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Renewable installation recap

  • Sept 2018 – Solar PV installed
  • Feb 2020 – First home batteries installed
  • Aug 2022 – EV delivered and myenergi Zappi charger installed
  • Winter 2021/2022- radiators upgraded to run combi boiler ‘heat pump ready’.
  • Feb 2022 – Mixergy water cylinder and myenergi Eddi diverter installed in custom plant room
  • Oct 2022 – Vaillant 5kW Arotherm Plus heat pump installed
  • Nov 2022 – Batteries upgraded to Hanchu models

Note: most of these items have their own blog entry.

Blog Summary

This summary is taking into account the first 6 months of heat pump data collected from my Open Energy Monitor and heat meter monitoring setup.

Note: this is another in-depth document, so grab a brew!!

You can read about the heat pump installation in more detail here:

My 5kW Vaillant Arotherm Heat Pump

For info, my water cylinder and this whole heat pump and heating project was undertaken, designed and installed by Damon Blakemore Plumbing Heating and Renewables as part of the Heat Geek Assured program.

Heating Season COP Results (Nov 2022 – end April 2023)

These measurements are taken from a heat meter and electric meter attached to Open Energy Monitor recording kit.

You can view complete daily recordings of my heat pump performance here:

Below is just the data formatted into a nice table.

Month / Year Electric Input (kWh) Heat Output (kWh) COP Outside Low Temp Outside Avg Temp Outside High Temp Avg Room Temp
Nov-2022 344 1356 3.94 2.1 8.8 15.2 20.2
Dec-2022 647 2195 3.39 -3 4.3 12.1 20.3
Jan-2023 569 2056 3.61 -3.2 4.9 11.1 20.3
Feb-2023 390 1471 3.77 -1.6 6.6 14.1 20.5
Mar-2023 393 1433 3.64 -3.9 6.3 15.5 20
Apr-2023 205 848 4.13 -1.4 8.4 17 19.6

Note: indoor temperature is showing lower than 20C in April as we were away for a week in case you were wondering.


  • Heat Output: 9359 kWh
  • Electric Input: 2548 kWh

Ratio of heating to hot water production

  • Heating Electric Input: 1942kWh (76%)
  • Water Electric Input: 593 kWh (24%)

COP (coefficient of performance) = Heat Output / Electric Input

So 9359 / 2548 = 3.67

  • Overall COP: 3.67
  • Heating COP 3.9
  • DHW COP: 2.7

So this meant that for every 1 unit of electricity we put into the heat pump we got 3.67 units of heat out across the whole 6 months.

Remember, a gas combi boiler has a COP of around 0.85; 1 unit of gas in and 0.85 units of heat out, i.e. 85% efficient.

Note: if you want to explore COP on your own Vaillant Arotherm heat pump, check out this article on the site.

How to Measure Vaillant Arotherm COP

I was hoping for a higher overall COP from these first 6 months across winter. But as you can see from the COP breakdown, it’s the hot water production that has really dragged the figures down.

But I know why this happened.

Over the winter I’ve been heating the cylinder to 55, 60 and even 65C.

I’ve had a tendency to make my hot water runs ‘hot’ as I wanted to try and fill the cylinder on the overnight cheap tariff to make the very best of the cheap energy and attempt to get through the whole day.

Whilst the Vaillant Arotherm heat pump is capable producing 70C water hot, it leads to much lower COP. 

The lower target temperature you can choose, the higher the COP will be.

Many folks are getting by with heating their hot water to 45C/50C, so are getting a good DHW COP.  But they need to do this a couple of times a day in some cases because they used their stored water much quicker because it is cooler.

Also, outside ambient temperature plays a big part in performance.  So my choice to do my hot water runs overnight (during my cheaper off peak electricity period) when it’s colder outside, makes a difference compared to doing the hot water runs during the day when it’s likely warmer.

But during midwinter, it can be advantageous to not have to do a hot water run during the day as you can’t heat the house at the same time as doing hot water runs (just like a combi).

There is a balance to be found for us, between doing things cheaply overnight, but also as efficiently as possible.  

I have also played with using the heat pump to heat the water to 50C (minimum that the Mixergy currently allows) and then taking it to 65C using the immersion. All during the cheaper off overnight period.

I will continue to experiment with the Mixergy setup I have, although it seems apparent that people with ‘standard’ cylinders that have larger heat pump ready coils inside seem to be getting better performance than I am currently getting.

