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The year's costs in retrospect

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Majordennisbloodnok
(@majordennisbloodnok)
Noble Member Contributor
4162 kWhs
Joined: 2 years ago
Posts: 372
Topic starter  

I have noticed many times that a lot of the discussions about relative costs of running heat pumps vs boilers rely on some form of extrapolation. Typically, a poster will have good historical data about their old way of doing things but will be posting a comparison with what they expect the heat pump costs to be based on extrapolating a month or two's data from a fairly newly installed unit.

Given a heat pump tends to be less efficient at times when it's most called for, the running costs of a heat pump are rather more seasonal than those of an oil or gas boiler. As a result, plenty of people are seeing a winter electricity bill for the heat pump and fainting into their sofa, without necessarily seeing the flip side - the lower costs during more moderate weather. In short, it makes it difficult for someone looking to make the heating switch to make a truly informed choice.

I've now passed a milestone. Although we've had our ASHP for well over a year, it took a while before UKPN could sort themselves out and provide us with the necessary details to allow us to export our excess solar energy to the grid. Nonetheless, 6th March saw the anniversary so I now have a full year's data of running our ASHP alongside a solar PV installation that can power our kit and export any excess under Octopus' agile export tariff. This means, of course, that I can now give a definitive answer of the effect of switching heating sources and of installing a solar PV system without having to resort to any extrapolation or guesstimates. Since I expected this would be useful to people, here are the key figures.

We used to have an oil boiler and consistently got through two tanks of oil each year; one delivery after 5 months and then one after 7 months, reflecting the greater usage during winter vs summer. Looking at the delivery dates and using the prices on those dates, I was able to work out that running our oil boiler over the past year would have cost £1,806.

Looking at our electricity bills for the same period, we have spent a net of £1,707.53 (importing £2,229.41 and being credited £521.88 for exported energy). However, this also includes non-heating related electrical usage that we would still have had to spend if we had our oil boiler. I'm happy to share my workings if people really want to see them, but suffice to say:

  • The cost of running the ASHP purely from grid-supplied electricity would have cost us £1,775.19, so simply switching from oil boiler to ASHP saved us £30.81 over the year.
  • The cost of running the ASHP using home-grown electricity and topping up from the grid cost us a nominal £1,274.87.
  • The cost of running the ASHP after solar exports have been taken into account (i.e. our net savings overall from the ASHP/solar PV combination) are £976.43, which is a saving of £829.57 over the year.

Obviously, this doesn't take into account the upfront installation costs (£3,500 after RHI for ASHP, £7,500 for solar array and battery), so there is definitely a time yet before we hit our theoretical break-even point, and nor does it take into account any value we may have added to our house by installing either or both new systems. Similarly obviously, the reasons for the big savings are split pretty evenly between moving to a heating technology that uses energy we can generate at home and money paid to us from a decent rate of export tariff. The markedly better efficiency of the ASHP has been almost exactly offset by the price differential between oil and electricity, so in fact doesn't make a significant difference.

All that said, those bills were definitely unevenly split with our December, January and February bills being £468, £493 and £352 respectively. I'm well aware many people will struggle with summer months with negative outflows and then be hit with £1,300 outgoings in just three months. That's an important consideration to bear in mind.

 

105 m2 bungalow in South East England
Mitsubishi Ecodan 8.5 kW air source heat pump
18 x 360W solar panels
1 x 6 kW GroWatt battery and inverter
Raised beds for home-grown veg and chickens for eggs

"Semper in excretia; suus solum profundum variat"


   
Mars, Marzipan71 and Kev M reacted
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Marzipan71
(@marzipan71)
Estimable Member Member
993 kWhs
Joined: 2 years ago
Posts: 101
 

Hi @majordennisbloodnok thanks for sharing these...I'm doing something similar myself as I have a full 2 years data (almost) for my ASHP and PV, and modelled data for this year. Do you have the relevant figures for your past year in kwh as well you could share? I've seen very few sets of data around the web on monthly ASHP consumption, solar production and consumption etc in kwh - I find this sort of thing is very useful https://renewableheatinghub.co.uk/tesla-powerwall plus context of house size etc. 


   
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Majordennisbloodnok
(@majordennisbloodnok)
Noble Member Contributor
4162 kWhs
Joined: 2 years ago
Posts: 372
Topic starter  

Yes, by all means, @marzipan71. However, I’m not near that file at the moment. PM me if you haven’t seen an attachment appearing here by the weekend.

105 m2 bungalow in South East England
Mitsubishi Ecodan 8.5 kW air source heat pump
18 x 360W solar panels
1 x 6 kW GroWatt battery and inverter
Raised beds for home-grown veg and chickens for eggs

"Semper in excretia; suus solum profundum variat"


   
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Majordennisbloodnok
(@majordennisbloodnok)
Noble Member Contributor
4162 kWhs
Joined: 2 years ago
Posts: 372
Topic starter  

Perhaps a bit late, but here's the data, @marzipan71.

 

105 m2 bungalow in South East England
Mitsubishi Ecodan 8.5 kW air source heat pump
18 x 360W solar panels
1 x 6 kW GroWatt battery and inverter
Raised beds for home-grown veg and chickens for eggs

"Semper in excretia; suus solum profundum variat"


   
Marzipan71 reacted
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Marzipan71
(@marzipan71)
Estimable Member Member
993 kWhs
Joined: 2 years ago
Posts: 101
 

@majordennisbloodnok many thanks - I'll take a look and share my numbers for the record


   
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Marzipan71
(@marzipan71)
Estimable Member Member
993 kWhs
Joined: 2 years ago
Posts: 101
 

Hi @majordennisbloodnok this was an interesting read, thanks. Its really useful to see the data from other installations to help understand my own data - both in terms of efficiency of the ASHP and the PV aspect also. For context our house here in Italy has an internal floor space of 240m2, we've a Daikin HT 16kW ASHP, 6.6kWp PV, and we run an unheated pool in summer - usage is the pool filter pump. I've attached my data below - actual data from 2021, when the ASHP was shiny and new and I was using it like a gas boiler due to my own ignorance (and for most of 2022), actual data for 2022, where you can clearly see the positive influence of the help of @derek-m from October 2022 onwards, and modelled data (pink) for 2023 (other than Jan and Feb) which is based on late 2022 data. I'm predicting for 2023 that we'll be using around 55% of the grid energy we used in 2021. I don't have any fancy devices that allow me to record consumption for background, DHW, or the ASHP itself so these are my best efforts based on my daily data - so a bit of a fudge but as close as I can get. May be of interest, maybe not 🙂 but thanks for sharing your data again.

 


   
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