#1 May 24, 2014 09:08:00

Dave Stevens
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Inverter size vs array size


Hi, 


I see quite a lot of systems have inverters that are smaller than the array size (apparently there is some industry recommendation to do this?). My own quote was for a 3.0 inverter which i challenged and changed to a 3.6 (£60 diff in price). 


The point is, I've already had several days in my first week where I'd have been inverter limited in my generation if I hadn't requested the bigger inverter.


How many other people get caught out by this?


I know there are other factors to consider (like startup voltage or whatever) which can make the smaller inverter more attractive (ie it kicks in at lower levels) but all else being equal, surely you should get one that can cope with your full output?


Dave.


URN 6737, 3.27kwp, 176deg facing, 30deg slope, no shade, installed 15/05/2014





Edited Dave Stevens (May 24, 2014 09:08:00)

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#2 May 26, 2014 17:35:00

NEC
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Inverter size vs array size


Thanks for posting this Dave - it has prompted me to write in about my issue. I have a 4kw system with an SMA 3800 inverter - initially the installer could only obtain panels of 245 hence the 3800 inverter. Since installation I have upgraded the panels to 250's and installed the 245's onto daughters roof!


My point is a neighbour is from a 4 K system (on really good days) getting over 4 K thro his inverter - I never get more than 3815 max - do I need to fit a 4k inverter is the question and get  the maximum possible from the array. Bearing in mind the performance of the array will start to run down over time anyway any thoughts from the ‘boffins’ please?


 


URN 6699, South facing, 27 degrees, part shade winter months, installed November 2011 - just before the huge FIT (bless Ed Milliband forever) started to shrink





Edited NEC (May 26, 2014 17:35:00)

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#3 May 27, 2014 13:38:00

Jamie
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Inverter size vs array size


Hi both,


Inverter under-sizing is common practice in PV installations, particularly in the UK climate. The main advantage to having an undersized inverter lies in the way an inverter's efficiency varies with the input power. For most inverters, the efficiency as a function of input power looks something like this. As you can see, the efficiency drops exponentially as the input power drops below ~20% of the inverter's rated power. Even though your PV array might be rated as 4kWp, in the UK's climate (which consists of roughly 50% diffuse irradiance) it's likely the power outputted by the array (inputted into the inverter) will generally be much less than 4kW ~90% of the time. As such, under-sizing the inverter means the overall efficiency will be greater since the input power spends more time around the plateau area of the graph referenced above and is less likely to drop to the low efficiencies associated with lower input powers.


More info here.


Hope this helps


Jamie (SSF Team)







Jamie Taylor (Sheffield Solar Team)

Edited Jamie (May 27, 2014 13:38:00)

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#4 May 29, 2014 15:37:00

NEC
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Inverter size vs array size


Jamie, thanks for that info and the useful reference. Makes sense to me now!





Edited NEC (May 29, 2014 15:37:00)

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#5 June 1, 2014 00:45:00

SunshineCoaster
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Inverter size vs array size


With a typical inverter max rating around 90% of the array peak rating. You have to ask/calculate if any “loss” in FiT payment during a few summer days (when a UK array might otherwise briefly exceed the inverter max limiting level) would cover the cost difference of a larger inverter, with the slightly lower efficiency at lower irradiation levels for the rest of the year also taken into account. The difference could be far less than first expected?


The inverter price may not be the only cost difference where the array is at the FiT rate-break of 4kWp, The installer will be concerned to not exceed 16A being pumped into your supply as this could affect more than just MCB/cable sizing, but also the application to the DNO (G83, or G59) who can charge for work associated with evaluation.


Nominally, 16A x 240V = 3.84kW, and although the inverter tending to pull the voltage up may mean at 250V you marginally, only just, deliver 4kW at 16A, you also have to consider the mains at a lower level and if around 230V, the 16A limit equates to a 3.68kW output. If the mains dropped to 220V, which could happen, you'd only get 3.52kW output at 16A and a limit set in the inverter could save you having to reset a breaker. Otherwise, unless the system is monitored very closely, going ‘too near the edge’ could defeat the object and increase chances of FiTs lost due to an MCB trip going unnoticed for a day or so!


If you wonder why this ‘low’ 16A level of current should much matter to the DNO (your house ring circuit probably has a 32A MCB) it is because an eventual large number of micro generators within a given area/sub-station will make it harder for the network operator to maintain power system stability.
See https://www.ofgem.gov.uk/ofgem-publications/43620/dnos.pdf









Edited SunshineCoaster (June 1, 2014 00:45:00)

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#6 June 1, 2014 14:26:00

SteveRogers
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Inverter size vs array size


If you are using/considering an SMA inverter you can download the SunnyDesign software for free from their website.  You can play around with their different size inverters matched to your panel type to see the differences it would make over a year.


