30kW PV, All electric heating, is this mad?

[DevonBarn]

Hi, we're planning a large barn conversion and I'd like some neutral advice on the energy system and heating.

 

It's about 400sqm and going through the calcs on aspect, location etc, I think I need somewhere between 30-40kW to cover all energy needed throughout the year if we want to get it to zero ongoing cost.

 

This would mean making much more than we needed, and assumes selling electricity back to eg Octopus for 15p and buying it for 30p when needed.

 

But, the idea of heating a barn in winter with electric radiators.. isn't this mad?! The sums add up but the common sense rings alarm bells.

 

I like the simplicity, low installation costs and controllability of electric radiators, and I don't want to have to have solid flooring for heat pumps to work effectively. 

 

An estimate for a 30kW system here is around £32k. It sounds like an air source heat pump added to this would double the price.

 

It's currently single phase. It may be possible to connect up a nearby 3-phase supply.

 

Advice please! Has this sort of scale PV been done for a single house?

 

Many thanks, 

Dan

 

 

Ref: This is the website I used to calc PV needed for X kWh: https://re.jrc.ec.europa.eu/pvg_tools/en/

[Conor]

Need to know details of the fabric of the build, energy storage (thermal/electric?), insulation, air tightness, energy demand etc to give informed advice. Those bi folds on the south elevation will bake you in the summer (and significant heatloss in winter) - have you considered shading and/or active cooling?

Edited June 22, 2023 by Conor

[AliG]

24 minutes ago, DevonBarn said:

It's about 400sqm and going through the calcs on aspect, location etc, I think I need somewhere between 30-40kW to cover all energy needed throughout the year if we want to get it to zero ongoing cost.

Before even considering this you might want to check if you would get permission for this. I think it is quite unlikely that the DNO would permit you to connect such a large system to the grid. I would have thought that the maximum you can export on a single phase connection is 18ishkW 230V*80A.

 

24 minutes ago, DevonBarn said:

An estimate for a 30kW system here is around £32k. It sounds like an air source heat pump added to this would double the price.

That is expensive for PV and crazy for an ASHP.

 

24 minutes ago, DevonBarn said:

I like the simplicity, low installation costs and controllability of electric radiators, and I don't want to have to have solid flooring for heat pumps to work effectively.

A heat pump can be used to run oversized lower temp radiators.

 

I am assuming no batteries are involved here. Heating requires a lot of energy relative to most battery systems capacity, especially direct heating.

 

The real problem is going to be that in winter you often get 10% or less of your rated output. It can be dark for 16 hours a day, indeed almost all the time if cloudy. Thus the time when you most need heating you could have zero output, depending on insulation levels you may well need the heating on during the night. You are trying to adjust for this with 30kW of capacity, but in reality the system will be too large most of the time and still not necessarily available when you need it.

 

[DevonBarn]

5 minutes ago, Conor said:

Need to know details of the fabric of the build, energy storage (thermal/electric?), insulation, air tightness, energy demand etc to give informed advice. Those bi folds on the south elevation will bake you in the summer (and significant heatloss in winter) - have you considered shading and/or active cooling?

Hi Conor, it's not so much whether a 30kW system would have the capacity (although that is a valid second question), but whether relying on buying in elec for winter heating is OTT?

 

The bi folds should be ok in winter (we've got smaller south facing triple glazed where we are currently), but you're right in the summer, gets pretty toasty so we are thinking of shading solutions.

 

For ref:

- details of the fabric of the build, - really well insulated cavity, pre-fab tata insulated roof panels, all new windows and doors so draughts shouldn't be an issue.

 

- energy storage - electric / immersion

 

- energy demand - it's a large 5 bed house, ignoring car charging at this stage.

[DevonBarn]

2 minutes ago, AliG said:

Before even considering this you might want to check if you would get permission for this. I think it is quite unlikely that the DNO would permit you to connect such a large system to the grid. I would have thought that the maximum you can export on a single phase connection is 18ishkW 230V*80A...

