ASHP and Solar PV, are they worth it? - my experience and data

[ghaynes]

Reposting without any links since that seems to get caught in the spam bot filter

The recent energy crisis and conflict in Ukraine has a lot of us worried about living costs especially energy consumption at home, which is often one of the highest regular expenses for many. Rates are shooting up and in my opinion will never return to what they previously were unless there are some drastic changes in the UK government's energy policy.

There have been many questions on /r/ukpersonalfinance and other UK subreddits about whether it is worth getting an air source heat pump (ASHP) or solar PV as a means of reducing energy bills. We moved into our new home last summer which has both an ASHP and a solar PV array, so whilst I can't share costs for retrofitting onto an existing property and return-on-investment times, I can hopefully provide data on how these systems perform in a suitably insulated property. It's going to be a long post with lots of numbers, but I'll summarise as best as I can at the end.

Those that aren't familiar with ASHP should check out the Youtube channel Technology Connections. He did a number of detailed videos about ASHP within a US context but it is the same technology and applications in the UK.

Key facts about our home (most taken from the EPC):

• 4 bedroom detached with a basement - 176 sq metres
• Air source heat pump (ASHP) - NIBE F2040-8
• Underfloor heating (UFH) - Heatmiser central controller, room thermostats and manifold / valves. Rooms are invidually zoned so I can control heating in each room independently.
• Hot taps are fed by a large hot water cylinder heated by the ASHP system
• Solar PV 3.2kWh max - 10 x 320Wh panels on South facing roof in Derbyshire
• Mechanical Heat Recovery ventilation + ducting and vents to all rooms - Franksiche profi-air 250
• EPC A (92 score)
• Wall - Average thermal transmittance 0.17 W/m²K
• Roof - Average thermal transmittance 0.14 W/m²K
• Floor - Average thermal transmittance 0.21 W/m²K
• Air tightness - Air permeability 1.8 m³/h.m² (as tested)
• Triple glazing on all windows

I've unfortunately only got data for the last 6 or so months since that's when I got all the monitoring set up for the smart meter and the solar PV, but it covers the coldest months so is indicative of the max usage. This was our first winter in this house and I was still getting familiar with the optimal settings for the ASHP and heating schedules so we probably used more energy early on in the winter than we would do in the future. I have a Google Sheet set up to read from the Octopus API and my solar API which gives me some nice charts and summary metrics which helped me immensely in fine tuning our energy usage patterns to minimise cost. I'm still developing it and aim to publish a template and guide in the future. We do not have an EV so electricity consumption is primarily heating rooms, the water tank and cooking appliances like the induction hob.

Month
average daily temp
kWh consumed
total solar kWh
average daily consumed
average daily solar

Aug 2021
16.1 C
229 kWh
no data
7.2 kWh
no data

Sept 2021
15.9 C
278 kWh
283 kWh (from 5th Sept)
9.2 kWh
10.9 kWh

Oct 2021
12.1 C
386 kWh
203 kWh
12.5 kWh
6.6 kWh

Nov 2021
7.8 C
530 kWh
118 kWh
17.7 kWh
4 kWh

Dec 2021
6.4 C
702 kWh
50 kWh
22.6 kWh
1.6 kWh

Jan 2022
4.6 C
698 kWh
151 kWh
22.9 kWh
4.9 kWh

Feb 2022
5.6 C
472 kWh
169 kWh
16.9 kWh
6.1 kWh

Mar 2022 (to date)
summary not published yet
353 kWh
334 kWh
12.6 kWh
11.7 kWh

Couple of additional notes: we are on Octopus Go smart tariff which charges 5p per kWh between 00:30 and 04:30 and 15.33p per kWh at all other times (25p daily standing charge). It is an electricity only tariff and you need a smart meter set to report readings every 30 minutes. After months of tweaking the heating settings and schedules (to my wife's annoyance), we've settled on a schedule that heats up the downstair areas to 20C during the day and the upstairs areas in the early evening and early morning. The hot water cylinder heats up between 00:30-04:00 to make the most of the cheaper rate and 11:00-14:00 to make use of any output from my solar array. I don't regularly WFH, but the house is pretty much always occupied so we don't really turn off the heating unless we're all on holiday.

