Chosing a PV inverter size.

ThisIsWeird

Hi.

I understand that inverters are, counter-intuitively, chosen to be pushed to their limits in order to maximise operational efficiency, and my 3.2kW system is, for this reason, handled by a 3kW Aurora. But just how much loss would there be to go the next size up, tho' - is there an actual %-age figure to suggest the loss in generation? And aren't there some 'pros', such as a 'stronger' machine, less likely to fail over time?

Thanks.

QrizB

I don't have an answer to your question but when the 2.5kW inverter on my 2.72kWp system failed earlier this year, I replaced it with a 3.6kW one. I could've bought a 3kW but at the time the bigger inverter was (counter-intuitively) somewhat cheaper.

I've definitely seen higher peak outputs on the current-clamp-type monitor I use on the V output. The old inverter would get to 2.68kW but the new one has, occasionally, exceeded 3kW (with cool panels and cloud-edge effects).

It's far too soon for me to have any comparative data, and I suspect any difference (for better or worse) will be lost in the year-to-year variability of the weather.

If you're interested, I bought a Solis S6 mini (I only have a single string, but needed an inverter with a DC switch). I also splurged another £65 to buy the data logging stick (so I don't really need the current clamp any more).

ThisIsWeird

QrizB said:

I don't have an answer to your question but when the 2.5kW inverter on my 2.72kWp system failed earlier this year, I replaced it with a 3.6kW one. I could've bought a 3kW but at the time the bigger inverter was (counter-intuitively) somewhat cheaper.

I've definitely seen higher peak outputs on the current-clamp-type monitor I use on the V output. The old inverter would get to 2.68kW but the new one has, occasionally, exceeded 3kW (with cool panels and cloud-edge effects).

It's far too soon for me to have any comparative data, and I suspect any difference (for better or worse) will be lost in the year-to-year variability of the weather.

If you're interested, I bought a Solis S6 mini (I only have a single string, but needed an inverter with a DC switch). I also splurged another £65 to buy the data logging stick (so I don't really need the current clamp any more).

Now that is interesting! 

It may well be - 'cos so many installers say so - that having an inverter running on the edge could be slightly more efficient overall, but your experience would seem to suggest that a smaller inverter's designed output would then limit the panels to this amount, even if the panels were capable of outputting more? So swings and roundabouts.

So, possibly a touch more efficiency overall, but ultimately limited to the inverter's max rating.

What make of inverter did you have that failed?

I read good things of the Solis, but would need a dual-MPPT, and that would be a higher kW model than my current 3kW Aurora - but happy to go for that.

Thanks.

QrizB

What make of inverter did you have that failed?

A Kaco Powador.

Not a major manufacturer, even back in 2012 when the system was installed.

It lasted 11.5 years which I was generally content with. I considered getting it repaired but I when I saw that I could have a brand new replacement for under £500 I stopped looking for a Kaco service agent!

Exiled_Tyke

Again I don't have an answer but I did read a report (annoyingly I can't find it anymore) from an industrial PV set up where they stated that inverter size was optimum at about 75-80% of maximum output. I suspect it also varies according to where you are. Regions where you're likely to spend more time near the max will do better with a larger inverter?

Martyn1981

ThisIsWeird said:

Hi.

I understand that inverters are, counter-intuitively, chosen to be pushed to their limits in order to maximise operational efficiency, and my 3.2kW system is, for this reason, handled by a 3kW Aurora. But just how much loss would there be to go the next size up, tho' - is there an actual %-age figure to suggest the loss in generation? And aren't there some 'pros', such as a 'stronger' machine, less likely to fail over time?

Thanks.

Hiya. Going back about 10yrs+, SMA the main supplier of inverters at the time, did some testing. I can't remember the exact numbers now, but for a matching 5kWp systems they used a 3600TL, a 3800TL and a 4000TL inverter (some of the best at the time).

As I recall, the 3800 had the highest generation over 12months, with the 3600 in second place, and the 4000 in last. But there wasn't a huge difference overall, memory fails me, but I think it was just a few percentage points.

The idea for UK style weather, is that the inverter won't be running hard most of the time, so having a smaller one, will allow it to operate at slightly greater efficiency during poor weather, and also start up sooner, and stay operating a little longer, when generation is very low. In sunnier climes, you wouldn't worry so much about low generation, and cloudy weather as much, and would want to match inverter to panel capacity.

For the UK, the rough guidance is to undersize the inverter by 10-20%, but even that will vary. A shaded E/W set up would allow for a smaller inverter, but south facing, on the Cornish coast, with great sun, and cooling breezes, would suggest minimal undersizing.

Undersizing will mean some capping when the sky is clear and temps are low - this is often best seen in April and October. But during long sunny days (May-Aug), the higher air and panel temps will probably mean that panel efficiency will be down 10%, and on very hot, still and clear days, especially in June and July, they may be down 20%-30%.

Only my guess, but for UK weather, the inverter may not be getting stressed hard enough / long enough to make too much difference, so length of service may be the main factor for failure, but tbh, really hard to know. But for your set up of 3kW / 3.2kWp, it probably doesn't matter too much, as it's a close match, so probably a good compromise either way, sitting nicely in the middle.


Edit - Also going back 10yrs+, there was the argument that panel degradation, say 1%pa, and losses from dirty panels, would also support the use of a smaller inverter. But 'lovely' UK weather (rain) seems to keep panels clean enough, and degradation seems to be tiny. After 12yrs, I still can't spot any degradation in my annual generation figures, so it must be tiny, perhaps 2% or less in total - much to my surprise and pleasure.

ThisIsWeird

That seems a good summation, Martyn, and it does make sense - thank you.

If the overall loss - through using an overrated inverter - is anything like 10%, then that's probably enough reason to not go there.

