Sizing the inverter

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Brimble
Brimble Posts: 81 Forumite
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edited 21 August 2022 at 1:20PM in Green & ethical MoneySaving

Having done a lot of research and analysis, I’m reasonably comfortable that the best solution for our house would be as follows:

16 x 400W panels on SSE-facing roof

6 x 400W panels on SWW-facing roof

c. 10kwh battery

iBoost

(N.B. brands tbc – that will be the next step once I’ve decided on inverter spec)

 

…but I’m still confused about the inverter for numerous reasons… I've read about over and under sizing, also I know the ‘no need for approval’ DNO limit is 3.68kw, but I still don’t really understand what that means.

 

  • If my PV totals 8.8kw, and I have a 3.68kw inverter and a battery as above, does that mean all my solar production beyond 3.68kw would be ‘lost’, or just my production over 3.68kw PLUS whatever the battery is absorbing?
  • Given our usage pattern and proposed system, we will export very little to the grid – should that influence the inverter decision? I.E. should the inverter decision mainly be influenced by expected export, or by overall production?
  • My concern is that DNO approval will either take many months, or will cost say £500-1000, or both, only to avoid losing out on a minimal extra export income which will never pay back the cost of getting the approval.
  • When I try to use some solar design apps to suggest inverters, they seem to default to 3-phase – presumably it’s most likely I’ll only have single phase, and therefore need a single phase inverter – are they powerful enough to manage an 8.8kw PV?
  • Am I better just to see what installers recommend then bring it back here for advice?
  • Generally can anyone recommend a good resource to learn about and select inverters?


Would be grateful if anyone can help to educate me on this topic - thanks. 😊


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  • Spies
    Spies Posts: 2,035 Forumite
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    With a 3.68kw inverter all generation in excess of that is lost. 

    What you want is an appropriately sized inverter and either apply for g99 or to limit export of inverter to 3.68kw
    4.29kWp Solar system, 45/55 South/West split in cloudy rainy Cumbria. 
  • gefnew
    gefnew Posts: 883 Forumite
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    A bit from a aus web site they seem to have a bit of info.
    Sizing inverters to optimise solar panel system efficiency - Solar Choice
  • Brimble
    Brimble Posts: 81 Forumite
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    Ok, so then presumably I need an inverter of around 6.5 - 7.0 kw. That being that case, would it normally be just one inverter, or two smaller ones? How many panels normally go in a string, and how many strings can one inverter take? Can it take strings from different roof set-ups, and therefore different peaks etc? Can a single inverter that is 6.5-7.0kw run on single phase power, or would I need to change to 3-phase (which could presumably be extremely expensive)?
  • Spies
    Spies Posts: 2,035 Forumite
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    Just the one,  dual Mppt so two completely different facing arrays can go into one inverter,  single phase is fine. 
    4.29kWp Solar system, 45/55 South/West split in cloudy rainy Cumbria. 
  • 70sbudgie
    70sbudgie Posts: 749 Forumite
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    My personal opinion is that for 8.8kWp, you shouldn't worry about trying to fit your generation onto a 3.6kW inverter. Even with batteries, you will be wasting a lot.

    Another thing to consider is the charge / discharge rate of your batteries. I understand, from reading comments on this forum, that 2.5kW is a common charge / discharge rate (though I believe that it may actually be measured in amps).

    So if you have a 3.6kW inverter and 2.5kW battery, you will always waste all generation above 6.1kW.  (This is assuming a hybrid inverter with DC connected batteries). And when your battery is full (after about 4 hrs), you will waste all generation above 3.6kW.

    As far as I am aware, most solar diverters are AC, so wouldn't contribute to increasing DC utilisation. (I use this term as the difference between panel output and inverter output).

    I would suggest phoning up your DNO. The cost of approval for a bigger system will depend on the network near you - not just the DNO supply equipment, but also how many other houses have (solar) generation. You may also need your cut out (the DNO "fuse" to your property) upgrading.