Our bills are generated on the 18th of every month. Our highest bill this winter was £100.84 for Dec to Jan, 2nd highest was Nov to Dec at £92.52. Jan to Feb was lower at £79.97 mostly because we were away on holiday for 5 days so everything was set to standby state, otherwise it woud probably have been roughly on par with the previous billing period.



Is ASHP worth it?

I'm going to give the cop-out answer and say it depends. ASHP as a technology is perfectly suited to our climate (not super cold winters) and even mid-range ASHPs can maintain relatviely high efficiencies or coefficient of performance (the ratio between the power kW that is drawn out of the heat pump as cooling or heat, and the power kW that is supplied to the compressor). My particular model has a COP of 2.68 @-7C, 3.76 @2C and 4.65@7C which are temperature ranges well within the winter temps we experience in the UK.

What makes ASHP less viable in the UK is the cost of electricity and the state of insulation in many UK properties. If your home leaks heat like a sieve, then no matter what heating technology you put in, you're going to be consuming more energy to maintain the internal temperature. Invest in your home's insulation first Our energy consumption / bills would not be so low if we didn't have very good insulation throughout the house, that includes doors and windows. If you want the best ROI, it's in improving insulation first.

As for the high cost of electricity, rates in the UK are expensive and one of the highest in Europe so relying on electricity to heat your home will be more expensive per kWh compared to natural gas for the foreseeable future assuming no changes in the UK's energy policy. The great thing about electricity is that it can be generated by multiple energy sources which include renewables and can include your own solar PV array (more on that later). The optimist in me hopes that recent events encourages greater investment in renewable energies so our energy prices are less reliant on global commodity prices and hopefully help reduce domestive supply rates. In 2019, 40.7% of electricity generation in the UK was from gas.

If your goal is to reduce your household energy consumption then, ASHP is worth it IF you also implement the other necessary improvements to make it as cost effective as possible to run i.e. improved insulation, air tightness etc. At our current rate of consumption we would probably use between 8000-9000 kWh a year which is almost half what Ofgen suggests the average UK household uses in gas and electricty per year (12000 kWh gas + 2900 kWh electricity).



Do I need underfloor heating with an ASHP?

ASHP are designed to heat the water in a wet central heating system to lower temps as a means of improving efficiency / COP. Underfloor heating is the ideal solution for delivering this lower heated water around a home BUT that does not mean you can't have the ASHP deliver higher temp water to standard radiators. My particular model of ASHP runs most efficiently delivering 35C but can also be configured to deliver upto 45C to feed radiators with only a small reduction in COP. Gas boilers typically deliver 60-65C water so an ASHP will obviously take longer to achieve desired temperatures which is why it is so important to make sure you have excellent insulation in place first. Less heat loss means less time to reheat. You may benefit from larger radiators to deliver more BTUs so you can minimise conversion costs, but first make sure you have sufficient insulation.



Is Solar PV worth it?

On its own, I don't think so. I'm sure it made a difference in reducing our overall energy consumption, but we all know that sunny days in the UK are few and far between and more importantly much less frequent in the winter months when a household like mine uses significantly more electricity for space heating. My data table shows how much daily solar production can drop in these months and maybe produce just enough to cover 20% of the demand. The other problem with solar PV by itself is that you obviously only get energy production during daylight hours which again in the winter are significantly shorter. Most of my household's usage occurs in the early morning and early evening, so we don't get the benefit of solar power when demand is highest.

