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
{ 35 comments… read them below or add one }
Hi,
Do these comments apply whether you are grid connected or not? We have a lot going on here and use about 60+ kW-hrs per day; I would like to substitute some with about 5kW renewable. Can I use a 5kW inverter and take what else I need from the mains, or will my inverter have to cope with my stick welder, water pump, air conditioner, pool filter, deep fryer etc. on its own?
Hi Colin. Thanks for the comment. I’m not quite sure what you mean. Have you already got a system installed? And where are you located? Depending on which state you are in you have either a net- or gross- feed-in tariff. With net feed-in tariffs, you will get greater returns if you use less electricity while you are producing it, whereas with gross you get paid a flat rate for the amount produced, regardless of how much you use or when.
The comments apply for both grid-connected and stand-alone systems. In grid-connected systems, following this advice will ensure you get the best returns from feed-in tariffs, and in stand-alone, that you get more power from your solar panels to run your electronic devices, etc. Of course if you have a stand-alone system you face the possibility of actually exhausting your power reserves and your devices not working because your batteries have died or your panels can’t produce enough to keep up with your consumption. Stand-alone systems need to be designed with your expected use in mind.
As long as you are grid-connected you will be provided with electricity, even if it is excess of what your system is producing. Your inverter’s job is simply to convert what your panels produce into usable AC electricity and feed it into the grid (or your batteries, if you have a stand-alone system.)
I recently installed a 1.52 PV solar system at home and a Efergy meter was added to the system as well. The inverter is a Latronic 2500 cause I plan to install extra panels later on to get a 3.0Kw system and that was that the company recommended.
I know that the system is not 100% efficient, and the output varies during the day, but even on sunny and relatively cool days the reading doesn’t get close enough to the 1.52 Kw.
Is this has to do with the oversize of the inverter or are the solar panels not effcicient enough.
I live in Springvale South, Melbourne, Vic.
Thanks for your comments.
Cheers
Julio
I have the same question as Julio, we have 6x 180 Watt Panels and a 3kW Sunny Roo inverter (for future expansion), however even with the sun measured at 90 degrees to the panels we still only get just barely over 1000 Watts out according to the inverters own meter, voltage is close to 320v. Panels are on a 22 degree roof 3 degrees east of North, location is 30km North of Brisbane. Is it due to an oversized inverter? Thanks
James
sorry, that was meant to be 8x 180 watt panels (1440 Watts total)
James
Hello James,
Your array is 8 panels x 180W, which would ideally give you 1440W if it were 100% efficient, which no system is. There are a number of factors that could be causing this unsatisfactory performance, and one of them is indeed the fact that your inverter is oversized. Looking at product specs for the inverter you specified (you can find your inverter listed on this page), it seems that in ideal conditions, i.e. if your inverter and panel array were perfectly matched, which they’re not, you could expect 96% efficiency from the inverter. That would mean an actual capacity of about 1380W, but as you can see in the chart at the top of the article, which although not specific to your inverter is a typical representation of an inverter efficiency curve, efficiency drops off fairly quickly the more undersized your array is compared to your inverter.
So in essence, factoring in other inefficiencies (such as derating for dirt on the panels, the effects of overheating, wiring losses, etc) in the system which are possibly exacerbated by those brought about due to the extra-large inverter, it is not unfeasible to imagine the sort of production that you have mentioned. Hopefully the efficiency of your system will improve significantly once you install the rest of the panels! Good luck with it!
I m looking at the same system as Julio, but the feedback I am getting from friends who have similar they are not happy at all with their systems.
What do you think… Regards Dennis
I have a new 10 kW system, but it is only peaking at about 6 to 7 kW in the middle of the day. The panels are on a tracker which is not yet able to rotate (technical issues are currently being fixed), but the panels have been manually aligned to their optimal true north facing angle for this time of year. The inverter is a CMS10000 which has a maximum capacity of up to 11000, and an efficiency of over 95%. There is no shading other than clouds.
Is it true that the voltage from the brand new transformer being too high, could decrease efficiency. Voltage is about 255 to 259 volts, and the transformer is due to be “tapped down” this week (hopefully) so that the voltage will only be +/- 5% from 240 volts. Will the optimal output increase once the transformer is “tapped down”?
So does that mean you are having the transformer within the inverter replaced when you say you’re going to have it ‘tapped down’, or are you having it adjusted internally?
After accounting for all the inefficiencies that exist in a solar power system like you have, it’s not really a huge surprise that you are producing only 7kW in peak conditions–presuming, of course, that by 10kW system, you mean that your solar array (the panels themselves) is 10kW?
