Solar Inverter Sizing

[DevlinCurse]

Wondering if anyone can clarify if the following statement I was given by a Solar installer is correct.

I have 14 solar panels (7 on an east facing roof and another 7 on a west facing roof). Per roof there is a theoretical 2.73kWh per roof surface. The installer has both these strings on a single 3.6kWh inverter.

I noted that on sunny days the amount of energy for around 4 hours hits the maximum of the inverter. I raised this with the installer to see if moving to a larger inverter would be a better option to take advantage of these periods rather than having it clipped. The installer told me that this would not be a good idea as a larger inverter would be less efficient in the Winter months that the 3.6kWh inverter is. Is this true? As logically that doesn't make sense to me, but I am not an expert in this field.

 

[binky]

They have used a 3.6 to make the DNO paperwork easy. I would defo fit a larger inverter.

 

[tims22]

Depends on the inverter make/model & specs. Some Hybrid units are able to provide more than their rated output by using multiple output channels - eg powering house loads and concurrently providing DC output to charge batteries. We originally had a 3.6kW Solis hybrid on a 6.8kW array, which worked absolutely fine until we reached summer months when we didn't have enough empty battery capacity to soak up power and couldn't use the full output power.. which basically resulted in it clipping generation on decent days and limiting export potential etc.

We've now had it replaced with a 6kW inverter (+ g99 etc) and the difference is remarkable.
For the small cost difference it may be worth considering a 5kW unit.. if you've got a max output of 5.46 kW a 5kW would be better placed to handle what the system can potentially generate on a bright sunny day. That said, if your roof is East/West it's possibly likely that you'll see much lower maximum outputs from the system through a day than the theoretical 5.46kW - which might well be why they've specified a 3.6kW unit as that would be able to handle one string at full output along with a fair amount of generation from the other.

Perhaps ask the installer to clarify and go from there?

As for easier paperwork, some DNOs (UK Power Networks for example) use an accelerated approval process for up to 5kW generation. They need you to submit a G99 A1.1 application but for 3.68 - 5kW they approve instantly.

See: https://www.ukpowernetworks.co.uk/e...installing-large-scale-distributed-generation

 

[binky]

at said, if your roof is East/West it's possibly likely that you'll see much lower maximum outputs from the system through a day than the theoretical 5.46kW - which might well be why they've specified a 3.6kW unit as that would be able to handle one string at full output along with a fair amount of generation from the other.

It will still see peak outputs for a couple hours per day.

 

[DevlinCurse]

Thanks for the feedback.

The point that I find most interesting is the claim that a larger inverter will convert less energy in Winter that a smaller inverter will. Surely that can't be true?

 

[tims22]

Thanks for the feedback.

The point that I find most interesting is the claim that a larger inverter will convert less energy in Winter that a smaller inverter will. Surely that can't be true?

Have a look at the startup voltage specs for the smaller & larger inverters you're looking at. Might be that the smaller inverter starts up at a lower voltage and therefore "in theory" might produce more energy on a dull winter morning because it can start generating with lower PV output than the larger unit. We were given the same line by our installers but after checking the specs concluded it was the same across the range.. and therefore shouldn't make any difference.

Maybe ask them to explain why they think there will be a difference in performance and see what they say?

 

[binky]

Thanks for the feedback.

The point that I find most interesting is the claim that a larger inverter will convert less energy in Winter that a smaller inverter will. Surely that can't be true?

Inverter efficiency doesn't drop below 92% these days.