Hi All My installer has just put up a 6 KW Solar PV array and is about to connect a Solis S6-GR1P5K inverter to it.  This is rated at a max input of 7.5KW DC but a max output of only 5KW AC even though efficiency is stated at 97%.  Does this make sense?  Shouldn't they fit an inverter with an output of 6KW?

Rapid response appreciated because the electrician is due this afternoon to fit it!

Many thanks  Tom

@trbob i’ll reply in more deteil when I get home but basically, this is a restriction placed on the grid by your DNO. Regards Toodles

Posted by: @tommcl

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Hi All My installer has just put up a 6 KW Solar PV array and is about to connect a Solis S6-GR1P5K inverter to it.  This is rated at a max input of 7.5KW DC but a max output of only 5KW AC even though efficiency is stated at 97%.  Does this make sense?  Shouldn't they fit an inverter with an output of 6KW?

Rapid response appreciated because the electrician is due this afternoon to fit it!

Many thanks  Tom

TBF you won't get 6 kW from your solar array for very long. You'll only get 6 kW if the sun and roof are at the right angle (South facing roof, around midday, during the height of summer). Any deviation from that and you won't be getting as much. For example, I have a South-East facing solar array that's 5 kWp (p = peak). But I only get 5 kW for a few hours a day, and only during the Summer months. If your solar array is, say, an East-West array, I doubt you'd get 6 kW at any time of the day/year, as it won't be facing the right way to produce that. There are websites I think where you can put your location, roof pitch, and panel orientation, and it will estimate how much power you'd produce.

The other thing to consider: is the inverter also connected to a battery, and is that connection a DC connection? In this case it's even less likely that you'll need the full 6 kW AC output, as some of the PV power will be remaining as DC and going into the battery before it's converted to AC and dumped into your house / grid.

Having said all that, I still insisted on an inverter that was capable of producing my 5 kW peak power, even though it won't make a difference 95% of the time. The difference in cost was some hundreds of pounds, which is a fraction of the total cost, and I just wanted the best setup. I got another quote from a different company for a 3.6 kW inverter, and their logic was as above (i.e. it won't make a difference most of the time), but I just wanted the best stuff. 5 kW is better than 3.6 kW so give me the 5 kW one please.

@scrchngwsl My setup is 8.1 kWp (with panels on various orientations) my installer applied on a G99 form to my DNO and was granted a 5 kW connection. Without DNO approval a rig would be limited to 3.68 kW. My grid connection allows the 5 kW to flow via my Tesla Gateway and powerwall (I have actually seen 5.1 kW of power flow to the grid but this could be measurement errors I suppose). My system has only been in use since Aug.13th. and the best day’s production was that very first day: 41 kW/h. Having the battery and an immersion heater plus an all electric kitchen (and soon to be all electric house when the ASHP is installed next month) I don’t have a need to export much power at present but, come the summer Octopus Agile Outgoing will pay me quite decently for anything I don’t consume myself. This exporting will be ‘banked’ for the increased consumption when I am all electric and am powering the ASHP. So you can export up to 5 kW of power but should your rig produce above this figure, you may care to employ a Myenergi Eddi power diverter to your immersion heater or any other resistive load. Regards, Toodles

Posted by: @scrchngwsl

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Posted by: @tommcl

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Hi All My installer has just put up a 6 KW Solar PV array and is about to connect a Solis S6-GR1P5K inverter to it.  This is rated at a max input of 7.5KW DC but a max output of only 5KW AC even though efficiency is stated at 97%.  Does this make sense?  Shouldn't they fit an inverter with an output of 6KW?

Rapid response appreciated because the electrician is due this afternoon to fit it!

Many thanks  Tom

TBF you won't get 6 kW from your solar array for very long. You'll only get 6 kW if the sun and roof are at the right angle (South facing roof, around midday, during the height of summer). Any deviation from that and you won't be getting as much. For example, I have a South-East facing solar array that's 5 kWp (p = peak). But I only get 5 kW for a few hours a day, and only during the Summer months. If your solar array is, say, an East-West array, I doubt you'd get 6 kW at any time of the day/year, as it won't be facing the right way to produce that. There are websites I think where you can put your location, roof pitch, and panel orientation, and it will estimate how much power you'd produce.

The other thing to consider: is the inverter also connected to a battery, and is that connection a DC connection? In this case it's even less likely that you'll need the full 6 kW AC output, as some of the PV power will be remaining as DC and going into the battery before it's converted to AC and dumped into your house / grid.

Having said all that, I still insisted on an inverter that was capable of producing my 5 kW peak power, even though it won't make a difference 95% of the time. The difference in cost was some hundreds of pounds, which is a fraction of the total cost, and I just wanted the best setup. I got another quote from a different company for a 3.6 kW inverter, and their logic was as above (i.e. it won't make a difference most of the time), but I just wanted the best stuff. 5 kW is better than 3.6 kW so give me the 5 kW one please.

 

Thanks Scrchngwsl.   My roof is due south and nicely pitched so should be able to hit peak pretty well in summer.  Diverter to water heater is in but the that is after the inverter.   Inverter is in the loft with AC connection back into the consumer unit.  Battery and/or EV is on the long term plan but not yet designed in.  I thought the limit from the DNO around here (mid Cheshire) was 6KW ?   Seems a shame to put that beautiful array and then clip the top 20% of performance.