It’s as if the plate heat exchanger that came with the Mixergy heat pump kit can’t compare with a big coil inside the cylinder, despite it being labelled as 3m sq, same as large coils.

I was pretty happy with heating COP though at 3.9.

Yes, there are some much higher performing Arotherm installations being monitored on with higher COP for heating.

But many of them have underfloor heating, so they are able to run much lower flow temperatures than I am able to with my radiator only system.

I do have a little plan to help with my heating performance, which I’ll describe later in the article.

All in all though, I think I’ve finally made peace with my heating COP and stopped comparing to those other high performing Arotherm installations……. well almost!! 🙂

Hot Water Performance Chart


Here is a graph showing the hot water performance across the whole of the 6 months from November 2022 to end of April 2023.

The axis are daily outside average temperature and COP for that day.

As you can see we have low COP of around 2.0 when averaging 0C outside.

Remember, this is still double the performance of what an immersion element would get you. 1 unit of electric in and 1 unit of heat out for an immersion.

Performance then starts to trend upwards with direct correlation with the rising outside temperature.

Note: I write this in the summer and this correlation continues upwards with hot water runs of COP 3.8 when 18C outside.

As stated above, I’ve been a little underwhelmed with my hot water performance, especially as other heat pump installations listed and monitored on with coils seem to perform better.

Heating Performance Chart

Here is a graph showing the heating performance across the whole of the 6 months from November 2022 to end of April 2023.

The axis are daily outside average temperature and COP for that day.

As you can see we have low COP of around 2.5 when averaging less than 0C outside.

Things then start to trend upwards with a direct correlation with the rising outside temperature.

We start to get COP 3 at just above 0C to 1C average outside.

The first day with COP 4 for heating is when average 4C outside.

And a COP of 5 comes when average 9C outside.

Heat Pump and Domestic Hot Water (DHW)  – Embrace the Change

Hot water production is the biggest difference you need to get used to when coming from a combi boiler.

A combi boiler can produce instant and almost limitless continuous hot water.  A heat pump can’t do that.  Instead, the water has to be preheated in a cylinder.

You really have to get your head around that this reheating of water could be a slow process.

I discuss this in more detail in my ‘what size heat pump’ article where I created this table.

What Size Heat Pump?


Time in minutes to heat water using various power output sizes (10C to 50C)

  3kW 5kW 7kW 10kW 12kW 14kW 16kW
50L 47 28 20 14 12 10 9
100L 94 56 40 28 24 20 18
150L 140 84 60 42 36 30 27
200L 187 112 80 56 48 40 36
250L 233 140 100 70 60 50 45
300L 280 168 120 84 72 60 54

So looking at the chart, you can see that a small output heat pump (say 5kW) in a high water usage household (300L cylinder or with multiple reheats on a smaller cylinder) could be a tough combination and one that you’d certainly need to adapt your mindset compared to a combi boiler.

One word to note on my 5kW Arotherm Plus.  It can output heat way past 5kW and performs more like a 7kW heat pump during hot water runs.  As shown in the water reheat table above, this comes in very handy.

I have no doubt that these Vaillant models would also ‘beat their badge’ in heating mode if required too.

This higher output capacity seems to be a feature on the whole Arotherm range. They always seem able to work at a higher level than their badge suggests.

But this isn’t the case on many other heat pumps.  Some are absolutely limited to the output their badge says and some can only reach their badge output at certain outdoor temperatures.  i.e. some heat pumps perform less than the output on the badge when it gets cold outside.

So be sure that you and your installer understand the capacities and potential limitations of the model chosen for you.  

Low Temperature Heating – Embrace the Comfort

Oh my, what a revelation low temperature and long running heating is to comfort in the house, it’s beautiful.

The house is just permanently comfortable.  No turning up, turning down or cold periods.

The recommended way to run a heat pump is long and low.  In our house the heat pump unit runs 24/7.

In the daytime I target an indoor temperature of 20C and overnight we have the setback temperature at 19C.  Unless we are all at work/school and I drop it down to setback 19C in the day.

You need to be careful not to make setback too low as it can take low temperature heating a long time to get back up to room temperature.  Having initially tried 2C-3C difference, I’ve found the 1C difference works best for us.

As you can see in this monitoring snapshot, the heat pump ran here for 20 hours straight whilst it was 2C to 6C outside and keeping the house between 19C and 21C indoors with a flow temperature into the radiators of around 36C/37C.