No it means that the transformer in the street has multiple “taps” for the incoming feed and they represent different numbers of windings being used in the transformer to get the right voltage out the other side.
The reason why all of you are getting less than max output from the inverter is that solar panels make their STC rated power under STC conditions, which in most instances is less than 0.1% of the time. The remainder of the time the light is less ~800W/m2 and the temperature is much higher -like 40degrees higher resulting in a ~20% loss in performance. (0.5%/degree of temp rise).
ORER quote an average of 4.4kWhrs/Kw of installed panels in Perth per day, if the system made a full kW from sunup to sundown that number would be 3-4times higher but it isnt because it takes into account real world performance and systems should be judged in comparison to ORER’s guide for your location, adjusted for knockdowns for tilt and orientation.
I have just had 16 x 190 watt solar panels fitted and using a Aurora 5000 enverter so I can add an additional 10 panels if I need more power. THE PANELS ARE SUPPOSE TO PRODUCE 3.04 KW. With the rated efficiency 90% this is reduced to 2.736kw. AND the inverter efficency down to 2.653 kw.
On the day it was installed and switch on in the pm it peaked at 2530 watts and produced 6kw that day then I had the following readings for the next 6 days the first 3 having a few clouds around and the last 3 days completly clear skys.
PEAK KW PRODUCED
2715 12.7
2780 13.8
2806 14.2
2256 14.4
2232 14.3
2197 14.1
Now the question why on the 3 completly clear days with lower peaks did the unit still only produce about the same kw? at the peak power days AND why on the completly clear days di the peak power output drop by around 600kw ? shouldn’t the peak be higher on clear days? One would think the peak and then the power output on a clear day should be higher? Even the electrical firm who installed it contracted to the company could not explaine this and the rep from the company was no better saying he would be the electrical company to come and check everything and for me to ring Power One for info on the Aurora inverter?
What do you think is the problem ?
What output in kw a day should I expect out of such a system.
Hi Laurence,
Thanks for the comment. That’s sounds a bit complicated, but there could be a good explanation.
First of all, the output you can expect from your array depends on your location–where are you located? Different locations get different amounts of sunlight.
Second, it’s important to distinguish between kW and kWh. A kilowatt is a measure of power (how much we can produce)–systems are usually rated in kilowatts. If the sun shines on (for example) a 1kW panel for one hour, your system will produce 1kWh of electricity. So you won’t measure the output of your system in kilowatts, but kilowatt hours. A kilowatt hour is a measure of energy–how much is produced.
The important numbers that you should be looking at daily are the kWh produced: this is how much you use can use for home consumption or get paid for on a feed-in tariff. The ‘peak’ production number is the highest
You have a relatively large system, and according to those numbers it seems to be operating more at its anticipated output–2.653kW. The ‘peak’ number is the highest point of production that your system reached in that day–it doesn’t measure any thing across the day, just one moment. The disparity between your ‘peak’ numbers in the left hand column and the kWh produced in the right hand column could be explained by partial cloud cover: the partially cloudy days may have been cooler, which would have meant better performance from your panels when the sun was shining, but less during the moments when the sun was blocked by the clouds. Hence your higher peak for the day but lower energy output on the first 3 days. Day three may have been cool with lots of sun.
You’ll also notice that the next 3 days are the opposite: low ‘peak’ numbers but high energy output (kWh). This could mean that that the high heat from the constant sun brought down the peak productivity of your system, but that you got steadier production through the day thanks to the lack of cloud cover.
This is just guesswork, as I haven’t seen your system, of course, but it could explain what happened. It doesn’t sound like your system is functioning badly at all, and it’s unlikely a problem with your inverter.
Hope you found this helpful.
Hi,
We live in Norwood SA and our ordered panels will be facing due north at 22 degrees pitch. My original quote was to install 8 x 190W MONOcrystalline panels with a 2500 watt inverter. They are not yet installed however, the company has been very keen to replace the inverter quoted with a 1700 watt inverter. First off they offered me $500 discount to take the smaller one, which I declined. Now they are offering me 8 x 205 W POLYcrystalline panels with the 1700 watt inverter with no price difference. They tell me that I will get 5% increased efficiency but won’t be able to add on to my panels. I have seen conflicting information on the efficiencies of both mono and poly crystalline panels. Would you be prepared to offer an opinion on whether it may be more beneficial to go with the larger inverter and put more panels into the system when I can afford it, or go with the 5% increase in efficiency and settle for the smaller inverter?