Is solar PV worth it with a battery? In my opinion, yes and in this case I can provide some numbers for ROI on a battery installation since we do not currently have a battery, but have one on order (8.2kWh from GivEnergy). The calculations are pretty simple:

• The battery, inverter and installation cost £5000 and comes with a 10 year warranty.
• I'm going to ignore solar production for the purpose of this calculation to keep it simple.
• My current Octopus Go rate will expire in August 2022, so I've calculated return based on the current Go rate which is 7.5p off and 34.43p on (!!!)
• I would have the battery charge during the off period which would cost £0.61 for 8.2kWh.
• That 8.2kWh is now available to be used during the on-peak hours when the rate is 34.43p per kWh, which equates to 34.34p x 8.2kWh = £2.54
• The difference between the two prices £1.93 is how much I save per day if I use at least 8.2kWh during on-peak time, which we do on average from Sept to Mar (7 months).
• Approximate winter savings per year is 1.93 x 210 = £405 per year
• I do not have any summer data for our consumption so as a rough estimate I've assumed that we would normally use 4kWh per day on-peak = £1.24 which would cost £0.3 to charge off-peak.
• The summer savings are therefore the difference = £0.94 for Apr to Aug (5 months)
• Approximate summer savings per year is 0.94 x 155 = £145 per year
• Total annual savings = £550
• ROI on battery = £5000 / £550 = 9 years

If I include my solar PV as a variable, then the savings in the summer and part of the winter could be greater since there may not be a need to charge the battery from the grid at all

I'm also betting on the fact that the electricity rates will continue to rise for some years so the savings will increase year on year



Summary

This took a while to type up and format correctly so I hope it was helpful to some people. Unfortunately there is no "magic-bullet" solution. Our old home was old, had poor insulation and would have cost 10's of thousands to get to a modestly energy efficient state. We sacrificed internal and external space when we sold it in 2019 and bought this new house because we were so worried about our energy bills which were already ridiculous at the time (£200 a month in the winter for gas and electricity). It made more sense to sell the house for a more efficient smaller house than it was to deal with cost and disruption to significantly modernise it. Whether ASHP / solar PV / battery makes sense for you depends on your property, your energy usage and ultimately what you can afford. If I had to give a summary it would be:

1. Get a smart meter (SMET2) - this will give you access to smart tariffs that I predict will be the more cost effective option for many people. It has also been amazing to be able to see our energy usage in 30 minute increments which really helped me understand how and when we use energy. I would not have been able to optimise our usage (and be tweaking it further once we get the battery) if I didn't have that granularity of data.
2. Review your energy usage regulary - are you consuming more than you would expect? Are there periods in the day when no-one is home and you're still consuming a significant amount of energy?
3. Understand your home's heating systems - learn how to control them, set timers, put them into standby mode etc. Google the controller model and 90% of the time you'll find a digital copy of the manual. Read it. Consider smart heating controls if your current ones are too basic. Simply being able to have setpoint temps and schedules will help you reduce unnecessary space heating.
4. Assess how good your insulation is (including doors and windows) and what options you have for improving it - the biggest ROI will be preventing heat from escaping your home.
5. ASHP - great technology, perfect for the UK's climate. Should be mandatory for all new-builds (as well as very high spec insulation and MHRV). Can be retrofitted, but deal with the basics first i.e. insulation and ventilation.
6. Solar PV - (my opinion) best paired with a battery for high electricity use households i.e. homes with electric heating or EV owners. Get the smart meter and analyse your electricity usages before investing.

 

[topolski18]

@ghaynes TL/DR: heat is expensive but heat pumps can be big. Low temperature is more efficient but achievable using using big, powerful fan-coils rather than rads/UFH. Insulate if it's easy but don't let that put you off heat pumps. A single box that replaces every boiler isn't possible but this doesn't matter. Cooling is cool.

first make sure you have sufficient insulation

This is certainly the conventional wisdom, but in many cases it is equivalent to 'don't ever do this'. The majority of houses in the UK will never reach your home's standard of insulation - demolition and rebuilding would be the only way to do this, and the cost and carbon emissions associated with this would make it a terrible option. Luckily it's not required.