Note: these are the monitoring screens from the Open Energy Monitor heat pump app.

You can view mine in detail here:

Running long and low means the thermal mass of the house retains the heat (the bricks, the walls, the whole ‘fabric’.  So the house then only needs a smaller amount of heat (and electrical input) to maintain the internal temperature than it would trying to heat up from cold.

Think of it like bringing a pan of water to boiling point (more energy needed) and then finding just enough heat to keep the water simmering (less energy needed).

We also run the system in weather compensation mode, so that the water temperature from the heat pump that goes into the house (via the radiators/UFH) rises and falls as the outside temperature changes.

You need a lot less heat (and energy input) to keep the house warm when it’s 10C outside than you do when it is -3C outdoors.

This is the beauty of automatic weather compensation.  

All of this is controlled by the Vaillant controls.  It’s always best to use the manufacturer’s controls in full weather compensation mode on a fully open heating circuit.

Having third party controls (Hive/Tado/Honeywell Evohome etc) telling the heat pump when to turn on and off is a recipe for disaster.  

Don’t do it!!

Final comment about controls, you barely have to do anything!!

And once your installer has setup your weather compensation curve to match the heat loss of the house you should barely have to touch the system.  It just sits there working alongside the outside temperature adjusting the heat output as required.  It’s brilliant.

Smart Controls – an admission of guilt

Now, after me just saying that third party controls are bad, I have to admit that I have had some smart controls in play in this first winter.

Rather than using standard mechanical TRV in the bedrooms to limit upper temperatures, i.e. close the rad down if the room gets to say above 21C, I have retained some of my Tado setup from when I had the gas combi boiler.

I use these smart TRV valves to close down the bedroom radiators in the early evening so the bedrooms start to cool down.

YES, I know that shutting down whole radiators goes against everything you should do; aim for an open circuit, fully flowing system etc.

And YES, I know shutting down some radiators probably hurts my COP and causes some system cycling (I’ve seen this in the monitoring).

BUT house comfort (and family happiness / me not getting it in the ear) is more important than COP chasing.  

We prefer it being slightly cooler in the bedrooms in the evening.  Using the smart TRV allows us to keep the downstairs open and targeting 20C, but allows the upstairs to drop towards 18C.

Another bonus of doing this is that by using the programmable smart TRV in the bedrooms (rather than mechanical TRV set at 18C) it allows us to run the bedrooms at 20C in the day.  So the kids playing in their bedrooms or me working from home benefit from the heat and a fully open system.

Remember, this is a real 1930’s semi-detached house that we have to live in, not a showroom.

The final point in my favour here is that the Tado smart TRV only turn on/off a small number of radiators independently, they have no direct on/off control of the heat pump.  

All heat pump control decisions are made solely by the Vaillant weather compensation controls.  As I said above, this is what you want, having the manufacturers controls calling the shots.

How did the heat loss sizing work out?

In the ‘What Size Heat Pump’ blog post, we talked a lot about sizing.  If you’re looking to get a heat pump i’d recommend you read it, especially the bits around pipe sizing and design as these are bits that elevate the good engineers.  And with that, well performing heat pumps.

What Size Heat Pump?

An MCS approved installation must have a full room by room heat loss calculation completed, which my installer did.

As well as this official sizing task I also looked at a variety of alternative methods of calculating the amount of heat required and so, the size of heat pump required.

The most intriguing one was HTC, or Heating Transfer Coefficient.

The method we looked at suggested using previous daily gas usage data on cold days in conjunction with the average outdoor temperature to give some indication of heat loss.

I used gas data from January 2022 it suggested I’d need 3.3kW of heat per hour to heat my house to 21C internally when it was -3C outside.

-3C being my DOT (Design Outside Temperature) and 21C as a representative indoor temperature as advised by MCS.

Similarly, the Michael De Podesta (Protons4B on Twitter) HTC variant (What heating power do I need to raise the temperature by 1 ℃? ) suggested i’d need 3.8kW using my heating gas data.

To see how these estimates fared versus real world heat pump data, let’s look at 3 of the coldest days over this first winter.