Thanks
Hi Andrew,
Thanks for the comment. Sounds like a tricky situation.
First of all, there is commonly a lot of misunderstanding about the difference between monocrystalline and polycrystalline solar panels–the difference between mono- and poly-crystalline technology is not so important as the merits of the particular brand and model of solar panel that you are considering. What brands are on offer to you? (You might be able to compare their performance on the Desert Knowledge Australia Solar Centre website.)
Secondly, whether to go with the smaller inverter or not will depend on your household energy consumption and the financial viability of adding on panels later in the life of the system. What state are you in? Be careful: if you are on a feed-in tariff, some states do not allow for a system to be upgraded later–doing so may actually void your eligibility for the feed-in tariff. This is an important thing to think about, especially considering the fact that taking the smaller inverter will (according to the installer) offer you greater efficiency, but it will also lock in the size of your system.
In summary, if you are going to be on a feed-in tariff and live in a state where feed-in tariff eligibility is not altered by upgrading the size of your system, it would probably be the best outcome for you to try to get a bigger inverter, provided you are seriously considering upgrading the size of your system in the future. Otherwise, you’ll have to be content the more efficient system–8 x 205W polycrystalline with the 1700W inverter. This is because a favourable feed-in tariff could tip the balance and make the smaller, non-expandable system the more financially attractive option.
I do not quite understand the chart above. Could you explain a little more about what would happened if the inverter is oversized. For example, we connect 2.75kw system to 4k inverter.
Thanks in advance.
Hi Ada,
The answer to your question depends on the actual specs of the inverter that you would be using. Some inverters are better at coping inputs lower than their rated capacity than others.
In essence, what happens is that the efficiency of your system as a whole drops due to the fact that your inverter is not optimised to use the electricity from your solar panel array–the input power from your solar panels is outside the inverter’s ‘sweet spot’. As I mentioned above, how much efficiency you lose will depend on the inverter in question. A 4kW inverter may have an optimal operating range where you get between 90 and 98% efficiency even if your panel array is not rated at exactly 4kW.
Can you give specific examples of the components you are using?
Thank you very much for your explaination.
I am using SMA sunny boy 3000us inverter. And I want to feed 1 string of 10 Centrosolar K230 modules. I saw the inverter efficiency profile chart on their product cut sheet too. However, I have no idea how to read it. Could you give some instruction on how to read the graph in your post?
Thanks for your time!
Ada
Hi Ada,
The important thing to remember is that the chart above in our article does not apply to all inverters. Each inverter has its own individual efficiency curve. I tried to find the efficiency curve for the SMA Sunny Boy 3000US, but was only able to find the curve for the 4000US, which is an inverter meant for bigger systems with more capacity in the array. You can see a chart for the 4000US inverter below. (You can also see a brochure here.)
On the left you see can see efficiency (%), on the bottom you can see power (in Watts–i.e. the power of your solar panel array). The lines show different voltages of systems. You can see that efficiency is greatest overall if you have a 250V system (the voltage of your system depends on panel configuration). So basically, if the system is 250V and has a power output of 1000W, this inverter (the 4000US) will perform at its optimum efficiency (about 97% in this case.)
The important thing to note here is that efficiency really does not drop significantly until your array output exceeds 4000W. Between 1000 and 4000W the inverter is still performing in the ‘sweet spot’ of 96-97% efficiency. Be aware, however, that during the day the output of any system varies depending on how much sun is shining, cell temperature, etc. So, because they usually have efficiency curves similar to the ones you can see here, the rule of thumb for inverters is to oversize them–by doing so you avert potential losses that occur when system output falls into the ‘dead zone’ below the sweet spot.
I hope that this helps. Let me know if you have any more questions, Ada.
gday i have a 3kw aurora and 12 x 250w panels im north facing 40deg pitch and west qld! im get full sun and 25 deg temp and my max power is around 1800-2000wats peak i dont understand this not a cloud in the sky bright as a button no shade the day instaled it peaked at 3024w and has not gone above 2200w since then its also not meeting the daily average of 12.6kw even in this type of weather !this doesnt seem right to me the inverter is aurora 3600 !cheers tom
Hi Tom,
You might be able to attribute the low output to the fact that you’re currently in winter and, as your panels are on an immobile tilt frame, they will necessarily produce less power than their rated capacity indicates they should.
You should also be aware that all systems have inefficiencies, and there are a number of places where these might occur. You can also check out our solar power troubleshooting blog for ideas about how to check and test your system.