With my 3.96 kWp system I have a 3.6 kW inverter (SMA 3600TL).  According to the predictons the equivalent 4kW inverter would produce, as I recall, around 30 to 50 kWh more per year.  However, as a previous post points out you need DNO approval to exceed 16A output.  I'm happy with my system.


Steve


URN 215 North Yorkshire 3.96 kWp, S facing, 40 deg tilt, minor shading





Edited SteveRogers (June 1, 2014 14:26:00)

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#7 July 1, 2014 11:50:00

TerFar
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Inverter size vs array size


It is strange that they deliberately seem to underrate the inverters. Maybe they are more efficient when running at max, so over time you do gain by having an underrated inverter because the efficientcy is higher on days without full sun.


That said, they installed a Samil Power Solar River 3680TL and my power meter has regularly seen 4280W generated. I was suspicious that the power meter was over-optimistic, so I counted and timed the pulses on the official generation meter and that concurred. I haven't corroborated this with the inverter because it's a long crawl down to the far end of the loft!





Edited TerFar (July 1, 2014 11:50:00)

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#8 July 2, 2014 07:16:00

Dave Stevens
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Inverter size vs array size


Systems running at max for long periods dont usually last as long as ones that dont, so an over-rated inverter will likely last longer than an under-rated one that is working at max more. Whether than is measured in months or years, the industry probably doesnt have enough data yet to determine, but this should possibly be a factor in inverter choice formsmaller systems (I appreciate systems nearer 4kw need to be limited by the grid so I'm discounting those).


my own 3.27kw system regularly hits the max on my oversized inverter, so an undersized one would probably have been running at max 3-4 times as often at least.


Comparing my system and others on PVOnline, over 30 day periods, my system outperforms most others by about 8% in efficiency terms (obviously I only have stats since 15th may for my system, and its summer where my system should be better with an overrated inverter, so it will be interesting to see how that works in winter).


to summarise my view, for systems approaching 4kw, I think under-sizing is a given. For system 3.3kw and under, i think its something worth a look, particularly if you have top end panels (like sunpower) likely to outperform if you allow them.


Dave. URN 6737, 3.27kwp, 174deg facing, 30deg slope, no shade, installed 15/05/2014





Edited Dave Stevens (July 2, 2014 07:16:00)

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#9 June 1, 2017 09:13:18

BackYardSolar
From: Surrey, on Gatwick flight path
Registered: 2015-05-30
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Inverter size vs array size

Optimising generation.

There is a lot of talk about the importance of optimising your solar generation usage and that is important and vital to get the best return from your investment.

And what would be the point of having a solar PV system if you didn't use it to its full potential?

But what about optimising your generation, so that you had more solar available for generation, throughout the day.

A inverter needs a minimum voltage available before it can start to generate. So you could have 6, 8 10kW of panels feeding a 4kW inverter. Provided the string arrangement keeps within the inverters maximum DC input voltage no problem.

Imagine 4kW of East facing panels, 4kW of South facing panels and 4kW of West facing panels. This would vastly extend the generating day by suppling your inverter with sufficient power to output 4kW all day. Generation of 50kWh a day should be possible in the summer and far superior outputs in the winter achieved.

This would be a far cheaper solution to fitting a battery, as solar panels have a 25 year life and are maintenance free. Anyone considering a battery would be better off with additional panels first. A combination of the two together would be even better as the battery may even get a charge in the winter.



SolarEdge SE4000 inverter, 16 x Romag powerglaz 235W panels. South facing at 30 degree angle in Surrey just East of Gatwick and under the flight path.

ESS AC coupled system using a Victron Multiplus 24V/1600VA Inverter/Charger - 8.16kW Trojan 875 battery bank.

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#10 Aug. 1, 2017 13:55:00

martint123
Registered: 2013-01-09
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Inverter size vs array size

BackYardSolar
So you could have 6, 8 10kW of panels feeding a 4kW inverter. Provided the string arrangement keeps within the inverters maximum DC input voltage no problem.Imagine 4kW of East facing panels, 4kW of South facing panels and 4kW of West facing panels. This would vastly extend the generating day by suppling your inverter with sufficient power to output 4kW all day. Generation of 50kWh a day should be possible in the summer and far superior outputs in the winter achieved.

I suspect this would fall foul of the “Total installed Capacity” limits - the definition of which seems to change often!

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