    Talking to the DNO should highlight where there are step changes in the capacity of the local network. For example, if the network has 6.9kW capacity and you ask for 7kW, you will have to pay to upgrade. Probably the same cost as if you asked for 8.5kW. So you can then decide whether to pay for 8.5kW, or reduce your request to 6.9kW. 

    You can probably get quite a bit of information from your DNO website, though it will probably require some digging and might be challenging if you're not familiar with most of the terminology. Once you know who your DNO is, I would suggest that you come back here for help.

    8.8kW would require a fuse of at least 37A. I'm not certain, but I think the max on a domestic single phase property is 32A. Again I'm not sure, but I think that means you should be able to 7.68kW onto a single phase supply.

    I think you will be surprised how much you can generate and therefore export to the grid. With 8.8kW, on a sunny day, you could possibly generate in excess of 50kWhrs. That would fill your battery, heat your hot water tank, power your house and then have plenty to export. So you would probably want to consider time of use electricity tariffs. That would change your financial calcs and may make the DNO approval for a larger inverter more attractive.
    4.3kW PV, 3.6kW inverter. Octopus Tracker. Zoe. Ripple x 2. Cheshire
  • Brimble
    Brimble Posts: 81 Forumite
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    70sbudgie said:
    My personal opinion is that for 8.8kWp, you shouldn't worry about trying to fit your generation onto a 3.6kW inverter. Even with batteries, you will be wasting a lot.

    Another thing to consider is the charge / discharge rate of your batteries. I understand, from reading comments on this forum, that 2.5kW is a common charge / discharge rate (though I believe that it may actually be measured in amps).

    So if you have a 3.6kW inverter and 2.5kW battery, you will always waste all generation above 6.1kW.  (This is assuming a hybrid inverter with DC connected batteries). And when your battery is full (after about 4 hrs), you will waste all generation above 3.6kW.

    As far as I am aware, most solar diverters are AC, so wouldn't contribute to increasing DC utilisation. (I use this term as the difference between panel output and inverter output).

    I would suggest phoning up your DNO. The cost of approval for a bigger system will depend on the network near you - not just the DNO supply equipment, but also how many other houses have (solar) generation. You may also need your cut out (the DNO "fuse" to your property) upgrading.

    Talking to the DNO should highlight where there are step changes in the capacity of the local network. For example, if the network has 6.9kW capacity and you ask for 7kW, you will have to pay to upgrade. Probably the same cost as if you asked for 8.5kW. So you can then decide whether to pay for 8.5kW, or reduce your request to 6.9kW. 

    You can probably get quite a bit of information from your DNO website, though it will probably require some digging and might be challenging if you're not familiar with most of the terminology. Once you know who your DNO is, I would suggest that you come back here for help.

    8.8kW would require a fuse of at least 37A. I'm not certain, but I think the max on a domestic single phase property is 32A. Again I'm not sure, but I think that means you should be able to 7.68kW onto a single phase supply.

    I think you will be surprised how much you can generate and therefore export to the grid. With 8.8kW, on a sunny day, you could possibly generate in excess of 50kWhrs. That would fill your battery, heat your hot water tank, power your house and then have plenty to export. So you would probably want to consider time of use electricity tariffs. That would change your financial calcs and may make the DNO approval for a larger inverter more attractive.
    That's incredibly helpful, thanks. My DNO is 'UK Power Networks' - does that help at all?

    When you say "8.8kW would require a fuse of at least 37A. I'm not certain, but I think the max on a domestic single phase property is 32A. Again I'm not sure, but I think that means you should be able to 7.68kW onto a single phase supply" - are you saying that I might be better off reducing the number of panels to create a 7.6kw system? (My original plan was 6.4kw, I only increased it to 8.8kw on the basis of so many people on here advising to maximise your panels pretty much above all else).
  • gefnew
    gefnew Posts: 883 Forumite
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    A bit more for you to read.
    Export Limitation | SMA Solar (sma-uk.com)
  • Martyn1981
    Martyn1981 Posts: 14,814 Forumite
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    edited 21 August 2022 at 4:15PM
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    I would definitely suggest going as big as possible, but the word 'possible' covers a lot of caveats. For 8.8kWp, as you say, you'd probably need an inverter of around 7kW*, which if capped to 3.68kW for DNO purposes would be too wasteful. So I think you need to ask if you can go bigger, before considering such a large system.