Electric In (kWh) Heat Out (kWh) Heating COP Lowest Temp that day Highest Temp that day Avg Outside Temp Avg Inside Temp Heat / 24hrs
Wed, 14 Dec 2022 32.8 87.5 2.7 -2.6 1.2 -0.8 20.4 3.65
Mon, 12 Dec 2022 33.8 85.7 2.5 -2.6 0.9 -0.6 20.2 3.57
Fri, 16 Dec 2022 30.2 85.9 2.8 -2.6 2.1 -0.2 20.2 3.58


Note: some ‘cold’ days were eliminated from the chart because setback had kicked in during the day (ie, we’d gone to work/school etc).

The 3 days here are proper full days heating (ie, working from home or similar), with only the overnight 19C setback in play.

As you can see the average outside temperature was -0.8C, -0.6C and -0.2C with average indoor temperatures 20.4C, 20.2C and 20.2C.

And the heat divided by 24hrs figure gives us a kW figure per hour based on those lows and highs.

So to match 21C and the outdoor to -3C (or the previous HTC calculations) we have to perform a couple of calculations ourselves.

  • Heat per Hour / (Avg Inside – Avg Outside) * 24

So for Wed 14 Dec that would be

  • 3.65 / (20.4 minus -0.8) x 24 = 4.13kW (let’s call this DOT calcs)

This 4.13kW figure gives us the amount of heat per hour required to keep the house at 21C internally whilst constantly -3C outside.

We can apply those calculations to all 3 days here

Avg Out Temp Avg Inside Temp Heat / 24hrs DOT Calcs
Wed, 14 Dec 2022 -0.8 20.4 3.65 4.13
Mon, 12 Dec 2022 -0.6 20.2 3.57 4.12
Fri, 16 Dec 2022 -0.2 20.2 3.58 4.21


You can see that this is consistent with the real world  heat pump data in this 1 hour snapshot.

This 1 hour extract from Sat 21st January shows that 4.1kW was needed to keep the room at 20C.

If we do ( 4.1 / 23 ) x 24 we get what we’d need to keep the room at 21C; 4.28kW

23 being difference between 20C and -3, and 24 the difference between 21C and -3C.

Which there or thereabouts matches the figures from the 3 days listed above.

And here is a trace of all the rooms in the house showing all were well up to temperature.

These are some Xiaomi Aqara Temperature sensors that I have in each room and reporting back to Home Assistant.

Looking at the graph, can you spot the rooms on the south side of the house?  21st January must have been sunny as we certainly got some solar gain there.

So how did all this compare to the pre-install estimates

Here’s the 4.1kW result shown alongside the original chart from the ‘What size heat pump’ article.

Conclusions then:

  • Our MCS room by room survey overestimated the heat loss by a little bit (perhaps some of the insulation under the bricks is performing better than we thought?  Who really knows what is really behind there?)
  • The Heat Geek ‘best’ calculation was almost spot on (but the other two way too high)
  • Some of Michael De Podesta calcs were just about right.
  • But on the same hand, some of us his others were well below.
  • Finally, the HTC only calculations I did from January 2022 gas data, again fell a little short.

So what to make of all this?

“Most” of the methods got us in the right ball park for heat pump size, around 5kW.

But the full room by room heat loss by your experienced installer is still the go-to way of designing the system.

The room by room part sizes the radiators or underfloor heating to match the heat loss of the room.  And plump for an experienced installer using something like Heat Engineer software for this work.

I will show how important room by room heat loss is in the next section where just 100W difference in output per room can make a difference to rad sizes and overall flow temperatures.

Bottom line, I’m delighted with our 5kW Arotherm choice and I’m happy it’s slightly oversized for two reasons.

  1. If the outside temperature drops to -5C all day I know it will have enough headroom to cope
  2. Hot water runs are quicker than say the 3.5kW model (obviously not as much of an issue if you’re putting a 10kW model in)

A single HTC figure is fine for the whole house heat loss and it’s good to use these tools to get you in the right ballpark.

.i.e. If your HTC calcs come out at say 6kW and an installer is suggesting 10kW unit then that could be a red flag.

Air Changes Per Hour (ACH)

A lot has been made on Twitter recently regarding the number of air changes per hour (ACH) when doing heat loss calculations. The biggest beef being that the defaults recommended by CIBSE (Chartered Institution of Building Services Engineers) that are used as part of the MCS process are way too high for most properties.  Using these defaults can lead to larger than required heat loss calculations and ultimately oversized heat pumps.

Here is the reference table from the MCS heat loss spreadsheet

For example, my 1930 semi comes under category A (built pre 2000). So the defaults are 1.5 – 3.0 ACH.  The reality is that with double glazing and new doors, most houses, even old ones aren’t this leaky.

