Air source heat pump electricity consumption

by Mars

I have been asked this question numerous times via YouTube comments and email: what is your actual air source heat pump electricity consumption? Buckle up, because I’m going to answer that right now.

Let’s get a bit of context first. At Home Farm we have an 18kW air source heat pump (ASHP) that drives our hot water demand and central heating (12 radiators, two towel rails and three underfloor heating zones). We also have our old oil boiler connected that gives the ASHP a boost from time to time.

If the temperature anywhere in the house drops below preset minimum levels (monitored by our smart TRVs and thermostats) the ASHP starts the heating. Our hot water is also reheated immediately after showers, baths or washing up.

Working in tandem with the ASHP, we have a 6.16kW solar PV array with an iBoost immersion heater that heats our hot water when we are producing excess solar energy. We allow this to heat our hot water to a maximum of 60C.

So let’s break down the electricity consumed by our air source heat pump so far in 2020.

Air source heat pump electricity consumption

MonthTotal (kWh)ASHP (kWh)Oil Boiler (kWh)Avg Temp (C)
January1713.101712.600.505.92
February1760.901760.200.705.27
March1650.901650.000.906
April731.90728.603.3011.12
May444.90441.703.2014.1
June215.90211.504.4015.79
July127.90125.502.4014.79

Our oil boiler ignites and helps the ASHP when it has to start operating from a standing start during warmer months, which is why we see more activity from April-July when the average outdoor temperature is in double digits and the house doesn’t need the central heating to be on for prolonged periods of time.

Conversely, from January-March our ASHP pretty much operates 24/7 so the oil boiler only kicks in when the pump needs to defrost. This happens mostly on cold days with high relative humidity.

From the table we can see that the pump alone has consumed 6,644.6kWh this year, at an average of just over 1,000kWh per month (34kWh per day). This creates an accurate snapshot of the year with three cold months, and three warm ones.

To complete the picture we must factor in our solar PV system. According to the SolarEdge app, we generated 3,919kWh of power in 2020. Of that, we exported 1.2MWh resulting in 2.72MWh being used to offset our total electricity usage.

This is worth mentioning because on days with decent levels of light (in winter or summer) our hot water and central heating is powered by renewable energy during daylight hours.

If you’d like more information on air source heat pump electricity consumption on cold sunny days versus warm overcast days during the winter you’ll find this post interesting.

18 comments

Eddy Winko 19 July 2020 - 07:08

Nice to see some figures to add to those you provided in an earlier post. Do you consider this to be good value?
The numbers look very high to me, although I appreciate that the size of house and other factors have to be considered.
I wonder if the addition of solar water tubes or panels would help to bring the cost down?

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Mars 19 July 2020 - 07:18

The consumption is high, but it’s relative to what we were paying for oil. Our oil consumption was high too, 500 litres per month, and we weren’t heating the entire property. With the ASHP we heat the entire property.

The biggest plus point, however, is not having diesel fumes coming out of the exhaust outside the property.

We’ve got a 6.16kW solar PV system.

What are solar water tubes?

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Eddy Winko 19 July 2020 - 08:57

https://www.powermyhome.uk/evacuated-solar-tubes-installations.php
That’s just the first in the google search.
We have 30 tubes that provide us with 300 litres hot water. During the clear summer months we rarely have to supplement the water heating. Even in the winter we can get the temperature up to 40c on a bright day. They even add heat on cloudy days which means less work for the wood fired boiler.
I did read that this was a good combination to use with an ASHP as getting the higher temperatures requires a lot of energy. It is worth noting that the only reason that water is heated to 60c in the UK (not sure about EU) is to eradicate the chance of Legionnaires disease, If you can put that fear away then you only need water around 45c for a conformable shower, bath, washing up. Our tubes often heat the water beyond 60c so I feel we have adequate protection.

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Mars 19 July 2020 - 10:24

Thanks Eddy. I’m going to check out the solar water tubes. We have got the iBoost which kind of does that job for us, but I’m still going to investigate. Thank you for bringing something new to my attention.

Our hot water, when the iBoost doesn’t kick in, is set to 42C, because what you said is spot on. Why waste energy getting water to 60C when it burns you and you need add loads of cold water to cool it down for showers, baths, washing up, etc. It doesn’t make sense.