 

 

Thanks Ali, yes we aren't sure we'd get permission esp on single phase. I read that paying for connecting 3 phase is possible, maybe around the £8k mark but all dependent on site?

 

Thanks re: quotes, the £32k included 2 inverters and batteries but nothing that would handle heating. We'd just be buying in elec in the winter, which would be covered cost wise by selling 2x the elec in the summer.

 

The initial quote for heat pump was for ground source but yes I'll be getting more quotes for sure!

[saveasteading]

29 minutes ago, DevonBarn said:

I like the simplicity, low installation costs and controllability of electric radiators,

But you won't like the cost. Ignore all marketing nonsense about it being economical or sustainable......simply not true.

 

Ashp will multiply the electric input into energy output by 4 or 5.   You can distribute the heat by rads or air ducting if you don't want ufh.

[joth]

Do you have a number for peak heating load (in kW) and annual demand (in kWh)?

If these are low enough, then electric resistive heating is just fine. Certainly £60k can massively reduce your heating demand but increasing insulation depths everywhere, and going for a hard airtightness target. And then you might only need a single 2kW radiator working on the coldest days. 

Are you  installing MVHR? This also reduces heating demand significantly (and is mandatory if you're going for a sensible airtightness goal and no trickle vents on windows)

 

[DevonBarn]

16 minutes ago, saveasteading said:

But you won't like the cost. Ignore all marketing nonsense about it being economical or sustainable......simply not true.

 

Ashp will multiply the electric input into energy output by 4 or 5.   You can distribute the heat by rads or air ducting if you don't want ufh.

 

Hi, true they aren't cheap (£250/rad) but cheaper than ASHP, and more importantly they'll definitely work when you really need them.

 

Sustainable if the elec was ""green"", and pretty much 100% efficient (almost any energy loss is lost as.. heat).

[Alan Ambrose]

Here's a graph for our little 100m m^2 medium-insulated barn from when I was figuring PV numbers...consumption is total energy use and we have electrical heating (well backed by a log burner). This is 15 kWp limited by roof area.

 

Generation in winter is about 20% of summer generation.

 

Are you figuring that you want to run your full electrical needs including heating totally from PV in the winter?

 

 

Edited June 22, 2023 by Alan Ambrose

[saveasteading]

1 minute ago, DevonBarn said:

true they aren't cheap (£250/rad

I meant running cost. A very expensive way to heat a space. .

Solar panels not much use on a winters night so mains power all the way.

 

Tell us more. Who has suggested using electric radiators?

[Thorfun]

i think you're crazy to try and install a 30kWp PV system especially as you're not off-grid! i would size more sensibly to allow you to run the house in the summer and charge up your batteries to run overnight and in the winter charge the batteries overnight on off-peak tariff for heating demands during the day. you'd be much better off using the money you were going to spend on the PV array to insulate and airtight to the n'th degree so you simply don't need that much heating! at the moment our 10.5kWp array is generating between 50 - 70kWh per day.

 

Also, i fail to see what's wrong with an ASHP and ufh as has been said above, you get 3-4x the energy out that you put in

[RobLe]

I think elec storage rads on Octopus Go right now work out a similar running cost to gas or an ashp.  However - they are an easy retrofit solution, so my crystal ball tells me there will be a lot more retrofitted, and a lot more elec cars, so the predictable 7.5/30p price ratio will reduce due to this.  Given starting from a clean slate, I would have some sort of wet distribution system, rads or underfloor.  You can use go for now to heat a huge tank somewhere, or have an ashp in future if go vanishes.  This all assumes the elec load can be taken in a 4 hour window.  If you’ve only got single phase that’s quite an ask - 80A gives 20kW*4=80kWh storage, allowing 80kwh/24h =3.3kW peak thermal losses - what do you expect?  
Buying ‘Green elec’ right now I think is like recycling and carbon offsetting - wishful thinking (I do them, but don’t hold out much hope). 

Whatever happens, I think you will benefit from 3 phase.

[DevonBarn]

14 minutes ago, joth said:

Do you have a number for peak heating load (in kW) and annual demand (in kWh)?

