is an index, developed by the World Health Organisation, called the 'Solar UV Index'. It shows the strength of the sun's ultraviolet (UV) radiation. The index is split into categories (Low, Moderate, High, Very High and Extreme). We have included advice about what you need to do to stay safe in the sun for each category.

and is 1 to 11, but it has warnings like "Take care during midday hours and do not spend too much time in the sun unprotected. Sunscreen advised." but as to how this relates to solar production not sure?

 

[StephenStephen]

I'm in a similar position, in that I have a solar immersion diverter which I never use.

I've also always assumed that (in my case gas rather than oil) is cheaper to heat the water than solar electric, but also never actually done the maths.

In our case the boiler is about 3 feet from the tank, we pay about 4.5p/kWh for gas, and get paid about 17.5p/kWh for solar export in the middle of the day.

Another factor is that there'll be some (minimal) loss from the tank itself over time, and we can heat the water using gas first thing in the morning, when showers are about to be used...

 

[Murdochcat]

We have PV , a solar divertor and no batteries.

Works well for us and if we get an EV, I would add a divertor so excess energy goes to the EV as the priority.

The immersion divertor makes sense for us because the hot tank and the radiators in the bathroom, ensuite and shower room all get heated by the boiler when heating hot water - so the boiler has to heat lots of water in the pipework around the house to heat hot water.

A single shower needs about 1 to 1.25 kwh of lecky to bring the hot water back up to fully hot - assuming the tank was completely hot during the boiler hot water heating during the night. When the daughter was at University, and during the summer months, we could rely on the solar divertor to provide all our hot water.

 

[Rodders53]

I'm in a similar position, in that I have a solar immersion diverter which I never use.

I've also always assumed that (in my case gas rather than oil) is cheaper to heat the water than solar electric, but also never actually done the maths.

In our case the boiler is about 3 feet from the tank, we pay about 4.5p/kWh for gas, and get paid about 17.5p/kWh for solar export in the middle of the day.

Another factor is that there'll be some (minimal) loss from the tank itself over time, and we can heat the water using gas first thing in the morning, when showers are about to be used...

No brainer to export as much as you can at those rates. Gas is circa 5.25p / kWh in to HW tank if 4.5p is the ex vat tariff rate (5p if inclusive). Reheat time is low with a boiler cf 3kW electric immersion too!
But your gas price is surely extraordinarily low? The capped price is to drop to circa 6 pence per kWh from April 1st.... down from 7.4 p today. Even at those prices the gas is a no brainer still.

NB Loss from my unvented 210 litre HW tank is 1.95 kWh per day. Which I'd not really call "minimal" (unless comparing to the room heating use in winter). Check the specs of your tank to see if the makers quote it.

Eric possibly needs to change from instant electric showers to stored hot water; though the batteries probably help minimise import while using them?

 

[JohnW2]

I've also always assumed that (in my case gas rather than oil) is cheaper to heat the water than solar electric, but also never actually done the maths. .... In our case the boiler is about 3 feet from the tank, we pay about 4.5p/kWh for gas, and get paid about 17.5p/kWh for solar export in the middle of the day.

I think one of the causes of confusion may be the fact that the conclusions per Physics are totally distorted by 'bureaucratic' considerations (details of tariffs).

In terms of Physics, it clearly takes the same amount of energy to heat a certain volume of water to a given extent (temp rise), so the cost of that heating simply reflects the actual cost of the energy - lowest for oil/gas, much higher for grid electricity and essentially zero for solar.

However, as I see it, the 'bureaucracy' in the form of payments for export, totally change that. If, had one not used it to heat water, the solar energy would have been exported for a 'negative cost' of 17.5p/kWh, that would effectively put the effective cost' of heating with solar about mid-way between the cost of oil/gas and grid electricity.

 

[StephenStephen]

... bureaucracy or politics or late-stage capitalism or whatever else you want to call it contributes greatly to the difference in cost between gas and electricity - not physics!

 

[JohnW2]

... bureaucracy or politics or late-stage capitalism or whatever else you want to call it contributes greatly to the difference in cost between gas and electricity - not physics!

Indeed so, but eric's argument/discussion sometimes seems to get the two a bit confused.

As you say, given the non-physics considerations the difference can be very great - since, using your figures, you are being paid nearly four times more for not using your solar energy than you have to pay for using gas instead - bizarre though that might seem!

This must be very confusing for some people, since I imagine that, since they pay nothing for it, many people will assume that it is always better (cheaper) to use their solar energy rather than some other form of energy that they do have to pay for!