The reason our iBoost is set to 60C is to combat Legionnaires disease as you have correctly pointed out, and we’re only using excess solar that would have gone back to the grid to get it to 60C. We also try to have a second shower if we’ve worked in the garden in the late afternoon so that the water gets reheated using solar energy.

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Eddy Winko 19 July 2020 - 11:51

I remember reading in “Sustainable Energy- without the hot air” that solar PV is technically inefficient and so is heating water with electricity, for this reason alone we went for the tubes. We have PV as well, but it is more efficient to sell it back to the grid for us than use it to heat water. Mind you the the return on investment can sometimes seem like a long way off.
I’m sure we will be considering a ground source heat pump in the future, but not until the government provide us with an incentive 🙂 I’m yet to be convinced that air source is the right way to go for us here in Poland as the climate is that much colder in the winter. Your articles on the subject have convinced me of that, thank you.

Mars 20 July 2020 - 07:04

Do you have any incentives for renewable heat and solar in Poland? GSHPs will definitely work in colder climates.

Getting back to the solar water tubes, do they require extensive additional plumbing because the area where we get the best solar is over 30m from where our hot water tank is?

Steve Elliott 19 July 2020 - 07:46

Hi Mars,
If my calculations are correct then taking the highest consumption month February and assuming it ran 24/7 then that 1761 kWh is equivalent to about 2.6 kW instantaneous power. So you’re putting in 2.6kW and getting out 18kW. So that means the output power is about 7 times the input power which must be good. Does that sound right?

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Mars 19 July 2020 - 08:43

Hi Steve. That’s sound logic, but not entirely correct.

We have additional metres in our utility room that allow us to know how much heat, in kWh, we’ve generated. These metres are used for us to get our payments from the RHI scheme.

I took our reading at the start of July, so this calculation will be a slightly skewed, but we generated 18,224 kWh of heat from January 1 to July 1, 2020. As stated in the post, we used 6,644.6kWh of electricity to generate that heat, which equates to a coefficient of performance (COP) of approximately 2.74. This is a decent ratio in real-world terms. There are certain manufactures that claim that they can deliver a COP of 4, but that would only happen in heavily insulated homes where heat cannot escape easily. We have a period property that is reasonably insulated, but we do lose heat, and that impacts on our COP.

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Steve Elliott 19 July 2020 - 10:07

Thanks Mars, so that figure of 18kW output for the heat pump is a maximum but I guess you don’t always need or use that much. I think that averages about 4kW output. It’s still good I think.

We’ve had leaflets from local installers who quote that figure (COP) are a percentage efficiency so they’d say that was 274% efficient. It just so happened that at the time I was doing an Open University science course and we’d just been taught that you can never have a heat engine with an efficiency more than 100% and in practice the maximum was more like 30% so I couldn’t understand how they could quote over 200%. When I queried it I realised that they were simply dividing output power by input power which is not the same as thermal efficiency of the machine. It turned out later that Trading Standards had come down on them because they hadn’t carried out any actual trials to prove that figure. It was just a theoretical best case figure.

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Mars 19 July 2020 - 10:34

To be honest Steve, until you actually raised this point, I’m not actually sure what the 18Kw in our ASHP actually means. I’m going to have revisit the literature tonight because it doesn’t actually appear to be working at any more than 4kWh as you’ve pointed out, and that’s the maximum heat it can produce at our 45C flow rate that’s set by RHI to get scheme payments. More on this later when I’v figured it out.

I’ve seen the percentage statements too. I think what they’re doing is taking a percentage of the overall COP ratio by equating that they’ve taken 1kWh of power and turned it into 2.7kWh heat hence the 270% efficiency. It’s a good marketing tool. Good to hear Trading Standards stopped it.

