Hot water debate 25 July 2022 at 3:19PM in Green & ethical MoneySaving 32 replies 783 views

Magnitio

I know this subject gets debated a lot, but I have a slightly different question that I haven't noticed discussed recently.

Back in February when I had solar installed, I also put in a 1.5kW immersion heater, controlled on a timer which has proved to be very effective at using our solar generation or off-peak electricity. Our gas rate is 7.34p/kWh and electricity was 5p/kWh overnight and 4.1p/kWh for export, so it made perfect sense.

Recently, I have changed to Agile Outgoing Octopus which has been paying an average of 20p/kWh over the past couple of months. Purely from a financial point-of-view, returning to the use of the gas boiler for our hot water makes sense. My rough calculations are as follows:

Immersion heater - 4kWh per day at 20p/kWh = £24 per month.

Gas heating - 6kWh per day of gas (allowing for losses) plus 300Wh per day of electricity (pump/boiler/controller) = £15 per month.

From an environmental point-of-view, until renewables represent a considerably greater percentage of UK power generation, any solar electricity that is exported should reduce the consumption of gas/coal in a power station. I presume that the energy losses between a power station burning gas and the resultant electricity arriving at our homes is considerably more than the difference between using gas and electric to heat our hot water cylinder.

So, should I return to using the gas boiler, or stick with the immersion?

Thoughts?

QrizB

Economically, you're right.

Environmentally, it's less clear cut. The carbon intensity of the UK grid is currently 136g/kWh (per the Drax site) while burning gas will release 227g/kWh. Right at the moment, you'll have a smaller carbon footprint from electric heating than you will from gas. This does change (slowly) in real time and there are certainly times when heating with gas is greener.

Mikeyboy01443

So would you pick whichever method was greener, even if it costs more? 
I wouldn't...
I get this is the green forum but i didnt get solar, EV etc to be 'green' or be an eco warrior, i just did it to save money...

QrizB

Mikeyboy01443 said:

So would you pick whichever method was greener, even if it costs more?

The OP asked for thoughts, and I shared some.

It's your choice which you do, not mine!

Magnitio

QrizB said:

Economically, you're right.

Environmentally, it's less clear cut. The carbon intensity of the UK grid is currently 136g/kWh (per the Drax site) while burning gas will release 227g/kWh. Right at the moment, you'll have a smaller carbon footprint from electric heating than you will from gas. This does change (slowly) in real time and there are certainly times when heating with gas is greener.

Thanks QrizB.

I prefer not to use the financial justification for firing up the boiler instead of using the immersion if it is at the expense of increased carbon emmissions. However, I can see that it's not straightforward, even using average carbon intensity. For the majority of the time, are any fluctuations in load placed on the grid generally met by generation that is more carbon-intense than the average?

Of course, I could take the savings and invest them in Ripple.

Magnitio

Mikeyboy01443 said:

So would you pick whichever method was greener, even if it costs more? 
I wouldn't...
I get this is the green forum but i didnt get solar, EV etc to be 'green' or be an eco warrior, i just did it to save money...

You are correct, this is the "Green and ethical Moneysaving" forum. I am keen not to waste money, but I also wish to understand the environmental impact of decisions I make.

Mikeyboy01443

QrizB said:

Mikeyboy01443 said:

So would you pick whichever method was greener, even if it costs more?

The OP asked for thoughts, and I shared some.

It's your choice which you do, not mine!

Yes i wasn't aiming my reply at anyone in particular...
Just saying to me i don't take anything into account other than cost ie. If i can heat water off solar i will do that when i can as its free, rather than gas which will cost a bit. 
But as i only have a 3kw immersion if i don't have excess solar or its in the evening i would have to knock on the gas to heat my water...

Martyn1981

Magnitio said:

QrizB said:

Economically, you're right.

