Inverter sizing for PV – to undersize or not?

[Dan G]

It seems to be something of an industry standard practice to under solar inverters relative to the power of the solar panels fitted, typically about 25% below the rated max of the panels. For example, I know of an 8 kW panel system facing due south with a 6 kW inverter (feeding home, EV, Powerwall, and grid); while at the other end of the scale (and country) the well-respected, long-established local company who installed my system insisted that a 1.6 kW inverter was the right size for 2.1 kW of panels (also south-facing). Both inverters are current gen Solis.

 

It was explained to me that the smaller inverter would be more efficient when the panel output voltage is lower on cloudy days, which are obviously the norm for most of the time in the UK. But it just bothers me when I see the system output pegged at 1.65 kW with the sun out...

 

When collecting lots of quotes for a larger rooftop project (20 kW) I noted that some installers were specifying undersized inverters and some were not. It tended to be the younger companies who were not. We've picked a company who happen not to be undersizing (on the basis of price per watt of panel, not that they didn't spec an undersized inverter).

 

What do you guys think? Undersizing inverters, yay or nay?

[JohnMo]

Different for hybrid or non hybrid.

 

G98 approval allows 3.6kW export with prior approval, so you would overclock the inverter, have more kW on the panel side, because 99% of the time it will not be at nameplate rating. So 3.6kW inverter with say 4kW panels.

 

When you get to hybrid, the kW rating also applies to how much kW you can transfer from battery to house. So here it's better for a bigger inverter. But then you also need G99 approval.

 

Horses for courses.

[SteamyTea]

2 hours ago, Dan G said:

yay or nay

How much do you think will be exported and how much will you get paid of it?

 

I have seen a 3 kW SMA inverter exporting 3.5 kW, the cooling fan was working hard.

[Dan G]

I thought this question would get more interest 😄

 

MCS guidance recommends undersizing, and I was told the Solis design software does too. Just wondering what others think.

[Galileo]

I'd slightly undersize as manufacturers panel specifications are in an ideal world scenario in peak optimal generating conditions, why bother paying for an inverter that will never reach it's MPO? Then there's mono vs poly, the former working better in lower light conditions etc. Sizing to match the location (I'm at 60 North, all of our sun arrives in the summer) and enviroment, if the sunshine records for your area averages out as 3 hours a day, not much point in going up to a bigger inverter for that etc.

 

[SteamyTea]

@Galileo

While I understand your thinking about new modules and mono v poly in low light.

I doubt there is enough difference to worry about.

 

The main thing is what size inverters you can actually buy, they are not sold in 100W increments, usually in approximately 500W increments on the small ones and the 1kW jumps.

 

The main thing that matters is that there is enough voltage for the inverter to start working, so the more modules in series the better.

But then even small shading can hurt the output.

 

If it is all too confusing to work out, then microinverters, 1 module, 1 inverter.

[joth]

Undersizing can help cost efficiency, but very marginal on energy efficiency. It depends which efficiency metric you care about.

A major downside or undersizing is it limits options to add more panels in future. A notable benefit is it maybe necessary to squeak in under G98/99 limits. I think these tradeoffs are much more relevant considerations in practice 

[Alan Ambrose]

There's another factor - these inverters are at times working very hard, and generating a lot of heat internally. Pretty much any bit of power electronics lasts longer when you don't beat the ... life ... out of it.

[TonyT]

https://www.ebay.co.uk/itm/254556014381?var=554264886840&gbraid=0AAAAAB_7XZzG61wSMyptt9T-Mmj6UU3oL&mkevt=1&mkcid=1&mkrid=710-53481-19255-0&campid=5338723872&toolid=20006&customid=0y64364ZAAAAYA5quGnTslCGAwQAAAAAAA
 

Connect a desk fan to it, and keep the magic smoke in the inverter

[billt]

33 minutes ago, Alan Ambrose said:

There's another factor - these inverters are at times working very hard, and generating a lot of heat internally. Pretty much any bit of power electronics lasts longer when you don't beat the ... life ... out of it.

 

Any decent inverter will cope easily with the internal heat, which won't be very much anyway.

 

My ancient SMA SB1200s have an operational temperature range of -25C - 60C and will generate a maximum of 110W at maximum output. That's a trivial amount of heat considering the size of the inverter and the heat sinking. A more modern SB5000TL has the same operating temperature range but is more efficient so would generate 138W at maximum output, but it's a bigger inverter so will run cooler.