If these are low enough, then electric resistive heating is just fine. Certainly £60k can massively reduce your heating demand but increasing insulation depths everywhere, and going for a hard airtightness target. And then you might only need a single 2kW radiator working on the coldest days. 

Are you  installing MVHR? This also reduces heating demand significantly (and is mandatory if you're going for a sensible airtightness goal and no trickle vents on windows)

 

Hi,

 

I used the energy use for our current house (mostly solid wall old cottage with single storey extension).. Surely it would be better than this in a new build!

 

Peak heating load: I've only got monthly kWh figures but worst is Dec: 3500 kWh for the month.

For ref: According to the site above I'd only get about 500 kWh from a 30kW system in Dec.

 

Annual demand: 23,500kWh / year. It's painful even writing it!

 

Honestly hadn't considered MVHR, thanks I'll look into it.

[saveasteading]

Our conversion is 400m2. Single phase appears to be plenty. We had an expensive option to convert to  3 phase but after study and advice (thanks @ProDavestuck with what we have.

 

It will cope comfortably without the aid  of solar, which we may integrate later.

 

I have seen some of the marketing for electric rads.....it was lying basically.  I had a business connection who was passionate about sustainability but she didnt understand the basic energy in = energy out principle. Talked her out of it eventually.    I think the misleading bits were, the abuse of the term 'efficiency'  and the constant references to aluminium, controllability, and ' German' which were all true, but not the main point.

[DevonBarn]

@Alan Ambrose Thanks for the graph Alan, no realistically we'll be buying in most of our electricity for heating in the winter (eg December, demand 3500 kWh, PV output 500 kWh). 

But as it would be such a large system, we'd be selling enough electricity back to the grid in the summer to pay for the winter use, at the prices paid / charged by Octopus.

 

@saveasteading Yes that's it, expensive compared to eg Gas, but covered by the income from the summer months.

 

@Thorfun Hi, yes I think it sounds crazy too, but why?! And yes we would be using off peak in the winter to charge batteries but they're a drop in the ocean compared to heating demand. Re: ASHP - it's fair to say they have mixed reviews, especially when you need them most when it's freezing. But we haven't ruled them out!

 

And yes we will be insulating/draft proofing to n'th degree as well, so I'm hoping the annual demand is way below the current estimate. 

[DevonBarn]

3 minutes ago, saveasteading said:

Our conversion is 400m2. Single phase appears to be plenty. We had an expensive option to convert to  3 phase but after study and advice (thanks @ProDavestuck with what we have.

 

It will cope comfortably without the aid  of solar, which we may integrate later.

 

I have seen some of the marketing for electric rads.....it was lying basically.  I had a business connection who was passionate about sustainability but she didnt understand the basic energy in = energy out principle. Talked her out of it eventually.    I think the misleading bits were, the abuse of the term 'efficiency'  and the constant references to aluminium, controllability, and ' German' which were all true, but not the main point.

That's really interesting, when you say 'cope' how are you heating yours?

[DougMLancs]

7 minutes ago, DevonBarn said:

@Alan Ambrose Re: ASHP - it's fair to say they have mixed reviews, especially when you need them most when it's freezing. But we haven't ruled them out!

The three countries with the highest number of heat pumps per head of population are Norway, Finland and Sweden- as a ex-Devon lad myself, I don’t think it gets as cold as them. I've seen several stone barns in Scotland running great on an ASHP

Edited June 22, 2023 by DougMLancs

[Iceverge]

Welcome @DevonBarn.

 

Plenty of barn conversations here to take inspiration from. 

 

@IanR has a really good example of what's possible in terms of airtighess, comfort insulation etc if you search his posts. 

 

3 minutes ago, DevonBarn said:

And yes we will be insulating/draft proofing to n'th degree as well, so I'm hoping the annual demand is way below the current estimate. 

 

 

Have you put some numbers on this? 

 

I would suggest 0.15W/m2K as a beginning for U values and 1ACH for airtightness. 

 

@Marvin has it summed up with his AIM APE priority list. 