So an experienced engineer doing the heat loss survey will make a judgement call and dial these down as appropriate.

In my heat loss, Damon did exactly this and dropped the ACH to around 1.0 for most rooms, whilst leaving the bathroom and kitchen a little higher due to extractors.

For a test in the heat loss software we dropped all the main rooms from 1.0 down to 0.5 and really, it didn’t make that much difference to the overall heat loss. It’s that initial drop from defaults that gets you towards heat loss reality.

So this is something to be very aware of when you are having your heat loss survey done. Make sure to query the air changes per hour with your installer, especially if they have left them at default values as above.

Planned Radiator Upgrades

Low temperature heating can be a fussy bugger!!

If you remember from my original layout diagram, the radiator outputs in both the Kitchen / Dining Room and Bedroom 2 were slightly under the heat loss of the room.

To be honest, I dismissed it and thought ‘we would be okay’, as they were almost there, only hundred watts or so.  My installer showed me the heat loss and radiator output figures and I accepted the risk.

But low temperature heating can really highlight the lack of heat output in a room.  And because of that, those two rooms had slightly lower running temperatures through the winter because of the undersized radiators. 

The rooms weren’t cold, they were like 0.5C to 1C lower than the rest of the house, but you could just about notice it.

To counter this, we increased the weather compensation curve to increase the flow temp to the radiators, which in turn raised the whole house temperature.  But this then put a little too much into the other rooms and the TRV kicked in.

Remember: radiators (or underfloor) in conjunction with weather compensation works best by putting just enough heat into the room to match the heat loss at each different outside temperature.  Finding that simmer point.

Here’s a breakdown of how the rooms with a shortfall looked.

Kitchen / Dining

  • Heat Loss: 1702W
  • Original Rads Output: 828W + 724W = 1552W
  • Difference: -150W (9%)

Bedroom Two

  • Heat Loss: 327W
  • Original Rad Output: 229W
  • Difference: -98W (30%)

Now 98W and 150W don’t seem like a lot, but you do notice it when you’re dealing in low temp / low output radiators.  The missing watts become a large percentage of the overall radiator output.

So my advice is really try and match the heat loss of each room to the output of the radiators as closely as you can.

You can’t magic missing output without upping the flow temperature of the whole system.

Oversizing a radiator in a room isn’t as much of a problem as under sizing, as you can turn the lock shield down on a radiator to reduce output.  Or use a TRV to limit the output.

The ultimate aim seems to be to have just the right amount of heat output capability in each room to match the heat loss, which makes for a level and comfortable house.

Obviously this is very tricky where there are rooms with high solar gain, ie south facing windows.  In this case, TRV is the best solution to close down radiators when the temperature goes above target.

So to try to remedy the heat output shortfall we plan to upgrade the radiators in those two rooms before next winter to more closely match heat output and heat losses.

We’ve added a new small 600 x 600 K2 radiator in the Kitchen / Dining Room and we’ve replaced K1 in Bedroom 2 with a higher output K2.

This is what the system will look like going into the winter.

The main benefit of doing this is to give us that room balance we are missing.

This should mean we can hopefully drop the weather comp curve down at tad, lowering the overall house flow temp, which will improve performance/cop and comfort.

How did the heat pump cope when it was -3C outside?

You obviously hear the horror stories that heat pumps can’t work in cold conditions (go tell them that in Sweden).  But no such worries here in Sheffield.

Here’s the monitoring graph from the day with the coldest average temperature across the winter, Sat 21st January.

And here the lowest temperature day I recorded, 11th March 2023, when it dropped to -4C.

Yes, I appreciate it was colder than this in many parts of the country last winter.

And a lovely snowy photo from the 10th of March.

First winter with my air source heat pump

Defrost Cycles

When the temperature drops below around 4C outside the evaporator on the outside heat pump unit can start to freeze up due to the decompression process of the refrigerant.

When the heat pump senses things are frosting up it puts the whole system in reverse, pulling warm water from the heating side, out of your radiators/UFH and into the outdoor unit to melt the frost.

When this happens you may see a plume of steam appear from the outdoor unit.

You can see from this snapshot that the unit went into defrost around every 90 minutes here.  But the timing and regularity depends greater on the outside temperature, humidity and how hard the heat pump is working.

