PV Sizing - Self consumption or oversize to export & is it financially worth it? Off grid or grid tied, oh i really dont know now!

[MechanicalBuilder]

Good Morning everybody,

 

I found this forum whilst researching power options for our self build dormer bungalow.

Originally our intention was to be partially off grid and self-install solar as we have previously self installed a 10kw off-grid system powering a workshop and static caravan for my parents.

This recently took a turn for the worse when the surveyor discovered we were going off-grid causing the lender after 6 months of knowing about being off-grid, deciding they would not mortgage our property without mains electricity. This is coming at a massive cost of £32000 due to the location of the property.

I was going to try and blag that we will have mains installed later but its looking like the surveyor is going to ask for certification or the MPAN number (possibly?????)

Its a shame as we could have saved a fortune self-installing our own system.

 

Anyway, the mortgage is now on its way and with the slab being poured this week, progress is being made!

We have pretty much everything in place now with just one bug hurdle to make decisions on.

The power.

 

The bungalow is being heated via 2 ashp. One 8kw ashp for the 300litre hot water tank and UFH with a smaller 5kw utilising 3x air to air units in the bedrooms. 

I'm fortunate that my best mate is a heat pump engineer so his boss has really done me a great deal sourcing and installing my system. 

He has advised that a worse case scenario the combined consumption will be 7500 kWh per annum.

Going off of this figure and what we consume in our current property I can only estimate we would possibly be looking at consuming 10000kWh per annum.

 

 

With the solar...... The roof is south facing with a 40' pitch and no obstruction whatsoever.

 

So we have had a few quotes now and the best of the bunch has proposed 2 options (this is subject to DNO)

 

1.

 

10k array, 8kw inverter and 19.4kwh battery storage.  £20,000.

 

2. This option being the 'best case scenario' we could possibly fit on our property

 

28kw array, 8kw inverter and 38.8 battery storage. £31,500.

 

Now, as you can see this has now gone from a cost of £10,000 ish if we self installed to a whopping 50-60k. 

 

I'm really in 2 minds what is best to do considering the DNO cap, export prices and what financially i will gain back over our lifetimes.

 

--Is the extra cost of going as large as possible really worth it with export prices? we would essentially never draw from the grid and only export but does the 4kw dno cap throttle this?

 

--As the mains isnt in yet we do have the option of going 3 phase thus increasing the amount we could export - at a cost obviously. At the moment the DNO is paying for the transformer upgrade but i think we may have to contribute if we want 3 phase. The only have to provide a suitable connection for free.

 

--Do we go with the optimal sized system and export a little back to the grid and offset our electricity payments which at todays prices are going to be 250ish a month.

 

--Do we have mains bought in but not use it, self install and stay off grid? 

 

This probably all boils down to whether we are ever going to see this 30-60k outlay for mains and solar back.

 

Thanks in advance and sorry if I have left anything out, we are 5 days overdue with our second child, we have a 3 year old and i'm trying to manage this build.

 

I CANT WAIT TO SLEEP AT NIGHT AGAIN hahaha

 

Phil 

Mechanic turned builder!

 

 

[JohnMo]

I would really look again at the heat pump set up your are considering. Sounds like overkill just install fan coils in the bedroom and run from a single heat pump. I'm using a 6kW one for heat, DHW and heating a summer house and it spends half its time off.

 

PV winter yield was most important for me, so I did vertical panels and they are working out to be good. 

 

I don't export on purpose and as it's all self installed, so get zero money when I do.

 

16 minutes ago, MechanicalBuilder said:

todays prices are going to be 250ish a month

Why that high, look at a ToU tariff.

[MechanicalBuilder]

Thanks John, 

 

Yes it is overkill but when I’m getting it pretty much for free I don’t mind it. The point of having 2 smaller units is that they won’t be working nearly as hard as a single 12kw heat pump plus of air conditioning in my bedrooms in the summer time 👍🏻 the bungalow isn’t quite as small as a summer house! 

 

the crux of the story convo is whether or not I’m ever going to see the money they want me to invest in mains and solar again? The ideal is to self install and stay off the grid but I still have to fork out £30k to have mains even if I don’t use it 😞 

[SteamyTea]

If your DNO is National Grid (used to be Western Power) then I think they are installing 3 Phase by default now.

 

Real shit that mortgage lenders do not allow you to borrow without a mains connection.

