So you already have a 3.68kW inverter / solar install?

Only a DNO application will tell you if you've missed the boat and what you are allowed., but you are correct in thinking it's kind of a first come first served thing. We were in a similar boat, with an existing 3.68kW G98 solar install, and applied for a PW3 and more solar under G99 and the DNO initially offered 7kW inverter limitation (to match the proposed 7.2kWp solar install) and a 1kW export limit. In other words, you can generate, store and consume, but we don't want your export.

The Tesla PW3 is somewhat unique in that it has an inbuilt inverter that can be software limited to an output of 3.68, 5, 6, 7, 8, 9, 10 or 11.04kW, so an installer could in theory replace your existing 3.68kW inverter with the PW3 (limited to 3.68kW) under your existing G98 (I'm not sure if you need any further application for the battery itself?).

Otherwise the installer must complete a G99 application before any work to connect to the grid can commence (whereas G98 is install and notify)

Some installers push to just install under G98/3.68kW as it's easy and quick for them, but I would not settle for anyone wanting to do this. Our DNO is National Grid, and I found we were able to engage in conversation with them, but ultimately they have to maintain the stability of the grid.

@old_scientist yes already have a 4kwp array on a venerable 11 year old Stecca 3.68kw inverter which is on a 2014 FIT, so don’t want to mess with it to be honest.

am I right in assuming then that the DNO can impose limits on your inverter output even if you’ve set it to a limited export power? That’s nuts, it doesn’t make any sense, I can understand an export limit being imposed to stop overloading the grid but a self use limit like wtf. Surely it won’t affect the grid if I self consume the power as the inverter will just match the incoming voltage so power doesn’t flow to the grid no?

Correct, the G99 imposes separate limits for the size of the generating equipment (inverter size) and an export limitation. For example, you can apply for an 8kW inverter and have a 5kW export limit imposed. Both are effectively limited. If you wanted to upgrade to a 10kW inverter, you've have to submit a new G99 application.

The PW3 is a somewhat unusual case in that it can be set at multiple settings from 11.04kW downward. Normally you would choose an inverter size to match your solar array. In our case with a proposed 7.2kWp of solar installation, an inverter rating of 7kWp seemed OK. I don't know if that's what they applied for or if they applied for the full 11.04kW max rating of the PW3 buy the DNO knocked it back to prevent more solar from being installed down the line.

I don't really understand the technical reasons for limiting an inverter rating in addition to limiting the export rating, but the Powerwall can certainly do both, easily set in the commissioning software, making it extremely versatile. It may kind of be unique to the Powerwall situation as it can be installed at multiple output ratings, whereas normally you'd just apply for a 10kW inverter and get a yes/no answer, and maybe have to go back and apply for a lower rating to gain approval, so the G99 approval itself is for a specific value inverter and is limiting in it's own right - if you have approval for a 5kW inverter, you can't just replace it with a 10kW inverter without a new G99 application.

You will need to take advice regarding your FIT, whether it is best to leave it on the original AC coupled inverter and add the battery (with more DC coupled solar if you wish), or whether it is advisable to replace the old inverter allowing the original solar to be DC coupled to the battery without affecting your FIT payments. I have no knowledge or personal experience in this area.

Please can you clarify what you'd ideally like to achieve. Are you simply seeking to add a battery to your existing setup, or are you also seeking to add more solar and a battery?

The inverter rating of the battery will limit at what rate the battery can charge and how much power it can output to the house. If you connect the battery with a 3.68kW inverter, the battery will only be able to supply a max of 3.68kW to meet the house load, and any additional load will met from the grid (or AC coupled solar), so understanding your house load is important.

For us, a background load in winter with the heating (ASHP) on might be 1.5kW, and if the oven is on to cook dinner (another 3kW) then a 5kW minimum would seem appropriate, but simultaneously using the kettle or microwave whilst cooking dinner would likely exceed that rating. Also, we use the Cosy ASHP tariff in winter which only has 2h or 3h periods of cheap rate, so it is important we can fully recharge the battery quickly, so a 5kW charge rate is required. We couldn't fully recharge our 13.5kWh battery in winter in a 3h window with a 3.68kW inverter rating, although it would not be far off.