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Steffan Cook 20 July 2020 - 11:15

This is very interesting, thank you for the post. I have to say though that you are pretty heavy users of electricity, with 6.5MWh for the heat pump so far this year and then another 4.5MWh for other consumption (according to your solar edge) you seem to be using almost a semi-commerical amount of power. Is this normal for life on a farm?
When it comes to heat-pumps, they are not heat engines (they are not generators) like Steve is suggesting. Instead heat pumps, move (pump) heat from one object to another, in this case moving heat from the surrounding air into the water system in the house. This allows them to have an efficiency over 100% and many of them do. In fact thinking in terms of efficiency is a bit strange for a heat pump, as the pump is always 100% efficient at pumping, the only terms that matter are how much heat has been moved, which is given by the co-efficient of performance (COP). Also, a leaky house has no effect on COP, as you measure heat output at the radiators, your internal heat loss is a property of the building not of the heat pump.
I would say that from a quick analysis your heat pump is working OK. If your pump is working 24/7 in January then the average input is 2.2KW, but as you mention your heat output is limited to 4KW, so thats a COP of 4/2.2=1.8. This is an OK amount but not great. Your house is quite well insulated if 4KW will keep it warm at such a high indoor temperatures, but strangely you mention the house is quite leaky (are you using a lot of direct electric heating?). If it was not strictly necessary to have the house at 26C indoors, then I would consider dropping it to less (21C for lounge areas is deemed comfortable, however 23C is the premium spec given by builders).
Your May numbers look a bit incorrect, you could always think about another monitoring solution like I have set-up for my house (https://biaenergyconsulting.com/2020/04/08/home-energy-part-1-the-monitoring-set-up/).
Thank again for your blog posts.

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Mars 21 July 2020 - 07:07

Thanks for the detailed reply Steffan. This answers a few questions and casts some new light. Greatly appreciated.

As a correction, the general temperature in the house is 21C (not 26C). We don’t have any zones that operate above this temperature preset. On the subject of COP, if the house is better insulated, does it not loose less yet, and the pump has to work less, improving the COP in the process?

I have one other question. What does the 18kW in the heat pump model really mean? As you’ve mentioned, the heat pump output is limited to 4kW, so why is it referred to as an 18kW model?

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Mars 21 July 2020 - 07:11

Also, why do you think our May numbers are wrong?

Ive had a look at your post, and the monitoring software is for PC only, and we run off Macs. Are there any Mac packages you’re aware of?

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Steffan 21 July 2020 - 11:36

“Also, why do you think our May numbers are wrong?”
only because your average air temp is up and down. If you know for sure it’s fixed to 21, then thats fine all the other data seems to make sense.
Are there any Mac packages you’re aware of?
The emonPi is web browser based so you can access it on a MAC or PC. It may however not be polished enough for you as a user yet…

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Steffan 21 July 2020 - 11:28

As a correction, the general temperature in the house is 21C (not 26C) –
I’m sure you top graph in the post (img-0244) suggests the average temp is around 26C(?) (the red line).

On the subject of COP, if the house is better insulated, does it not loose less yet, and the pump has to work less, improving the COP in the process?
In theory,Yes. If your house was to lose less heat then you could run a lower ‘flow’ tempearture and the pump could operate at a higher COP. However, best not to think about it that way…the COP is a test result given by a lab that belongs to the heat pump under standard conditions. The heat loss in your home is something independent of that.
What does the 18kW in the heat pump model really mean?
The name is just a name, however most of the time it refers to the highest possible output of the heat pump. In this case the manufacturer is suggesting that the heat pump can output 18KW of heat. However, according to the spec sheet (https://globalenergysystems.co.uk/wp-content/uploads/2019/03/GES-DATSHT-001-27-02-2019-Caernarfon-data-sheet.pdf) the max output under standard conditions for the heat pump is around 15KW, so a bit short of that.

As you’ve mentioned, the heat pump output is limited to 4kW, so why is it referred to as an 18kW model?
I doubt very much that your HP is giving you only 4KW. You need a post 1990’s well maintained small (<90m2 floor space) well insulated home to get that small amount of heat loss (newer homes are even better insulated though). It's possible you are only getting RHI for up to 4KW of heat demand, and that you get no subsidy for the rest (the subsidy RHI limit is for 20,000KWh annually – it's likely you are above that limit).
Don't get me wrong, the heat pump was the way to go and is the way forward for houses in the countryside that are off-gas grid. Fairplay to the manufacturer for offering larger heat pumps and for the awesome names.

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Roger Alan Young 21 July 2020 - 12:54

May I ask how large your property is in terms of cubic metres (if you have the figure) or square metres of floor area? It would help me to make a comparison with our own house. It would probably help other users of your site as well.

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Mars 21 July 2020 - 12:59

We’re heating over 4,000 square feet. That’s a very good question actually, because that’ll put the consumption in context.

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Mark Crooks 14 August 2020 - 07:34

372 sq m in new money!

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