Just goes to show how little the finance industry thinks of reducing domestic energy usage.

[JohnMo]

39 minutes ago, MechanicalBuilder said:

the bungalow isn’t quite as small as a summer house

I am heating 195m2 bungalow (3kW max) and a summer house (1kW max). Why does a new build need 12kW? Unless 500m2? If it does you should really concentrate on insulation and airtightness, moderate PV and battery. When the sun isn't out no matter how much PV you have, the production is rubbish. Plus Dec and Jan when heat demand is highest you have the lowest yields from PV and shortest production time.

[MechanicalBuilder]

47 minutes ago, SteamyTea said:

If your DNO is National Grid (used to be Western Power) then I think they are installing 3 Phase by default now.

 

Real shit that mortgage lenders do not allow you to borrow without a mains connection.

Just goes to show how little the finance industry thinks of reducing domestic energy usage.

Yes it’s national grid but they have only quoted to install single phase to the property as that what I requested at the time. The chap has been pretty helpful, I can ask him what transformer they intend on installing if it’s really worth me having 3 phase to export on.

If the information the solar company is providing is correct and I can expect to see the circa £3000 a year return for exporting then it will be worth the investing.

 

i just don’t believe it! 

[MechanicalBuilder]

48 minutes ago, JohnMo said:

I am heating 195m2 bungalow (3kW max) and a summer house (1kW max). Why does a new build need 12kW? Unless 500m2? If it does you should really concentrate on insulation and airtightness, moderate PV and battery. When the sun isn't out no matter how much PV you have, the production is rubbish. Plus Dec and Jan when heat demand is highest you have the lowest yields from PV and shortest production time.

The idea is that the upstairs will hardly

ever be on whilst the downstairs is a little oversized so it’s not running flat out to provide heating and hot water. The bungalow is 300m2 and will be as well insulated and sealed as possible. The issue isn’t going to be solar production or storage for ourselves, it’s whether to bother with the extra expense of having more to export 

[JohnMo]

Why pay a supplement to have PV installed, so you can export? As much as Octopus says they accept non MCS installs it's only a trial and you have to pay a fee, and jump through hoops.

 

Plenty on here don't have heat for bedrooms at all, when they are upstairs. 

 

I have found the cheapest way to run my heating is.

 

UFH run at a fixed 35 Deg demand flow temp, mostly on E7. The floor is 100mm concrete so a big lump to heat, flow temp never really exceeds 33 degs before a thermostat switches it off. Run time depends on outside temp etc. sometimes it goes off at 7am other times it runs until lunch time, controlled by thermostat. Then it's off until 0.30am.

 

We have a GivEnergy All in one battery. We automate the charge % via home assistant based on solar forecast, so last night it charged to 64% on E7. Sunny day today, but only 4 degs currently and -4 last night. Total import 20kWh (UFH and battery from 38 to 64%) home usage so far 24kWh.  Heat pump is off and battery being charged at 3.5kW. Unlikely to use much, if any high rate electric. Our typical running cost for all electricity at the moment (Feb) averaging at about £4, in Dec nearly twice that.

[MechanicalBuilder]

We are kind of veering off subject a bit here as I’ve used extreme worst case consumption figures. I get what your saying but you don’t know the heat loss calcs of the build, the room sizings, how many occupants or our living style? It’s different strokes for different folks. We may well be having 3 baths a day and heating the 300litre tank twice.. the heat pump system has been designed by a multi-million pound air conditioning and heat pump company that aren’t trying to sell me something for a profit… 

 

What i’m wanting to really know is how realistic the figures are that have been provided by the solar company selling me the system. They have estimated we will consume 6000kWh and be up by £4000 as a result of exporting. Is this realistic?

 

I have to pay £30000 to have a mains connection which isn’t debatable so why not spend the extra £10k for solar installation over a self install and be connected to the grid and export if I’m standing to be up £4000 a year??

[Thorfun]

11 minutes ago, MechanicalBuilder said:

We are kind of veering off subject a bit here

happens a lot on here but sometimes you get real gems of information by that off-topic veering so don't knock it and do, as you did, gently veer it back on track. 😉 

 

12 minutes ago, MechanicalBuilder said:

What i’m wanting to really know is how realistic the figures are that have been provided by the solar company selling me the system. They have estimated we will consume 6000kWh and be up by £4000 as a result of exporting. Is this realistic?