As you can see from this zoomed in view, the defrost cycle lasted around 7 minutes.  Once completed the unit just went back to normal running by increasing the flow temperature to start heating the house again.

How did the Plant Room fare?

As you may remember, I took great pride in the amount of insulation and taping that we put into the plant room where my water cylinder is stored.

You can read all about the creation of that in its own dedicated blog post.

Plant Room Project


Here is a chart showing outdoor temperatures along with the corresponding temperatures inside the plant room during the very cold first half of March 2023.

As you can see, despite it being sub zero outside for long periods, the inside of the plant room never dropped below 17C.

So I’m pretty happy with how the whole mini plant room project is playing out.

Running costs of the heat pump versus gas

From Jan 2023 to Apr 2023 these were the gas and electricity prices from the government energy price cap.

  • Gas: 10.33p per unit
  • Electricity: 34.04p per unit

Source: Martin Lewis website (

Looking back, the November and December rates were very similar too, so we can also use those for these calculations.

So the ratio difference there is 34.04 (elec) / 10.33 (gas) = 3.29

Over the whole of the 6 months monitored (1st Nov 2022 to 30th April 2023) we have the following numbers for the heat pump.

  • Electricity in: 2418kWh
  • Heat Out: 8709kWh

8709kWh / 2418kWh = 3.6 (which is our overall COP for both heating and hot water)

Electricity cost of running the heat pump: 2418kWh x 34.04p per unit = £823.08

Equivalent gas cost: (8709kWh x 1.15) = 10015 x 10.33p per unit = £1034.54

Note: we use x 1.15 multiplier to account for 85% efficiency of the gas boiler.  I.e. We would need 10015kWh units of gas to produce 8709kWh of heat on an 85% efficient boiler (to match the 8709kWh of heat produced by the heat pump).

Obviously, as we head into Winter 2023, prices are predicted to be 8p for gas and 33p for electricity.

Based on those prices, the above calculations would be £797 for electricity and £801 for gas.

The government, policy makers or whoever controls the price differential really needs to address this disparity in gas and electricity pricing if they want to promote the electrification of heat.

Bottom line, money talks and the general public will only come over to heat pumps if it works out cheaper.

I’ve watched followers of Energy Stats enough over the years now to see them hop from one energy tariff to the next.  And 9 times out of 10 they move tariffs because it is cheaper.

You would hope that as more renewables come onto the grid over the coming years the price discrepancy would eventually swing in the favour of electricity to make heat pumps a no brainer.

How did the Solar, Batteries and Time of Use tariffs help?

As detailed at the start of the article, I’ve invested heavily in renewable technology for our house.  An early adopter you might say.

This investment has allowed us to generate lots of our own energy via the solar panels.  I can’t say ‘free’ energy as the panels and installation was an upfront cost.

Using the EV and the home batteries also allows us to make use of time of use tariffs. 

In our case, Octopus Go Faster where there was a cheaper 5 hour period overnight.

  • Off Peak (01:30 to 06:30): 8.25p/kWh
  • Peak (06:30 to 01:30): 38.82p/kWh

You can find the latest Go tariff pricing here:

Octopus Go Yorkshire

The Go time of use tariff allows us to charge the batteries, charge the car and do our main hot water runs via the heat pump during the cheaper overnight period.

Shifting all these heavy loads to the cheaper (and greener) overnight period really helps us drive our average unit rate down each month.

Here’s a breakdown of the monthly average unit costs this winter.

  • Nov 2022: 16.12p per kWh
  • Dec 2022: 17.18p per kWh
  • Jan 2023: 14.58p per kWh
  • Feb 2023: 12.63p per kWh
  • Mar 2023: 12.46p per kWh
  • Apr 2023: 10.60p per kWh

Average Unit Price (Nov to Apr): 14.70p per kWh

Here is a snapshot from the Carbon Coop PowerShaper smart meter tool that shows the average utilisation across all 30 minute periods through the day across all 6 months of November to April.

And below is a small snippet of a report from the excellent Octopus Watch app by Smarthound.

This shows that over 91% of my electricity bought in those 6 months was off peak.

Heat Pump running costs using Solar, Batteries and Time of Use tariffs

So using the average unit price of 14.7p that we were able to get from the solar, batteries and time of use tariff we can look at the cost of running the heat pump like this.

