Optimizing Solar Panel System Efficiency through Inverter Sizing

Inverter-Efficiency-Profile

by Solar Choice Staff on February 11, 2011

in Balance of System,Installation advice,Inverters

The efficiency of the inverter drives the efficiency of a solar panel system because inverters convert Direct Current (DC) (as produced by the solar panels), into Alternating Current (AC) (as used by the electric grid). This leads many to wonder what effect over-sizing or under-sizing an inverter will have on overall system efficiency. This entry sheds some light on this issue, hopefully helping you to make better decisions with regard to your current or future photovoltaic installation.

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Similar to any other component in the solar power system, the inverter has an optimal operational window within which it performs at its most efficient point. With respect to the graph above, where the red-line represents an average inverter efficiency and the green arrow suggests the size of power output from your solar panels, the grey box becomes the operational window of the inverter based on the input from the solar panel and the predetermined inverter efficiency.

Under-sizing your inverter

Using the graph above as an example, under-sizing your inverter will mean that the maximum performance of your system will be dictated by the size of your inverter and regardless of how big the output of your solar panels is, the output will be cut-off by the inverter. On the other hand, if the inverter is sized with the ‘average power received on-site’ then some money can be saved by choosing a smaller inverter (‘average power received’ being a calculation based on meteorological data and estimated solar panel efficiency at those conditions). However, under-sizing your inverter can also lead to overheating and shorter life-span of the components in the system.

Over-sizing your inverter

When over-sizing your inverter, the efficiency of the system (as seen in the example graph) depends on what your average point of operation is (which in turn is based on the size of your solar panels). Thankfully, most inverters have been designed for this. This means that when the inverters were designed, the inverter companies predicted that a natural choice would be to oversize the inverters and, as seen in the graph above, in this case this means it is in the optimal operation range between 85-95% efficiency. Furthermore, if you do decide to add more panels to your existing installation the most you’ll loose is 10-15% of total system efficiency.

Written by Prateek Chourdia

MEngSc – Photovoltaics and Solar Energy, UNSW

Solar Energy Analyst

Solar Choice

© 2010 Solar Choice Pty Ltd

{ 34 comments… read them below or add one }

graeme May 29, 2016 at 7:39 pm

I have 3kw inverter and 3 kw panels all facing north in melb.
In mid summer the sun sets west/southwest (WSW) so can I instal a second (1.5 KW) group of panels to that same inverter? The panels would be facing that WSW direction to operate while the north facing 3kw panels are functioning at low /very low capacity late in afternoon. Facing that direction I expect the inverter to be not overloaded. Your thoughts please? (Both technically and regulatory).
Thankyou.
Graeme.

Reply

Solar Choice Staff May 30, 2016 at 9:28 am

Hi Graeme,

The first question you need an answer to is whether your inverter has an input for a second string of panels (and associated Maximum Power Point Tracking – MPPT). If your inverter only has one string input (or the current array uses both inputs), it’s not a good idea to add more panels facing a different direction – you’ll end up losing efficiency. If there is an additional MPPT input sitting unused, however, it might be possible and even worthwhile – consult with an installer about the technical details (although I have doubts that you’d be able to add on another full 1.5kW, as 30% ‘overcapacity’ is the maximum they recommend in the Australian standards for PV installations).

Another option would be to install a second, separate system facing west with its own dedicated inverter, but adding on to the existing system (as discussed above) would be preferable, finance-wise.

As for the regulatory aspects, you won’t be eligible for the federal STC ‘discount’ if you add new panels, but it will apply if you add a whole second system (including inverter). Additionally, if you’re currently on a state-sponsored solar feed-in tariff then you migh forfeit it by increasing your existing system size or adding on another system – it depends on the rules in your state.

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HEMANT May 12, 2016 at 4:01 pm

I intend to put up a hybrid system which shall have dependence on solar as well as grid power. Due to space constraints I may be able to install only 1 KWp solar panels. In this scenario, is it OK if I have 1 KWp solar array but 3 KVA solar inverter (with battery bank). Any deficit in solar energy shall be met from grid.

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Solar Choice Staff May 30, 2016 at 11:17 am

Hi Hemant,

Probably not a good idea to undersize your solar array too much – most inverters require a certain power signal from the panels in order to ‘switch on’, so the efficiency losses could be significant. Best to speak with a certified solar installer, but my feeling is that you should opt for a smaller capacity inverter for the PV – and maybe a separate battery inverter for the battery bank.

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Fanie Coetzee February 22, 2016 at 4:33 pm

Dear Sir / Madam

Why do you need a PV Array with a total voltage output of 600VDC supply a 380VAC; 11kW Inverter? What is the importance of the 600VDC?