[saveasteading]

1 minute ago, DougMLancs said:

ASHP - it's fair to say they have mixed reviews, 

I don't agree. Perhaps it depends where you ask.

The vast majority of reviews are favourable, not that the electric rad industry would agree publicly.

[joe90]

As an aside and someone who successfully heated my new build with a 4kW ASHP with MVHR, regarding the comment above about your bifolds overheating you, I was told this with my huge south facing conservatory and bifolds into the kitchen and lounge, yes it was quite warm for a couple of weeks (similar temps to a holiday in Italy 🤷‍♂️) but I have to say for the rest of the year it mostly heated the house!!!!, I had a rule, if the temp in the conservatory was below 19’ the bifolds into the house were kept shut, if over but below 22’ they were open, if the house was getting too warm they were closed. I think this saved a great deal in heating costs.

[joe90]

8 minutes ago, saveasteading said:

ASHP - it's fair to say they have mixed reviews, 

No @DougMLancs said this, however I think you will find this forum,s consensus is that only badly installed/set up ASHP,s don’t work well.

[DevonBarn]

Thanks @joe90, no we'd definitely do with a bit of overhang for the bi folds next time, we might get one of those sails. They are great for heat generally, but there were a few days last summer where we had curtains draped over the outside of the bi folds! Also noted re: ASHP, I need to get a few more quotes in.

 

Hi @Iceverge that's really useful thanks, I'll plug that in to the spreadsheet, should get me a better estimate. Also do you know how to find @Marvin's AIM APE? Also what's AIM? And what's APE?!😅

[Thorfun]

44 minutes ago, DevonBarn said:

Hi,

 

I used the energy use for our current house (mostly solid wall old cottage with single storey extension).. Surely it would be better than this in a new build!

 

Peak heating load: I've only got monthly kWh figures but worst is Dec: 3500 kWh for the month.

For ref: According to the site above I'd only get about 500 kWh from a 30kW system in Dec.

 

Annual demand: 23,500kWh / year. It's painful even writing it!

 

Honestly hadn't considered MVHR, thanks I'll look into it.

rather than guesstimating use the heat loss spreadsheet that @Jeremy Harris wrote. you should find it here

 

that should give you a much more accurate heating demand. it will also give an indication as to how adding extra insulation/airtightness detailing will change the heat requirements.

[Conor]

Take a step back, look at what you are actually trying to achieve, see what building fabric you need to use to achieve this.

 

I've been playing round with our energy consumption and generation in a spreadsheet since reading this thread. Our 4.5pkW (limited to 3.8kW) system generated 4.5MWh from april-april, and our imported electric from the grid was 4.4MWh for the same period. In reality, we had a massive excess in the summer, and defecit in the winter. We're still better off by £1k a year with the array. Payback 5 years. Do a similar exercise yourself and play around with different scenarios and see what makes sense for you. 

 

In terms of heatpump Vs direct electric for us... Quickly calculated our heating season consumption, and it's about 21kWh per day, so about £400 for the heating season. If we had gone with direct electric, that would be more like £1000. Factor in the capital costs of the unit Vs a direct electric setup (no UFH, storage rads, direct heated water) and payback is about 6.5 years. That's with a pessimistic COP of 2.5.

Edited June 22, 2023 by Conor

[SteamyTea]

If you say you use 40 kWh/m².year for space heating, that is 16000 kWh.

60% of that will be December and January, so call that 10000 kWh (7W/m² , I use around 8 W/m², larger places use disproportionately less, but my house is terraced) when the PV is producing the littlest. How does PVGIS line up with that, especially when you take annual variance into account?

An A2AHP may halve that in December and January, but water usage may become your biggest problem.

To get what you are after, you need to model it as an off grid house, then see what you need to import, and when. You may find that having a relatively small amount of battery storage is helpful, but you will need a generator as well.

 

Which bit of hell are you in, on the coast or up the moor, temperatures are quite different, as are wind speeds. But not as wet as here, we take that away from you.

Edited June 22, 2023 by SteamyTea