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Peak generation of a north facing array
Posted: Fri Apr 25, 2025 12:47 pm
by SimonSays
With arrays facing the sun, we can usually work out what the peak generation is likely to be. I don't think it's possible to calculate this (at least, I can't see a way) for north facing arrays that never receive direct sun.
Is there a way to do this without waiting for mid-summer?
The back story is that I need a new dual MPPT inverter. My 3.6kW Growatt is clipping most days from about 10.15 to 2.45 with both arrays connected. To me, it seems to be a question of sizing a new inverter with a tradeoff between a little better efficiency in lower light and a little potential clipping vs an oversized inverter with potentially a little less efficiency in lower light.
Am I overthinking this?
(the back, back story is that I purchased a lightly used Growatt 5k hybrid from a business on ebay which turned out to have the strangest issue. The seller told me to dispose of it, so I gave it away to a member here. The seller then sent a 6k hybrid which had only one MPPT working. Now waiting for a refund)
Re: Peak generation of a north facing array
Posted: Fri Apr 25, 2025 1:06 pm
by Stinsy
SimonSays wrote: ↑Fri Apr 25, 2025 12:47 pm
With arrays facing the sun, we can usually work out what the peak generation is likely to be. I don't think it's possible to calculate this (at least, I can't see a way) for north facing arrays that never receive direct sun.
Is there a way to do this without waiting for mid-summer?
The back story is that I need a new dual MPPT inverter. My 3.6kW Growatt is clipping most days from about 10.15 to 2.45 with both arrays connected. To me, it seems to be a question of sizing a new inverter with a tradeoff between a little better efficiency in lower light and a little potential clipping vs an oversized inverter with potentially a little less efficiency in lower light.
Am I overthinking this?
(the back, back story is that I purchased a lightly used Growatt 5k hybrid from a business on ebay which turned out to have the strangest issue. The seller told me to dispose of it, so I gave it away to a member here. The seller then sent a 6k hybrid which had only one MPPT working. Now waiting for a refund)
IMO a well-designed system does involve clipping and it isn't a sign of some kind of fault that needs rectifying. Solar panels are so cheap that it is economically viable to size the array for bright-overcast conditions and just accept that you're leaving some energy on the table in (rare?) bright sunshine.
If you do want to make more use of the power then you could maybe buy a 2nd solar inverter and have one of your arrays AC coupled. This would result in you taking a bit more of Octopus' 15p on the sunniest days. This could possibly be worthwhile if you're comfortable buying used and DIY-ing the job. However the recent sunny weather is unprecedented, in other years you wouldn't have been clipping this time of year...
Re: Peak generation of a north facing array
Posted: Sat Apr 26, 2025 6:43 pm
by SimonSays
Thanks Stinsy. I’m happy to DIY house electrics.
I bought this
this which I think will be well sized to the array’s maximum generation and I’ll get it logging into Home Assistant.
Despite it’s 2000 moniker it can apparently output up to 2.2kW.
Re: Peak generation of a north facing array
Posted: Sat Apr 26, 2025 8:08 pm
by nowty
That sounds like an optimal solution, both technical and financial.
Re: Peak generation of a north facing array
Posted: Sat Apr 26, 2025 8:29 pm
by AGT
Doesn’t PVGIS let you pick orientation?
Been a long time since I’ve used it though
Re: Peak generation of a north facing array
Posted: Sat Apr 26, 2025 10:05 pm
by nowty
AGT wrote: ↑Sat Apr 26, 2025 8:29 pm
Doesn’t PVGIS let you pick orientation?
Been a long time since I’ve used it though
It does in terms of yield but what SimonSays wants is the peak power output which is a bit more complicated. I think the old version used to have more options to give more detailed info on the theoretical watts / m2 verses time of year and day.
As he is already clipping, he needs a bit extra inverter power, probably about 50% more of the north facing panel STC panel ratings which he is now going to get with the extra inverter.
Re: Peak generation of a north facing array
Posted: Sun Apr 27, 2025 9:05 am
by Stinsy
SimonSays wrote: ↑Sat Apr 26, 2025 6:43 pm
Thanks Stinsy. I’m happy to DIY house electrics.
I bought this
this which I think will be well sized to the array’s maximum generation and I’ll get it logging into Home Assistant.
Despite it’s 2000 moniker it can apparently output up to 2.2kW.
That'll do the job nicely and is priced low enough that you'll see the money back, particularly if you see the labour cost as being a "hobby" that you get satisfaction from.
What are your plans for installation? What size breaker is your existing inverter on? What is the cable size?
Re: Peak generation of a north facing array
Posted: Sun Apr 27, 2025 12:51 pm
by John_S
SimonSays wrote: ↑Fri Apr 25, 2025 12:47 pm
With arrays facing the sun, we can usually work out what the peak generation is likely to be. I don't think it's possible to calculate this (at least, I can't see a way) for north facing arrays that never receive direct sun.
Is there a way to do this without waiting for mid-summer?
The back story is that I need a new dual MPPT inverter. My 3.6kW Growatt is clipping most days from about 10.15 to 2.45 with both arrays connected. To me, it seems to be a question of sizing a new inverter with a tradeoff between a little better efficiency in lower light and a little potential clipping vs an oversized inverter with potentially a little less efficiency in lower light.
Am I overthinking this?
(the back, back story is that I purchased a lightly used Growatt 5k hybrid from a business on ebay which turned out to have the strangest issue. The seller told me to dispose of it, so I gave it away to a member here. The seller then sent a 6k hybrid which had only one MPPT working. Now waiting for a refund)
I assume that you are aware that you will need DNO approval if your inverter is capable of exporting more that 3.68kW and possible limiting to maximum export of 3.68kW.
Re: Peak generation of a north facing array
Posted: Sun Apr 27, 2025 2:30 pm
by Stinsy
John_S wrote: ↑Sun Apr 27, 2025 12:51 pm
I assume that you are aware that you will need DNO approval if your inverter is capable of exporting more that 3.68kW and possible limiting to maximum export of 3.68kW.
We're sticklers for that in these parts...
Re: Peak generation of a north facing array
Posted: Mon Apr 28, 2025 9:36 am
by SimonSays
John_S wrote: ↑Sun Apr 27, 2025 12:51 pm
I assume that you are aware that you will need DNO approval if your inverter is capable of exporting more that 3.68kW and possible limiting to maximum export of 3.68kW.
I've done a couple of G99s when I received the higher capacity hybrids (but that turned out to not work)
Our DNO, UK Power Networks, have an online form which is dead simple. UKPN have automated approval at up to 5kW for G99.
When I did the G99 for the 6kW Growatt I found that was also auto approved with no export restrictions. I spent a few days waiting for the approval email to arrive only to later find it had been sent alongside another email that contained details of what I'd applied for. I'd only seen the first of the two emails and didn't notice the second email.
So I'll do a G99 today for the additional inverter but I'm not expecting any issues at all.