If you want to know whether or not the problem has something to do with your inverter, you’ll probably want to get in touch with your installer or the inverter manufacturer. Are you signed up for any post-installation customer service?
we have six monocrystalline 170 watt panels (1020 watts) and have an Orion 2.2 Kw inverter. Can we add another six panels to our system or would the voltage more too high?
Hi Kathleen,
What is the brand of your panels? Different brands have different voltages per panel, although this is unlikely to exceed the input voltage of your inverter. A spec sheet would be useful in answering your question.
In principle, adding 6 more of the same brand of panels should not be a problem, but you might need to ensure that they are arranged into strings so that the basic conditions (voltage, power) are met for proper inverter operation, and that the panels are arranged so that they are getting roughly the same amount of sun. Having the same brand and model of panels is also important for this reason–output discrepancies between panels can result in power loss. It seems, however, that your inverter comes equipped with a maximum power point tracker (MPPT), which means that even if there are some small inconsistencies between panels, it will not cause any major issues with array output.
We have 11x215W Hyundai monocrystalline panels (2.365) with a Fronius IG20. The salesman encouraged to go for as many panels as we could afford, which we did, but then they supplied a 2kW inverter. Max we are producing at the inverter is 1700w even on a bright day (Scotland, facing perfect south). I know its not peak summer and the sun will be lower but I’m worried we’re losing out. Would an IG30 make better use of the capacity? We won’t be adding further panels in future.
Many thanks for your informative blog.
Graham
Thanks for the comment, Graham. We don’t get many inquiries from your side of the world.
I’m having trouble finding any graphs of inverter efficiency curves for the inverter you’ve mentioned. If you have the spec sheet, that could indicate where things are going wrong with your system. You may not actually see a significant loss in efficiency if your input voltage is just a bit over the nominal voltage of the inverter–in some cases it can actually be good to have a slightly undersized inverter. Without this information it is hard to say definitively if the problem is likely to be occurring with the inverter.
We also have an article about how to troubleshoot grid-connected solar systems , which might be worth a read.
Hi I am looking at a system that has a growatt 4200MTL and enough panels to produce 2.2 kw
if all conditions are perfect (sun beaming at the correct angle, temperature correct etc.) would this run at max efficiency?
Hi Aaron,
Generally speaking, we don’t recommend oversizing a system’s inverter unless the owner is intending to increase the solar panel array size to roughly match the inverter’s capacity within about 3-6 months from installation. The inverter is the ‘brain’ of a solar system, and needs to be selected carefully. Have you looked at an efficiency curve chart for the Growatt 2400MTL?
I have 12 Suniva 240 Mono in a single string with a SMA 4K inverter. Is this a good fit for an expansion system?
Hi Cosmic,
You would be able to expand your panel array by another 1.2kW.
we are looking at a 5kw system utilizing 20 x Q245 monos from Qcells they are offering two types of inverters the sunny boy 5000tl or a growatt sungold 5000tl. which system would produce better efficiency and return. We are in perth central true north facing 25* pitch clear.
cheers in advance
Hi Laurence,
SMA Sunny Boy inverters are some of the the most widely used and most trusted inverters in the world, with extremely low failure rates. Paying a bit extra for an inverter makes a difference, if you can afford it.
If you would like more quotes on systems, a number of the installers in the Solar Choice network use both SMA and Q-cells products. Request a Solar Quote Comparison by filling out the form to the right of this page, or call us on 1300 78 72 73 for more details. Our service is 100% free to our customers.
We have a 100kw system under contract with the possibility of adding another 100+ in the near future. We would like to use a SMA 250U inverter we have in stock. We were going to use MOTECH IM 72 (295W) -31 strings of 11. Nominal power ratio is 258% but all other voltages check out on sunny design. Would it be safe to use this inv. or am I losing too much in efficiencies? Thank you.
Hello John,
I understand your predicament but unfortunately I haven’t got the technical expertise to advise on this matter. A 258% power ratio is huge, however. However, it could theoretically work as long as the inverter’s efficiency curve doesn’t cut your output so much as to make it not worthwhile to do at that low a wattage. Good luck!
We are using 18 X 240W total 4.3KW PV array system and power conditioning unit of 4 KW which will charge the batteries also. We have connected load 2 KW. My question is can we use 2 KW PCU instead of 4 KW ? What happens actually if 2 KW inverter is connected to 4 KW PV system.
Hi Harshad,
Thanks for the comment. Generally speaking, system output will be severely limited by having an undersized inverter, as you suggest. Depending on the specifications of the inverter in question, you will see very little generation above 2kW, even if you have 4kW of solar panels. We do not recommend it.
Thanks