@old_scientist mainly just add a battery so I can make the heat pump equation work when argued against a gas boiler. How does the gov expect people to green there heating and house use if they are putting obstacles in the way like ridiculous electric pricing and inverter limits.

i might add some more solar 2kwh soon but a major uplift I’m likely to leave until the FIT finishes in 9 years at which point I’ll likely remove the old panels and max the roof.

Posted by: @adamk

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@old_scientist mainly just add a battery so I can make the heat pump equation work when argued against a gas boiler. How does the gov expect people to green there heating and house use if they are putting obstacles in the way like ridiculous electric pricing and inverter limits.

i might add some more solar 2kwh soon but a major uplift I’m likely to leave until the FIT finishes in 9 years at which point I’ll likely remove the old panels and max the roof.

OK, so if it were a Tesla Powerwall I'd be presenting arguments to the DNO for the full 11.04kWh inverter rating for the battery, keeping the solar on the existing inverter and AC coupled, with no additional export allowed from the battery. That way you keep the flexibility of the battery being able to supply high demand loads within the house (when battery capacity is available) without having to pull from the grid (that has to be good for the grid, reducing demand at peak times?) and you are not seeking to increase your export, if anything you may decrease the amount you export as you will now self-consume a greater percentage of what you generate through storage and later use (which again should benefit the grid?)

On the Tesla Powerwall you can independently limit any/all of the following in software:

• Inverter rating (output current in Amps) from 16A (3.68kW) up to 48A (11.04kW)
• Import limitation (only really applicable if you have multiple PW's installed on a single phase)
• Permission to Export ON/OFF
• Export limitation, set in Watts
• Export from Solar only allowed or Export from Solar and Battery allowed

The difficulty will likely come if you were to increase your solar and want to increase your current 3.68kW export limit. If you can, I'd apply for extra solar now (attached to the battery system) if you can afford it. Things will only likely get worse and it costs you nothing to apply. If they knock you back, you can always ask the installer to reapply for the battery only solution. That extra solar will also benefit you in winter as you may produce enough from a large array, even on a cloudy day, to cover a heat pump ticking over. Mine runs at around 1.2-1.5kW in winter and it's a great feeling when it's heating the house off solar!

@old_scientist one installer who wanted to use sigenergi and 2 8kwh batteries was also going to fit 1.875kwp of solar on the garage roof where the battery will live, but I’m not sure at that low amount it would be worth the expense. I had hoped to get 2.5kwp on there but he’s worries about it being too close to the edges and the wind catching it. I do wonder if it’s possible to blank off the gap between the panels and the metal roof sheets, that way the wind can’t get under them.

i think I will seek to get maybe a 8kwp+ export limit, that would give me some room for expansion of the solar in 9 years. I think I can jam 8kwp on the house and 2kwp on the garage. If I up rate the cable to the workshop might even get 3kwp on that.

There are MCS guidelines about how close to the edge/ridge of a roof panels can go, and if they are MCS accredited then they will have to stick to those guidelines, and you need the MCS certificate for your energy supplier for the export. I think it's 300mm or 400mm clearance.

You can get panels of different physical size (older panels are smaller, and there's also large format panels about 1.5x the size of normal panels), so best to pick an installer whose able to work with you to maximise your available roof space (if that is an issue for you) rather than one who has bought 10 containers of a certain solar panel so it's that or nothing. I agree you want to get as much capacity as possible on your available roof space. Nobody ever says I wish I'd installed less solar panels.

I do not think you will get a notional export amount. You have to apply for what you want to connect to the grid, receive an offer and then notify the DNO within 3 months that it has been installed and commissioned, otherwise the offer expires. So if you apply for, and are granted permission for 8kW export, then you will need to install that within 3 months.