Best regards
Fanie

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gabriel January 19, 2016 at 5:15 am

Sir,
Please,can I use 2.2kw iron on 5kva 48v inverter comfortably for laundry work.

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Solar Choice Staff January 19, 2016 at 11:36 am

Hi Gabriel,

It would depend on the time of day and where you’re located. If you’ve got a lot of sun and your panels are at peak production (about 2 hours on either side of midday) you should have enough energy to run the iron. Of course you might have problems if the sun passes behind a cloud or on days with inclement weather. Batteries could be a solution for you.

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S Kayani November 1, 2015 at 1:33 am

I have 20 panels of 350 w each, which comes to 5000 with. Can I install 2 hybrid inverters of 5 kva each to enhance the output

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Solar Choice Staff November 2, 2015 at 9:44 am

If your panel array’s peak output is 5kW (5000W), then you should have an inverter that is roughly classed for that figure. It’s not uncommon practice for installers to oversize a solar panel array relative to the capacity of the inverter, but it’s doubtful that many of them would recommend 2x 5kVA (~10kVA) for a 5kW solar array – an example of gross inverter oversizing – unless the plan was to eventually fit the system out with more panel capacity.

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Trevor October 31, 2015 at 2:21 pm

Hi…I have a 3kw system with 12x250w panels and a 3.3kw inverter(panels facing north for almost all day sun,dropping off in arvo).
the most I have ever seen it produce at once is 2.2kw (14-16kw a day)..The inverter is about 15 to 20 metres from panels and have been told this is the reason it only produces that…I am able to add on more panels (another 5 im thinking on afternoon sun only roof)as the other panels drop thes will pick up in arvo, my only worry is this going to damage inverter if it sends too much dc current to it…its only converting 2.2kw now out of 3kw panels..they told me excess would just get dumped ?
please advise….thankyou

Reply

Solar Choice Staff November 2, 2015 at 9:36 am

Hi Trevor,

You’re correct that the distance between panels and inverter and the resultant voltage drop could be the cause of the relative inefficiency of your system. We ordinarily estimate that a solar energy system is about 80% efficient from panel to socket – for a 3kW system, that translates into a peak output of about 2.4kW. The voltage drop could be further dragging it down to the 2.2kW that you’ve been witnessing – a total panel-to-socket efficiency of about 73%, which is not outlandish at all.

Oversizing the panel array to maximise and ‘flatten’ a system’s output is a fairly common practice. This could be an option for you – especially if your inverter has a second MPPT (maximum power point tracker) input which is currently unused. I would recommend speaking to an installer or post-installation specialist (SolarSafe is one company that we know of that provides post-installation services) about exactly what your options are and what the benefits would be.

Best of luck!

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Anthony October 20, 2015 at 5:28 pm

Have a question, we had a company install a 4KW 16 panel system on our house last year, this is not leased. So far we have seen none of the savings they promised, per the solar company our peak usage has increased which is causing the lack of savings. We have not changed any of our daily or house utilities and we are on a set thermostat and have added, blinds, insulation and anything else. Any ideas or suggestions to debate them? Any additional suggestions are appreciated as we are not paying the same utility bill on top of paying the bill for the system.

Thank you for you time.

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Solar Choice Staff October 28, 2015 at 12:22 pm

Hi Anthony,

Sorry to hear about your issues with your system.

Do you have anything in place to monitor your electricity usage throughout the day? For example, a monitoring system? This would provide you with some data to present to them.

Could you possibly switch over to a flat-rate retail tariff instead of time-of-use? Generally speaking, that is the best way to get the most out of a standard solar energy system, provided you shift your electricity usage to the sunlight hours.

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shivakumar October 16, 2015 at 10:34 pm

hello i am designing a solar station of 3kw for the load demand of 5940Kwh/ day with an autonomy of 3 days from 48V, 600Ah battery bank. is there any fault in the design? plz justify

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Merv October 13, 2015 at 1:33 pm

Hi,

Hope you can spread some light on an issue that I am having. We have just connected my power monitor to our meter box after getting solar installed (approx 6months). My reason for this is we were getting massive bills after the digital meter was installed, I have been using an iPad APP to record the numbers everyday and the numbers are not making any sense. Here are some details of our setup (in basic terms..lol)

Solar Spec:
SMA Sunnyboy Inverter (I believe from memory is a 3.5Kw Inverter)
24 Panels in 2 x arays (NNE and NNW) as the house is not quite N.
Produce rought 20Kw / day (as of the last 2 weeks)

Inverter reads: Producing – 2945W
Monitor reads: Solar – 2867W
: Household – 2390 (day)
: Household – 17-23 (night)

My issue is that with the inverter turned on and producing power our consumption goes up as well. The monitor with the solar turned off we are only using about 14W/h.