    *If I'm reading your info right, then you have ESE and WSW roofs, so only a 90d difference (edit - sorry, may be 120d or so), so they'll be peaking close together, whereas, for instance, my roofs are 180d different so the peak is a smaller percentage of the total. But even then, in June and July when the sun is highest, both arrays can see the sun at the same time, even if the angle isn't great.

    70sbudgie raises a great point about the battery, it'll fill too fast, so unless you can go for a much bigger DC side battery, the battery will top out and the DNO cap will kick in perhaps by the time the system is really starting to peak, and capping will be huge.

    There may be another option, if your DNO has approved/certified export limited solutions. These don't cap the inverter at the agreed limit (say 3.68kW), but cap export, so you'd be able to deduct home consumption, such as a charging BEV, or AC side batteries, where I think you'll have more choice to go bigger (or even DC and AC side batts, I suppose(?)) I don't know much about export limiters, and apologise if this is a wild goose chase.
    Mart. Cardiff. 5.58 kWp PV systems (3.58 ESE & 2.0 WNW). Two A2A units for cleaner heating.

    For general PV advice please see the PV FAQ thread on the Green & Ethical Board.
  • 70sbudgie
    70sbudgie Posts: 749 Forumite
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    Brimble said:

    When you say "8.8kW would require a fuse of at least 37A. I'm not certain, but I think the max on a domestic single phase property is 32A. Again I'm not sure, but I think that means you should be able to 7.68kW onto a single phase supply" - are you saying that I might be better off reducing the number of panels to create a 7.6kw system? (My original plan was 6.4kw, I only increased it to 8.8kw on the basis of so many people on here advising to maximise your panels pretty much above all else).
    No, I don't mean reduce your panels (I am one of the people advocating as many panels as possible!). I meant that a 7.6kW inverter for 8.8kW of panels seems like an ok amount of oversizing (of the panels). 

    Also, I don't think it should cost you if your cut out needs upgrading. Not all properties have a cut out, so when I got my EV charger installed, I had to get my DNO to come and fit one. I'm fairly sure they gave me a 100A one for my 7kW charger (I think they are usually either 60A or 100A) and I wasn't charged for anything. 

    I think an export limitation scheme / device is a good idea. That would allow you to have AC batteries as Martyn1981 suggested (these would then have a separate inverter) and with the IBoost as well (immersion heater is typically 3kW), you would have two good sources of large load to use your generation before any export goes to the grid.

    And if you decide to get an EV in the future, you would also be able to install a charger like the Zappi and charge your car from your own PV generation. (And I would be very jealous as this is getting close to my ideal set up!)

    The export limitation only has to limit how much goes through the meter, whereas the inverter limits how much feeds into your consumer unit.

    I think I have read a post on this forum from someone with a large PV array, batteries and export limitation, but I can't remember who it is.
    4.3kW PV, 3.6kW inverter. Octopus Tracker. Zoe. Ripple x 2. Cheshire
  • 70sbudgie
    70sbudgie Posts: 749 Forumite
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    Links to the relevant bits of the UKPN website:
    For upgrading the fuse https://www.ukpowernetworks.co.uk/electricity/fuse-upgrade

    UKPN also have a web portal called Smart Connect. 
    https://www.ukpowernetworks.co.uk/smart-connect

    This looks like it will answer all your DNO questions in advance and that you might be able to register yourself and therefore get the answers before deciding on an installer. However, I think it is normal practice for the installer to include for notifying / liasing with the DNO, so some installers might use you having made contact as an excuse to be funny.
    4.3kW PV, 3.6kW inverter. Octopus Tracker. Zoe. Ripple x 2. Cheshire
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