Solar panel inverter - How to work out the appropriate size to match my new installation

  • 12 July 2022
  • 5 replies

Userlevel 4
Badge +1

Hi there,


I just had solar installed over the last few days. It was 14 panels which is 5,460 watts. The inverter is a 5k inverter. I know nothing but queried this, how can the inverter be smaller than the max consumption? They said its normal to under spec the inverter by up to 20% as you never get the max output. When then panels get hot they are less efficient etc. 

Anyway, when they get up on the roof they say there is actually room for 2 more panels, do I want them? I said yes thats great, but surely now we need a bigger inverter? Now the max output is 6,240W. The inverter is 5k. No they say, you will never get to the max, maybe once or twice a year.


So here is the data from today. the very first day. Clearly you can see its bumping up against the max across about 2 hours in the middle of the day. I know its very hot, but I’m told the heat actually makes the panels less efficient?!

So my question is is this normal, that I’m creating DC i can’t convert to AC? The 6k inverter is about £60 more expensive. This really seems messed up to me, but I though I would ask here before I push it with the company.


Many thanks



5 replies

Userlevel 6
Badge +3

For £60 it would make sense to upsize the inverter if that option is available to you.  There is also no problem having more panels than the max rating of the inverter as long as the inverter is capable of it (most are). 

Userlevel 5
Badge +4

An American article but the principles are the same on either side of the pond

Under sizing can make the system more efficient when the system isn’t running under optimal conditions. 

Did you get a battery to dump all the electricity in to?

Userlevel 1
Badge +3

Are your panels all on one aspect?


Fwiw, I've got 6kwp on a 5kw inverter, but my panels are split over 3 aspects, so I really don't have any hope of hitting 6kw.

I've seen it spike to just over 5kw, but only when the panels are cool from cloud cover and then the sun comes out.

My normal peak on a clear day with hot panels is around 4 to 4.4kw, but that's fair for my split array.

Userlevel 4
Badge +1

@hydrosam thanks for the link and thanks all for other replies. That link is interesting and suggests I’m wrong and just don’t understand (which is great).

This is the bit I don't get:

“Undersizing will reduce the system’s power output under conditions that would result in the system reaching its peak output, but that would be true for only a couple of hours in the day. But an interesting thing happens with undersized inverters in the mornings and afternoons. Undersized inverters will ramp up quicker in the mornings, and ramp down slower in the afternoons. If you graph the power output, you’ll see a slightly lower peak production, but higher morning and evening production, resulting in a fatter power production curve. The result of this is that the undersized system would produce more power in total than a system that wasn’t undersized.



I would be less concerned with the peak and more with the daily yield. I have 4kW nominal panel installation and a 4kW solar edge inverter.

My panels all face South West with a little shading in the morning so I get the characteristic single hump curve. At this time of year on a clear day it can peak at just over 3kW. I would generate more if I had extra panels facing North East as I don’t produce much at that time and could do with filling that gap.

I find I produce most on a slightly cloudy day when you get direct sunlight plus some extra reflected from clouds. I would also wait for a few months data before drawing any conclusions.