We need to separate the three problems here:

• Energy is expensive, we want to use less heat therefore we need more insulation. But note: a well-designed ASHP will cost about the same as gas per unit of energy. Insulation does not change the cost of one kWh of heat, it just means you use less. If there are easy wins like loft insulation and cavity wall insulation (where suitable), installing these is a no-brainer because it's cheap and pays off quickly. But if those aren't possible, you can still use a heat pump. It just needs some different design choices to allow efficient operation.
• Heat pumps are more efficient with low-temperature emitters. One way to achieve this is to leave the radiators and improve insulation. Another is to use larger radiators or other emitters.
• Heat pumps are expensive, particularly because of installation costs. Variable costs can be very low but the initial investment is large, and this can be reduced somewhat by reducing the heating demand (but a 10kW heat pump does not cost twice as much as a 5kW one).

From the perspective of carbon emissions and gas usage, the best thing you can do is get a heat pump right now, even if it means getting a large unit and emitters. We use less gas by burning it in a CCGT and powering heat pumps than burning gas in buildings. The same would be true for hydrogen. And of course we have wind, nuclear, solar etc. as well, allowing 70-80% reductions in CO2 right now and 90%+ within a decade, all going well.

Regarding cost, until recently it was almost always cheaper to burn mains gas in your home, except for a handful of very efficient heat pump installations. This has changed a bit as gas has got more expensive more quickly than electricity (just about) and heat pumps have got more efficient but the system is still biased against using electricity. This is a government policy that can be changed.

Most heat pump based systems in the UK have been designed around new builds and other very well insulated houses so UFH and normal sized radiators have worked fine. This approach won't work for the UK's large stock of less efficient houses. But there are good solutions that allow efficient operation without e.g. solid wall insulation, retrofitted UFH etc. Mini-split air-to-air heat pumps can easily provide enough heat per internal unit to keep even a very large, poorly insulated room warm on the coldest day. These are already very common across Europe, Japan and the US. And you get cooling as well. A very efficient Daikin 10kW external unit with 5 internal units can be had for £5k; much cheaper options exist. It's a job for an air conditioning installer rather than a plumber which is perhaps why the plumbing-industrial complex hasn't been promoting these so much! Hot water is a separate problem that generally uses much less heat than space heating and is more about space inside; an immersion heater might be fine for most, or keep a combi boiler for a while, or a heat pump water heater, or whatever.

Saying that, most houses with cavity wall insulation, loft insulation and double glazing will be fine with a standard air to water heat pump (probably bigger than OP's) and some new convector radiators, probably leaving the existing pipes in place. A bit of a faff, but a £5k grant will be enough to make this comparable in cost to a gas boiler system once the installer base has scaled up. Plus more cost if you need a new water tank, but that was true in going from grotty boxes in the loft to modern tanks as well.

 

[rom1974]

@topolski18 I really like either the mini split or high temperature hydronic options. The latter also because it can be a drop in replacement and most boiler replacements are distress purchases so need to work with existing emitters and building fabric. Can always turn the temp down later after next natural renovation point improves building fabric and puts in UFH.

 

[topolski18]

@rom1974 Yeah, some of the hydronic heat pumps have got to a point where a SCOP of 3 is possible even with a peak flow temperature over 50C, which is just about doable with most existing central heating systems.

Still, mini splits are unreasonably effective and super quick to install - I'm always surprised they're not more common. My parents had a 2kW unit installed about a decade ago and it's basically completely replaced central heating and a wood burner in the rooms they spend most time in (about 40m[sup]2[/sup] of poorly insulated, open-plan 1960s house with big, old windows). It would be a struggle to heat that space efficiently with reasonably sized radiators but the mini-split just sits on one wall taking up no floor space. Much cheaper than LPG, less work and cleaner than burning wood, and the benefit of having an air conditioned space during heatwaves is hard to understand until you've experienced it. They also have PV so the cooling is effectively free.