Martyn1981

ThisIsWeird said:

That seems a good summation, Martyn, and it does make sense - thank you.

If the overall loss - through using an overrated inverter - is anything like 10%, then that's probably enough reason to not go there.

Have tried to see if I can find the old SMA test, but I suspect I saw the details on a now defunct renewable energy site, and can't find it. But did find an interesting / fun old study from SMA in 2015:

7 Reasons Why You Should Oversize Your PV Array

This bit surprised me:

However by oversizing PV arrays, it may be possible to achieve almost the same annual energy output with fewer installed inverters. For example, using Sunny Design, a 100kWp PV array with three STP25000TL-30 inverters (i.e. 75kW of inverters) would only produce ~2% less annual energy compared to the same PV array with  four STP25000TL-30 inverters (i.e. 100kW of inverters). This means that there is only a ~2% lower energy output for 25% fewer inverters.

I appreciate that 75-100kW of inverters on a 100kWp array, is not quite what we're talking about, and this example is taking things to more of an extreme with 25% undersizing - but I was surprised to see the difference (loss in this case) is only 2%.

I think the set up comparison can be compared to running 33.3kWp of PV through a 25kW inverter, v's 25kWp through a 25kW inverter. It suggests to me that the sweet spot is somewhere above 75%, but below 100%, in this example.


Ironically, due to the sizes available at the time, my two inverters are slightly oversized, with 2.2kW on the 2kWp system, and 3.68kW on the 3.58kWp system. And me, just being me, would have thought it better to pay a little more, to slightly oversize an inverter ........ perhaps like yourself, before I learnt more, especially when we take British weather into account.  
 

ThisIsWeird

Thanks again. 

Really interesting, and I'd love to know the physics behind it - tho' probably OTT.

To sum up, it looks as tho' you can significantly under-size the inverters with very little overall loss of generation (and potentially save a bit on the inverter cost...), but are there similar studies to give an idea of the potential loss of generation by going oversized with an inverter?!

I'm guessing you haven't noticed any significant change to your system, having gone 'oversized'? (Impossible to tell with weather variations, I'm sure!)

Martyn1981

Yep, impossible to tell, without an exact match to test against. But I do have generation estimates from PVGIS, and over the last 12 years my actual generation has been between 97% and 110% of estimate, with 3 years under, and 10yrs (including 2023 so far) over. PVGIS is slightly generous, as it estimates losses a bit high on the default settings, but at the same time, I haven't allowed for any shading, despite both my systems having a bit. So looks good, even if it might have been a tad better.

In fact, thinking about it, two of my low years are 2011 and 2012 with 97% and 99% respectively. Since then I've replaced the two inverters on my 3.58kWp (a 2.5kW and a 1.2kW) with that single 3.68kW unit. The larger inverter has an estimated efficiency of 98%, whereas the weighted average of the smaller two, was 94%.


That's a fun question about over-sizing, and you sent me down a rabbit hole. I can't seem to find numbers for comparisons with an over-sized inverter, as the tests seem to show that some undersizing is better than a 1:1 ratio. So that may mean that the assumption is that it will only get worse, as larger inverters gets less and less efficient.

I eventually found one comment 'when is it a good idea to oversize an inverter' ....... hooray ..... but the answer was simply, when you plan to expand the PV array in the near future. 


I thought this paper was interesting, especially the number of caveats, desperately trying to point out that there is a lot to consider, no 'one-size-fits-all' solution.

Why array oversizing makes financial sense

[Note - Yes, I know that I keep saying 'interesting', whilst most people would not find any of this interesting.  ]

ThisIsWeird

It appears that it comes down to individual circumstances, such as panel orientation and where in the country or world you live.

From what I understand, inverters work most efficiently when run at above 80%-ish of their capacity. So, for a 3kW PV array connected to a 3kW inverter, that would be when the PVs are outputting ~2.4kW and above into the inverter. Below that output, efficiency is less than optimal. 

For much/most of the day - morns, late afas and eves - this max efficiency is therefore unlikely to occur, at least for most of the year. For the times around noon, it hopefully will. 

But if you fit a lower rated inverter, then the '80%' threshold will be correspondingly lower, so it'll be running at its max efficiency at lower PV outputs - it'll maximise morning, afa and eve efficiencies and gains, and may even switch on earlier, and turn off later.

The obvious counter to this is, at peak generation times - noons - the smaller inverter's capacity will now be be exceeded. How does the inverter cope? Does it go fizzzz? No, it 'clips' the incoming power to the level it can handle. They can cope with some flurries above its rated size, but usually only for short periods. So, they won't - shouldn't - fail from being overdriven, but it also won't be capturing the highest noon PV outputs as these will have been 'clipped' and effectively lost.

In essence, it's widening the max efficiency band away from just the noon peaks to grab more from either side, but in turn sacrificing the very max output from the panels during the noon peaks. Swings and horses. It's 'reckoned' that slightly undersizing the inverter will maximise overall gains.

My take on this is that undersizing is best left to those who chase max outputs, and is unlikely to make a significant difference to the annual production. And, since the smaller inverters can only cope with peaks above their rated capacity for short periods - often just blasts of 10 minutes or so - it's pretty clear that this is planting more electrical and heat stress on the components, which is just not a good thing in my book.

Anyhoo, it looks as tho' I have just bought a brand new 3kW Aurora Power One - the same as my current faulty unit - for £50! It was collection only, which really cuts down the market, but he was happy for me to arrange and pay for a courier to collect it from him (it's safe in its original box). I also told him I wanted to give him £150 for it, as it was just too cheap - he's never had anyone try and up the sold price of anything before. A bargain for me too - I was anticipating having to pay more than that for a 2nd-hand unit.