Surely you could calculate this yourself? you can use pvgis to get an estimate of your solar yield and then calculate your daily/monthly usage for day and night and battery storage size and then calculate how much will be exported and how much you will use overnight to size your battery accordingly.

 

I wouldn't blindly trust what installers are saying tbh. they want to sell a system and make money. unless they're guaranteeing a figure and will repay you if you don't make that (as if anyone would do that though!) then either calculating it yourself or just not worrying about it an do what you want is the only way forward.

[MechanicalBuilder]

I do value people’s opinions and replies, always interesting to hear others stories! There shouldn’t be any reason why we couldn’t export the capped 4kw for 10hrs a day for let’s say 15p and be looking at an extra couple hundred quid a month on top of the money we will save paying for power

[Dillsue]

To get £4k at 15p/unit you'd need to export 26666 units. Our 4kw system limited to 3.68 kw output is forecast to produce 3600 units and actually does a touch more than that. 4kw for 10 hour/day x 365 gives 14000 units. Unless Im missing something I can't see how you'll get anything like £4k export payments from PV over the year.

 

If you start playing with peaktime export then rates go up quite a bit but its hard to see how you'll make £4k.

 

Has the installer given you calcs to show how you'll make this?

Edited February 25 by Dillsue

[S2D2]

3 hours ago, MechanicalBuilder said:

I do value people’s opinions and replies, always interesting to hear others stories! There shouldn’t be any reason why we couldn’t export the capped 4kw for 10hrs a day for let’s say 15p and be looking at an extra couple hundred quid a month on top of the money we will save paying for power

Don't expect the 15p rate to be around for a lot longer, tracker import is down to around 17p.

[JohnMo]

9 hours ago, MechanicalBuilder said:

 

1. 10k array, 8kw inverter and 19.4kwh battery storage.  £20,000.

 

2. This option being the 'best case scenario' we could possibly fit on our property

 

28kw array, 8kw inverter and 38.8 battery storage. £31,500.

So about 73,000kWh required to be exported to break even difference between option 1 and 2. So about 15 years with an average 15kWh exported 365 days of the year. That's at 15p per kWh and I would assume that price will drop in April by at least 20%, so then 18 years to break even.

[PNAmble]

Interesting discussion.  I’m currently confirming spec of my PV panels.  We’ve built a flat roof in Northumberland so limited to 15degrees due to wind, but they will be pointing due south.  
 

Roof size and shape meant we are at 3 rows of 4,5 or 6. At 4.5/5.4/6.5 kWp.  Difference was approx £200 per panel (fitted)

We used PVGIS to look at generation, we are also speccing at 9.5kW battery;  My decision is based upon the shoulder months production not peak.  Payback to max out at 18 panels is about 3 years more than the 12 panels. We don’t intend to export, we have a battery, solar boost (hw) and EV.   
 

In my opinion building a business case around export costs is a fools game, scope what you need  and use to understand payback.

 

 

 

[JohnMo]

10 hours ago, MechanicalBuilder said:

deciding they would not mortgage our property without mains electricity

Now you have mortgage, why not look for an alternative mortgage supplier, that will mortgage without mains electricity. You have £32k to save.

 

Or at least look at different ways of getting electric from A to B. We employed our own digger driver for a couple of days, cost a £1 000, saved £10 000+

[MechanicalBuilder]

Unfortunately our power has to come up through the middle of the road so the contestable works limit who i can use. Ive got someone quoting this week but a couple of years ago when we first enquired he was only a couple of grand cheaper and advised the saving wasnt worth the hassle the national grid would give us!

 

I'm looking into our plan of staying off-grid, we have a self build mortgage so I need to know when they are going to ask about the mains. I think its at first fix and we cant afford for them to hold any money back. I had another advisor whos a friend look into off-grid mortgages for us, she could get one through ecology but did warn that with the residential mortgager they kind of have a monopoly over us knowing we have limited options so may end up paying quite a bit more in repayments

[MechanicalBuilder]

1 hour ago, PNAmble said:

Interesting discussion.  I’m currently confirming spec of my PV panels.  We’ve built a flat roof in Northumberland so limited to 15degrees due to wind, but they will be pointing due south.  
 