  • 2418kWh x 14.70p = £355

Compare that to the pricing to run the heat pump from above:

  • Electric at 34p: £823
  • Gas at 10p: £1034
  • Electric at 14.7p: £355

£1034 versus £823 versus £355.

So if you apply the 3.6 COP to that time of use tariff inspired unit price, it’s 14.70p / 3.6 = 4.08p per unit of heat.

Compared to gas price calculation: 10.33p x 1.15 = 11.87p per unit of heat on an 85% efficient combi boiler.

Solar generation per month

  • Nov 2022: 101 kWh
  • Dec 2022: 93 kWh
  • Jan 2023: 144 kWh
  • Feb 2023: 189 kWh
  • Mar 2023: 279 kWh
  • Apr 2023: 439 kWh

Total Solar generated (Nov to Apr): 1245kWh

So my solar generated 33% of what my heat pump needing across the 6 months (806 / 2418) x 100 = 51.5%.  Granted, some of this is skewed towards the brighter months of March and April.

Return on Investment (ROI)

Over the 6 months we are looking at here, the investment in renewables allowed me to save £468 on heat pump running costs by getting a lower unit price of imported electricity.

You could also say that the 1245kWh of juice the solar generated saved me from importing another 1245 kWh of electricity at 34p across the 6 months.

1245 kWh x 34p = £423.30

Our total electricity imports across the whole 6 months was 5505 kWh.

That’s the heat pump, whole house usage and the EV…. basically everything except a remaining gas cooker.  Yes, that’s on the list to get electrified.

5505 kWh x 34p = £1871 (if all imported electricity bought at 34p per unt) – 

5505 kWh x 14.7p = £809 (cost the electricity actually cost us at 14p per unit)

£1062 saved?  Plus the £423 saved by the solar?  Almost £1500 in 6 months?

People will always want to know the ROI (return on their investment).

I’m giving you a 6 month snapshot here, real world facts and figures.

I’ve had my solar for 5 years, batteries for 3, EV for 2 years and heat pump for 6 months.

With each element providing more and more value as time goes on.

In the 4 full years we’ve had the solar it has generated over 17,000 kWh of electricity for us.

You can see a monthly breakdown of our solar figures going back to 2019 here:

My Solar Generation History

You could break each year down to the following unit prices (as electricity hasn’t always been 34p as it is now)

  • 2019: 4250 x 15p =  £637
  • 2020: 4250 x 15p = £637
  • 2021: 4250 x 25p = £1062
  • 2022: 4250 x 34p = £1445

These figures are from looking back at my bills across that period.

The 4 years above show over £3700 in electricity generated.  If electricity prices stay as high as the past two years, my solar install will have paid for itself in a total of just over 5 years.

  • The EV has slashed our running costs versus petrol
  • The Home Batteries have allowed us to use almost all our generated solar and make best use of cheaper/greener overnight electricity.
  • The Zappi EV charger and Eddi PV diverter also make sure we use the PV we generate onsite when the home batteries are full.

You may not want, or can use all of what you generate onsite, maybe you could make more of your investment selling back to the grid using one of Octopus Energy export/outgoing tariffs?

Octopus Outgoing Yorkshire

I’ve said this before, but you can call the combination of Heat Pump, Solar and Batteries “The Holy Trinity” of renewable tech.

I also think that adding an EV and then Time of Use tariffs bring a massive benefit to proceedings too as I proved.

But I’m yet to come with a quite a snappy quintet / pentagon based version of the Holy Trinity. 🙂

Closing Thoughts

ROI is tricky and potentially argumentative.

No one asks what the ROI is on a gas combi boiler?

It provides a service in the house, and keeps you warm.

In the same way you don’t ask what the ROI on a fridge or a washing machine is. 

So I’m not sure why we are levelling the same ROI questions at heat pumps?

I see my heating system upgrade as an investment into the property and its infrastructure.  I’ve created a low carbon home.  Hopefully something that is desirable to house buyers in the future?

Heat pumps are not a get rich quick scheme.  No investment in renewables is, you are in it for the long haul.

I’m doing it because I think it’s the right thing to do.

I think it’s the right thing for my family and the message I want to send to my kids. “Daddy, what did you do?”

I also want to reduce our gas usage and try to do our bit for the planet.

I also enjoy the way I’m gamifying our home energy use.  Seeing how I can get the very best out of every element.  You can’t keep an old skool 80s gamer down I suppose.

Can you put an ROI figure on “doing the right thing if you’re able to”?

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