Environmentally, it's less clear cut. The carbon intensity of the UK grid is currently 136g/kWh (per the Drax site) while burning gas will release 227g/kWh. Right at the moment, you'll have a smaller carbon footprint from electric heating than you will from gas. This does change (slowly) in real time and there are certainly times when heating with gas is greener.

Thanks QrizB.

I prefer not to use the financial justification for firing up the boiler instead of using the immersion if it is at the expense of increased carbon emmissions. However, I can see that it's not straightforward, even using average carbon intensity. For the majority of the time, are any fluctuations in load placed on the grid generally met by generation that is more carbon-intense than the average?

Of course, I could take the savings and invest them in Ripple.

Hi, just to complicate things further, let's say your boiler is new and 'upto' 90% efficient, then it can take 1kWh of gas and produce 0.9kWh(t). So if you use PV export to heat the water, then that lost leccy will probably/possibly be supplied by gas generation at around 55-60% efficiency, plus transmission losses, so perhaps 2kWh of gas will get burned.

So it may appear greener to export your excess, with little losses to your neighbours and offset some gas generation.

But, in the summer (or to be more precise, any non-heating periods), your boiler will need to run 'just' for the hot water, so isn't going to be anywhere near as efficient. Plus all of the heat losses from the pipework/joints between the boiler and tank. Those aren't losses during central heating periods as they are within the property envelope.

So ..... hard to guestimate, but may be a draw in the summer, so not to worry about it, from an environmental aspect.

An additional expenditure consideration, might be longevity of the boiler. I'm way out of my depth now, but many people with pv diverters suggest that their boilers don't need to run for most of the non-heating period, which may be a cost (maintenance) saving?

QrizB

Magnitio said:

However, I can see that it's not straightforward, even using average carbon intensity.

Looking at last week, for a lot of the time the grid carbon intensity was higher than 227g/kWh so burning gas to heat water would have been the greener option. There's a forecast at https://www.carbonintensity.org.uk/ and eg. it's saying that on Wednesday it will always be greener to heat with gas.

For the majority of the time, are any fluctuations in load placed on the grid generally met by generation that is more carbon-intense than the average?

Marginal load is an interesting concept but isn't especially useful when you're talking about a single household. For you on your local circuit, your personal choice to import or export is unlikely to have a direct impact on generation; it's possible that your local voltage will rise or fall a fraction without a perceptible effect on the wider grid.

If you were part of a group with 10,000 other owners and you all decided to change your habits, each drawing 3kW from the grid for a total of 30MW, that might be noticeable and could result in a CCGT burning slightly more gas than before. But on a windy night it could mean30MW less abatement (where wind turbines are switched off to prevent grid instability). And I guess this is the idea of Octopus Go; it provides a larger load at night when wind can make up a bigger proportion of demand and helps prevent abatement.

Martyn1981

QrizB said:

If you were part of a group with 10,000 other owners and you all decided to change your habits, each drawing 3kW from the grid for a total of 30MW, that might be noticeable and could result in a CCGT burning slightly more gas than before. But on a windy night it could mean30MW less abatement (where wind turbines are switched off to prevent grid instability). And I guess this is the idea of Octopus Go; it provides a larger load at night when wind can make up a bigger proportion of demand and helps prevent abatement.

Hiya. That's a really good point, and reminded me that hot water (domestic or commercial) is a potential energy storage medium. Pretty much as you've described, smart devices could be used to mop up extra RE generation and store it as hot water, heat pumps adding more heat, even air cons (in hotter countries) doing some extra cooling. All taking advantage of low cost leccy during excess supply.

Screwdriva

I've been waiting for a thread that touched on this very topic. I'm also on Outgoing Agile and find that the tariff is lowest during the peak generation hours (between 12-2pm) for us. This is also the only time our solar PV produces more than 3kWh, removing all likelihood that we will import during operation of the 3kW immersion heater. 

We time the use of our other appliances either before or after this window to maximize earnings, almost never running any heavy draw appliances after 4pm, which is when the export tariff often doubles if not more.