Hi. I'm new to forums so please forgive me if I've got this wrong but I'm hoping to put a thermal ground loop in our concrete foundations. As part of our building project we're hoping to use the Zehnder ComfoFond heat pump to cool/heat the incoming air from the mvhr. Wondering if anyone has knowledge of using concrete foundations for this purpose please. Regards, She's

Posted by: @scrchngwsl

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I still insisted on an inverter that was capable of producing my 5 kW peak power, even though it won't make a difference 95% of the time. The difference in cost was some hundreds of pounds, which is a fraction of the total cost, and I just wanted the best setup. I got another quote from a different company for a 3.6 kW inverter, and their logic was as above...

The installer should not have used a 5kW grid-tied inverter under these circumstances.
It's not their decision - because they are required to operate within the permission granted by the Distribution Network Operator for your region.
Were they installing without having obtained the relevant G98 or G99 approval?

Have a look at the diagrams I've just posted over here for the benefit of @tommcl

If you'd like assistance to ensure that your installation is legal, then please ask for help over on that other topic, rather than here in the Welcome area.
I do have solutions to offer if you'd like.

No worries, they got G99 approval from the DNO for the 5kW system.

Posted by: @alexm

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Hi. I'm new to forums so please forgive me if I've got this wrong but I'm hoping to put a thermal ground loop in our concrete foundations. As part of our building project we're hoping to use the Zehnder ComfoFond heat pump to cool/heat the incoming air from the mvhr. Wondering if anyone has knowledge of using concrete foundations for this purpose please. Regards, She's

According to the manual the 'brine' (Water and Anti-freeze mixture) pipework should be installed 1.2 to 1.6 metres below ground, because the temperature at that depth is a fairly constant 8C to 10C.

Unless I am looking at the wrong manual, the unit mentioned is a heat exchanger rather than a heat pump, and can be used to pre-heat or pre-cool the outside air being supplied to the MVHR unit. I assume that you have an additional form of heating for the Winter months.

Thanks Derek. Yes, in open ground the pipework should be 1.2m+ below the surface. We unfortunately can't get a long enough trench at those depths but do currently have more shallow trenches dug for our foundations. An insulated floor slab is going to be built off the foundations which might prevent the foundations from getting heated by the sun etc and so they might maintain a constant temperature even though they're not that deep in the ground.  

Thanks for clarifying @alexm but I don't think that would work.

Firstly I would expect any GSHP installer to provide a desk-study/survey based on the characteristics of your local geology. From that you would get an idea of the lengths of 'slinky'-style collector coils which you'd require. But I would expect the required trenching to be many times greater than that which you would create for a strip-foundation on a domestic property.

Secondly, the body of ground from which you will be drawing heat needs to be sufficiently large that there isn't going to be the risk of the water/glycol mix in the pipes from simply freezing the ground which surrounds them!
If you wanted to try limiting the amount of ground in which the pipe coils are laid, then you'd need to be on a site which constantly replenishes that space with fresh 'heat'. In that situation you'd be looking to use a sloping site with a high water table and water slowly percolating across the layer holding the pipes.

Sloping sites, high water tables and moving ground-water are features which tend to be avoided if you're using strip foundations! 😲 

If a site without sufficient horizontal space for pipe loops is to have a GSHP installed, then there are two ways forward:

• drill a borehole at least 200m deep (150mm diameter)
• find an adjacent abandoned mine-shaft and throw the heat collector coil into the flooded workings

Kensa heat pumps in Cornwall do precisely that to provide heat for their own offices and manufacturing site.
Talk to them. These guys know what they're doing!

Posted by: @transparent

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Thanks for clarifying @alexm but I don't think that would work.

Firstly I would expect any GSHP installer to provide a desk-study/survey based on the characteristics of your local geology. From that you would get an idea of the lengths of 'slinky'-style collector coils which you'd require. But I would expect the required trenching to be many times greater than that which you would create for a strip-foundation on a domestic property.

Secondly, the body of ground from which you will be drawing heat needs to be sufficiently large that there isn't going to be the risk of the water/glycol mix in the pipes from simply freezing the ground which surrounds them!
If you wanted to try limiting the amount of ground in which the pipe coils are laid, then you'd need to be on a site which constantly replenishes that space with fresh 'heat'. In that situation you'd be looking to use a sloping site with a high water table and water slowly percolating across the layer holding the pipes.

Sloping sites, high water tables and moving ground-water are features which tend to be avoided if you're using strip foundations! 😲 

If a site without sufficient horizontal space for pipe loops is to have a GSHP installed, then there are two ways forward:

• drill a borehole at least 200m deep (150mm diameter)
• find an adjacent abandoned mine-shaft and throw the heat collector coil into the flooded workings

Kensa heat pumps in Cornwall do precisely that to provide heat for their own offices and manufacturing site.
Talk to them. These guys know what they're doing!

It is not a GSHP, so there is very little chance of the pipework freezing underground. It is actually a heat exchanger to heat or cool the intake air to a MVHR system.