Is this showing correct numbers or do we need to contact the installers and have them out again. Hopefully not as we made a poor choice on installers and they haven’t been the best of businesses to deal with.

Thanks for you help.
Merv.

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Maggie July 27, 2015 at 12:44 pm

Hello,

We currently have 3 x 12volt solar panel for our boat (two x 80 watts and one x 60 watts).

We are planning to expand the solar panel by installing a 190watts solar panel. The question is, what should the size of inverter be for optimum result?

Thanks very much

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Kavunja August 13, 2015 at 11:32 am

Specifying the value of the panels in volts will be a bit confusing. The possibility of maintaining 12V DEPENDS ON THE KIND OF INTERCONNECTION whether series or parallel. Your current 260w panels added to 190w will be optimised if you contact your installation tech and explain to them what your expansion needs are. Thanks Maggie

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Nico July 6, 2015 at 5:20 pm

Hello.

Energy Australia sold us a 3.9Kw system (15*260W panels) These were fitted with Enphase M215 inverters. I was playing around with API calls to my Envoy unit and the Enphase website, and the system reports it’s size as a 3.225Kw system. (This is exactly 15* 215W). During summer months my generation flatlines at 3.23 Kw per hour, which seems to indicate that the inverters are the limiting factors in my energy production.

Questions:
Should this legally be sold as a 3.9Kw system, with it having a 3.2Kw inverter, and effectively being able to only ever produce 3.2Kw?

The inverters are rated roughly 17.5% lower than the panels. Is this a significantly large enough number to cause overheating and/or other issues with any components?

Thanks,
Nico

Reply

Solar Choice Staff July 7, 2015 at 6:00 pm

Hi Nico,

You’ve probably already read our response to Robert’s comment above – it is acceptable to have the panel array oversized to ‘flatten out’ and ‘fatten’ production, even if this means clipping peak production. The Clean Energy Council’s guidelines recommend that inverters should be rated at no less than 75% of panel capacity, and it sounds like what you have is within this limit.

The above aside, we can see why you might not like the idea of a system being labelled as 3.9kW in capacity when it never reaches that level of power production. But hopefully the installation company knew what they were doing and sized the system as they did in order to maximise overall power output. What are the daily production numbers for your system in kWh, as opposed to kW?

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Joseph August 22, 2015 at 1:14 am

Hi Nico,

I just had *installer name removed* install 8 JA Solar panels with 8 x M15 micro inverters and said my system is a 2.15kw when really it is a 1.72kw system as the inverters are rated at 215 watts. Out of curiousity how much did you pay for your system? I paid $4875.

Cheers,

Joseph

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Robert May 18, 2015 at 12:16 am

My installer just installed a 7.6kw inverter. My system I had installed is a 9.6kw. Is this the right inverter for this size system?

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Solar Choice Staff May 18, 2015 at 12:09 pm

Hi Robert,

We’ve just had a chat with Nigel Morris of Solar Business Services about this and here’s his response:

Yes oversizing the array is ok in some circumstances.

Its ok as long as the current limit of the inverter is not exceeded.

The good part is that is can flatten out the generation curve and thus maximise production, albeit at the expense of clipping some power during the middle of the day.

So basically, array oversizing is a common practice, but whether it’s being done correctly in your situation would depend on the specifics of your system and components. If you’re concerned, we’d recommend getting a second opinion from another installer.

You can read more about array oversizing in this informative article from Climate Spectator.

Best of luck with your system.

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Joseph April 18, 2015 at 2:36 pm

I have 14 -CS6X Canadian Solar 300P Modules with a Sunny boy 3800 Inverter. This peaks early in the day and stays their for a a good while, I this to small of an Inverter ?
Thank you,
Joseph.

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Solar Choice Staff May 5, 2015 at 11:01 am

Hi Joseph,

Thanks for your comment. We’d advise you get in touch with your installer if you think there might be some kind of issue – but yes, there is a possibility that your inverter is limiting the output of your solar array.

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Trevor Stover March 17, 2015 at 8:39 pm

we have a 5kw system and my power bill went from $600 to $1200 and know one can tell why

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Solar Choice Staff March 18, 2015 at 10:46 am

Hi Trevor,

Can you describe in further detail when your bill went from $600 to $1200? Did this change happen between your last bill without a solar system and your first bill with a solar system? Or are you referring to a sudden drop in solar electricity output (and associated increased power bill) after you had the system was installed and presumably working without problems for a while?