 

[rom1974]

@topolski18 Would be curious to see what the average water temp is for SCOP of 3. Must be much lower than 50 but the U.K. has long shoulder seasons where with weather compensation you can use traditional rads at lower temps and get good results. Most spaces are massively over-radiatored anyway so the lower temp still works pretty well and with dynamic controls every future fabric upgrade gets you automatic SCOP improvement.

PV starts to get way better ROI once you have cooling loads since the U.K. is basically never overcast and hot at the same time so your self consumption goes way up.

 

[topolski18]

@rom1974 Yes, the very long shoulder season in the UK is what enables these high SCOPs.

I'm sure there's an 'official' method, but for a quick look I used PVGIS to generate a typical meteorological year's worth of hourly temperatures for somewhere in the middle of England. Ranked those and assumed a base temperature of 14.2°C to calculate heating degree hours based on (this paper). Somewhat crudely assumed radiators with DT50 output of 9.5 12.8kW, peak heating demand of 6kW, constant 20°C indoor temp and a 5°C temperature drop over the radiators when using a heat pump, which nicely works out to a peak flow temperature of 50°C. The ranking of flow temp ends up looking like this this. In this case, the 50th percentile heating hour has a flow temperature of 29 35°C, an outdoor temperature of 8.3°C and heat demand a bit over 2kW.

Lots of complications could be added like setbacks, cycling, thermal inertia, more realistic indoor temperature variation, higher temperature runs for hot water etc. But I think the basic message is that most of the time the UK is a very mild place so even a heat pump that looks like it's going to have to work very hard will be quite efficient as long as it's capable of modulating down enough in power and temperature.

 

[rom1974]

@topolski18 Interesting, thanks. How did you adjust for the power law relationship between radiator/room temperature delta and peak heat rate? I guess that as long as the room can be kept at target temp with 50C flow it doesn’t matter?

This is the problem I tend to run in with my modelling which focuses on peaking behaviour: not being able to keep up with fabric losses on cold days without either very high temps or very large radiating surfaces.

 

[topolski18]

@rom1974 In response to the question of peaks - it is an issue, but I think it's less of an issue than it's made out to be. In practice most people are OK with their houses being a couple of degrees colder on a few days of the year. Maybe that's undesirable, but it would be a change to expect heat pumps to do what current heating systems don't do.

Then there's building heat demand - it's common to size systems assuming the coldest, windiest, least sunny time all happen at the same time when people are at home and not in bed, but that's very rare. Usually when it's very cold the house has warmed up a bit with some sun and doesn't lose quite as much heat to ventilation as assumed in the worst case. Thermal mass averages out temperatures over ~12 hours, people like even lower temperatures overnight, etc.

Then there's the tendency for people to use supplemental heat sources. Perhaps ideally we'd have everyone running heat pumps for everything, but even now there are lots of homes with wood burners, electric heaters in single rooms, etc.

As an engineer I'd rather everyone had a well designed heating system that worked efficiently all the time. But I think in practice it's probably better that 90% people have something that works 95% of the time and we'll figure out what to do with the last bit as and when it becomes a problem. Which it might not. Maybe high temperature heat pumps will become more efficient or mini splits will get more common or the continued trend of people wanting their houses hotter will reverse. Maybe something we haven't thought of, or maybe people will figure out that having a few single panel radiators swapped for doubles isn't actually that disruptive.

 

[topolski18]

@rom1974 I fucked up my power is what I did. Never accept any calculations done on a Friday afternoon.

Let me just redo that one.

I assumed heating power of 350 * (heating degrees) to make the peak demand fit, average radiator temperature of 2.5 degrees below flow temperature and (T_rad - T_room) as 50 x (Q/Q_DT50)[sup]0.75[/sup]

Edit: 35°C feels more sensible anyway. And you'd need to start off with radiators oversized by about a factor of about 2 for peak heat demand at 70C average temperature, which isn't that unrealistic for a house that has been gradually upgraded over the years and heated intermittently.