Roof size and shape meant we are at 3 rows of 4,5 or 6. At 4.5/5.4/6.5 kWp.  Difference was approx £200 per panel (fitted)

We used PVGIS to look at generation, we are also speccing at 9.5kW battery;  My decision is based upon the shoulder months production not peak.  Payback to max out at 18 panels is about 3 years more than the 12 panels. We don’t intend to export, we have a battery, solar boost (hw) and EV.   
 

In my opinion building a business case around export costs is a fools game, scope what you need  and use to understand payback.

 

 

 

Exporting wasnt our intention at all, we want to be off-grid but our hand has been forced so I need to find a way of making the most of a bad situation!

[MechanicalBuilder]

5 hours ago, Dillsue said:

To get £4k at 15p/unit you'd need to export 26666 units. Our 4kw system limited to 3.68 kw output is forecast to produce 3600 units and actually does a touch more than that. 4kw for 10 hour/day x 365 gives 14000 units. Unless Im missing something I can't see how you'll get anything like £4k export payments from PV over the year.

 

If you start playing with peaktime export then rates go up quite a bit but its hard to see how you'll make £4k.

 

Has the installer given you calcs to show how you'll make this?

This is what they have given- 

 

Est. Annual consumption 6500kWh 

Est. PV output of 25682 kWh

 

Peak Rate 0.43

Off Peak Rate 0.19

Export rate 0.21

Standing Charge 0.51

 

Annual bill with solar -£3,842 

Annual bill without solar £2,839

Theyve assumed that we will be able to export 18,295 kWh or 50kWh a day for 365 days at 0.21p a unit. 

 

 

[Roger440]

Im with some of the posters above.

 

Just forget export. You have no guarantee over the rate at which you will be paid. Its totally outside your control. Could easily double you payback, maybe worse.

[MechanicalBuilder]

I priced up a couple of options last night to get down to the nitty gritty of what the extra cost of being grid tied is going to set us back.

The difference over staying off grid and self installing a system adequate for our own needs or being grid tied will cost us about £18k plus the standing charges and interest in mortgage repayments on the money. 
I was just interested to see if we would ever see that extra cost back!

 

[LiamJones]

I have a grid connection and opted to self install solar because MCS + export makes no financial sense. Install only what you need. Even the octopus non MCS export doesn’t make sense to me with the fees involved. 

[Alan Ambrose]

>>>

Peak Rate 0.43

Off Peak Rate 0.19

Export rate 0.21

Standing Charge 0.51

<<<
 

For these calcs I guess you need to use something like ‘average rate expected over your payback period’ and be aware that you’ve excluded inflation from the pricing. My average import rate using Agile was 18p in Jan and I suspect due to reduce.  I believe the highest export rates are 15p atm but likely going down sharply as the market normalises a bit this year. Now it’s anybody’s guess what electricity prices are over the next 15-20 years but my thoughts are summer prices going much lower with greater grid PV and hopefully normalising once the current conflicts are resolved.

 

Of course, the installers want to paint a positive picture, but you don’t want it too skewed.

[JohnMo]

10 hours ago, MechanicalBuilder said:

PV output of 25682 kWh

Sorry don't believe those figures are possible with an 8kW inverter. PV panels maybe able to produce that many kWh, but an 8kW inverter will not let that energy out. Especially if a G98 approved inverter is installed.

 

25682/365, is 70kWh a day on average, which is a production of 8kW constantly for 9 hours every day, have you actually done any checks on this to see if they are realistic?

10 hours ago, MechanicalBuilder said:

Peak Rate 0.43

Off Peak Rate 0.19

Export rate 0.21

Standing Charge 0.51

Those figures are well out of date. Your export rate is likely to 40 to 50% lower in 5 weeks time.

 

22 hours ago, MechanicalBuilder said:

28kw array, 8kw inverter

 

Edited February 26 by JohnMo

[Conor]

In winter, during a prolonged cold snap, it's not unreasonable to assume you could be consuming 20kWh a day. And your PV could only be generating 1 or 2kWh. 2kWh (based on my 5kW producing as low as 200Wh on a dark day in November and December.) You'll need an alternative power source.

 

Re the mortgage issue. You'll potentially run across this every time you remortgage, and will not get the best rates. Also, what if you want / need to sell in the future? How many people are prepared to pay full market rate for an off-grid house?

 

Finally, you'll be way, way, way over producing in the summer with all that electric going to waste. 

 

If it were me, I'd still go for a decent array, but get the grid connection as well. I hope you can get the price down.