It’s hard to help you figure out what the problem might be without knowing these things!

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Vivette March 9, 2015 at 5:10 pm

HI ,we are currently somewhat confused. We engaged a solar company to install a system with the intent of connecting stand alone battery system in a year or two, because it was indicated to us that the cost of the batteries would come down significantly due to an American company setting up for large scale production. Our average useage on our power bill was 65kwh /day, so a 15Kw system has been installed, with a ZED controller however we were not informed of the issues around export control. We are on 3 phase power and are told that the export has to be restricted to the lowest use phase. System is currently limited it seems to covering about one third of power consumption.
The installation company has now offered to remove the installed system and install a 5Kw system instead with 5kw Ingeteam inverter and EPC control unit (more efficient they say). We are unsure now as to how to proceed, do you have any comments or advice you can offer in this regard?. Do you have any current knowledge around the quality , availabiltiy, pricing, future pricing of the stand alone/ off grid battery power storage packs?.

Reply

Solar Choice Staff March 13, 2015 at 1:39 pm

Hi Vivette,

Sounds like a confusing situation indeed.

First of all – it sounds like the system was inappropriately sized from the outset. We constantly emphaise that a solar system should be sized to meet only the daytime electricity needs of a home – this is because no utility offers a reasonable rate for excess solar power these days (and in fact, utilities in Queensland actually prohibit excess solar power from being exported to the grid).

Unless you were going to have an energy storage system installed at the same time as the panels, it really wouldn’t be worth your while to wait for batteries to become more affordable – that’s about 2+ years of ‘wasted’ excess solar power. And as of now, it is still a ‘wait and see’ game for energy storage affordability – despite lots of talk about the upcoming viability of energy storage, there aren’t many homes or businesses yet making the decision to have batteries installed.

If they are offering to do so at no additional cost (and refund you the difference on the price between the two systems), it would probably be best for you to have them install the 5kW system in the place of the current system – unless you find an energy storage option that makes financial sense for you.

Best of luck!

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Mick Ward January 28, 2015 at 5:18 am

Is there any law to stop me having a 4KW system and a 4KW inverter on a domestic setting

Cheers

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Solar Choice Staff January 28, 2015 at 12:17 pm

Hi Mick,

There certainly shouldn’t be anything that prevents you from having a system of this size – in fact, 4kW solar systems are quite common for domestic applications in Australia.

Depending on where you live, you may need to install some sort of export control device. This would be if you’re in regional parts of WA or Queensland. But provided you meet the local requirements it shouldn’t be a problem to have a 4kW system installed.

Are you running into some kind of issue?

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Dishant January 8, 2015 at 7:54 pm

I am thinking of buying 5KW inverter for 3 KW SYSTEM, to begin with just so in future if i choose to upgrade, i have an option. My question is, does it affect the performance of panel n inverter if its not used at its full capasity??? In my case 5KW inverter bein used for 3KW system. Let me know please. Thanks.

Reply

Solar Choice Staff January 19, 2015 at 1:48 am

Hi Dishant,

We generally advise against getting an oversized inverter for future expansion of your solar panel array. There will be efficiency losses when you do this, and it’s highly unlikely that most people will actually expand their system in the future. On top of this, if you do want to expand the system, you’d have to try to make sure you get exactly the same brand & output of panels–something that might be hard to do further down the line.

Read more on this topic here.

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Bill Fletcher January 5, 2015 at 12:08 pm

I did only read about 1/2 the above but now I really have a concern.
3kw panels with 3kw inverter has been performing great last 3 years with max kw noticed last summer at 3900kw, all well and good, but now this summer I have suddenly noticed over 4kw with full sun on every day, I have watched when clouds have been active and kw down to 1+kw but then up to over 3 and 4 kw and staying there. I know enough about electricity to know what I don’t know and this has me confused and concerned, I don’t understand how, help please

Reply

Solar Choice Staff January 6, 2015 at 3:58 pm

Hi Bill,

Thanks for reading and for your comment. That does sound pretty unusual – if you only have a 3kW array you shouldn’t be producing more power than that. Your inverter is highly unlikely to be able to handle more than its rated capacity, although it would really depend on the specs of the model that you have. At the risk of sounding slightly silly (please don’t take offense), are you sure you have a 3kW system and not a 4 or 5kW system? Just taking a shot in the dark here, but usually system will not perform above their rated power output, especially for extended periods as yours appears to be doing.

An alternative explanation might be that you’re somehow reading the DC output of the panels as opposed to the AC output of the inverter? This is also pretty unlikely, however.

We’d recommend you get back in touch with the company that installed your system or the inverter manufacturer – they should be better equipped to address your concerns.

Reply

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