 

[fellkone]

@ghaynes Very interesting read. Thanks for spending the time to write it and I am sure lots of people will find it of interest.

 

[newcreation312]

@ghaynes Great write up, thank you for doing this!

 

[nishaphilips85]

@ghaynes This is a great write-up, thanks for taking the time.

You clearly seem like someone interested in data and home control. Have you looked into home automation? I built Home Assistant on a Raspberry Pi 4 during lockdown v2.0 and have been upgrading it ever since. r/homeassistant

Because I have all my smart switches/lights etc linked to it, as well as my smart meter, I can see what devices use the most power. (Not to mention the fact that I have automated everything from heating to a sunrise alarm to the plants watering and feeding themselves).

 

[ghaynes]

@nishaphilips85 I definitely started looking at Home Assistant to automate my home and had all sorts of ideas about automated blinds etc but I ended up just setting up Eufy cameras and some Hue lights and motion sensors. All that can be configured in app so I didn't set up a Home Assistant controller. To be honest once I figured out the APIs for my energy supplier and solar array it wasn't difficult to optimise the schedules and setpoints for my heating so we had comfortable internal temps for minimal cost. I probably won't touch them until both my kids are in school full time and my wife can start working again. The house would be empty during the day so we don't need the heating on.

One day I'd love to figure out a way to use the next day weather forecast to figure out whether to charge the battery overnight or use the excess solar. It would also be amazing to adaptively program the battery to charge up to the amount we'd likely use e.g. less in the summer, more in the winter.

 

[apples1500]

@ghaynes About 10 years ago, when I worked in green building design, we did a building in South Africa that used weather forecasts to control a hydronic cooling system.

The system basically relied on the much cooler night temperatures to chill the concrete floor/ceiling slabs based on the next days predicted heat load.

I'm pretty sure it was a custom configuration (I wasnt really involved with the cooling design) but the logic definitely worked.

 

[fudge]

@ghaynes I've looked into Solar and it just doesn't stack up. Really long ROI and that is ignoring the opportunity cost. 10 year time frame in an index tracker puts solar way behind.

Initial cost of solar has to come way down to be viable.

 

[ladyjones]

@fudge Or the cost of electricity has to go up!

 

[rainy214]

@fudge Home insulation once you get past the easy stuff doesn't seem to stack up for the same reasons. Based on the latest tariffs, I am looking at around £800 a year for used Gas (not including the standing charge).

Lets be imaginative and say that super duper insulation can drop that by 50%, i.e. £400 saving, and lets be (hopefully) pessimistic on the Index tracker front and assume 5% return.

So from a purely financial point of view, I need to spend less than £400/5% = £8k on my house fixes for it to be worthwhile. OP's triple glazing alone would blow that budget, let alone doing anything about my 1920s solid walls.

 

[apples1500]

@rainy214 I think this is where something like the Enerphit retrofit plan needs to be the way forward.

Sure the ROI is horrible if you look at it from the perspective of I'm going rip out my perfectly good single/double glazed windows to replace with triple glazed windows or I'm going to tear down perfectly good internal plaster to apply solid wall insulation.

But, what is the ROI on the additional cost of using insulated lime render vs un-insulated lime render to redo the problematic blown cement render that someone added to your solid walls in the 60's, or replacing those rotten single glazed windows with triple glazed rather than double glazed. Or when you rewire and destroy your internal plaster work anyway using insulating plaster or fitting insulation panels (obviously of a breathable type for a solid wall).

Sometimes it still wont make sense, but that has to be the logic that is encouraged.

 

[jaminko]

@apples1500 How can you tell if your lime render is insulated or not? My parents had it done sometime in the last decade but they probably didn't know they could get insulated lime render then.

 

[apples1500]

@jaminko Odds are pretty low that it would be insulated and they didn't know/specify it. It has a different aggregate in it, but I think would probably need analysis/someone experienced working with it to identify it.