Why Your 5kW Solar System Output Is Lower Than You Expected

Cleaning a dirty sold panel

Dirt is one of 5 losses

Is your new solar system producing the power output it should? 

A very common question I get from people who have just got a new solar system is: “Now the solar panels are on my roof, how do I know what the solar system output should be?”.

The solar system owners are usually a bit concerned because they have bought, for example, a  5kW system but their inverter is telling them that they only got 4kW of peak power  yesterday! Where’s the missing 1kW? Most people expect a 5kW solar system output!

Here’s an email that I got last week from Hans that is a perfect example:

Hi Finn,

Thanks for your advice. Much appreciated.

I had a system installed via you, all very nice and well installed on Thursday 15/3 and very pleasant people to deal with.

5 KW , 20 x  250 W ET panels with a Sunnyboy inverter.

During the test we have measured at peak at 14:20 4.7 kW and we were very pleased.  After that day we had some hotter and  more sunny  days but the capacity of the system always stays below 4 KW at peak. never go above it, strange.

Today a hot clear day 25kWh at peak 4kw.

May I have your opinion about this and can i expect to reach the 4.7 KW again?

What Solar System Output should Hans be getting from his 5kW system?

A fact that isn’t advertised widely in the solar industry is that most solar systems in Australian conditions will usually peak at only 80% of their specified peak power.

This is actually completely normal in almost any industry that sells hardware that has associated specs. Think about the last time you bought a car with a specified fuel consumption from the manufacturer. Did your car actually ever achieve that fuel efficiency on a day-to-day basis? Thought not!

Solar panels systems usually reach only 80% of their specified peak due to “system losses“. So what are these system losses, and how can you work them out for the solar system sitting on your roof?

In Hans’ case he is using ET Solar 250W solar panels. He hasn’t told me the model number, but there are 3 matching panels in my Solar Panel Comparison tool. I’ll assume he is using a ET-P672250 and use those specs in the calcs.

The 5 Losses in Every Solar System

A flowchart showing how the losses in a solar power system add up.

Solar System Losses Add Up!

1. Manufacturer’s power tolerance.  (1%)

All panels have a power tolerance. In the panels that hans is using, this is -1%/+3%. So worst case is that the specifiedpower output is actually 1% lower than spec.

2. Temperature Loss. (10%)

I wrote an entire blog post on solar panel temperature losses. To cut a long story short, solar panels don’t like to be hot. Most solar panels lose about 10% of their rated power on a 25°C day, more if it is hotter. Let’s assume 10% for this estimate.

3. Dirt (5%)

When your solar panels are put on your roof, airborne particulates like dust will settle on the panels’ glass. These particulates block the amount of sunlight reaching the solar cells behind the glass reducing your power. The reduction in power from particulate build up typically lies in the 5%-15% range.  Hans’ panels have just been installed so we’ll assume only a 5% loss.

4. Wiring Losses (voltage drops) (2%)

All the solar panels on your roof are interconnected with wires, then a long pair of DC wires connects the final solar panel to your inverter. All these wires have a small electrical resistance, which means the electricity flowing through them will suffer a voltage drop. This will reduce your power proportionally, typically by around 2%.

5. Inverter Efficiency (4%)

Everything goes through your inverter so the inverter efficiency will directly affect your system output. Hans is using an SMA Sunny Boy inverter. If he has a modern, transformer less model, he can expect an inverter efficiency of about 96%, giving a 4% loss.

Multiply all those together (see the pic on the right)  and you are looking at 20% total losses, giving a real world peak power of 4kW – which is exactly what Hans was seeing.

But what about the 4.7kW he saw on the first day? 

That was probably down to 3 of things. Firstly Hans tells us that the days following this reading were hotter. So there would have been higher temperature losses later on. Also, after the first day, a thin, almost invisible layer of dust probably quickly settled on the panels increasing the dirt losses from a starting point of zero. Thirdly, the panels can initially degrade a percent or two in the first day or so, before they “settle in”, after which they should degrade a lot more slowly (about 0.5% per year). Take all these into account and it would explain why Hans has not seen his 4.7kW peak power since.


About Finn Peacock

I'm a Chartered Electrical Engineer, Solar and Energy Efficiency nut, dad, and founder of SolarQuotes.com.au. My last "real job" was working for the CSIRO in their renewable energy division.


  1. Thanks Finn,
    All clear , i had again 4.5 last week on a cool but sunny day
    I am getting the average daily output in line with forecast/expectations , sunny days 25-28Kwh

  2. Bill Bromley says:

    Dear Sir, my panels produced 94kw according to my last bill and my useage is 224kw in the house. Origin paid me 6c. credit for the 94kw, what I would like to know is how do I get Origin to subtract power generated subtracted from power used ie 224kw-94 kw = 130kw and then be billed accordingly.

    Regards, Bill Bromley.

  3. mario rom says:

    her in the Philippines sun light is good and my work here as electrician i installed several solar panel and the same problem i encounter 270 watts grace solar

  4. Hi, would it be a good idea to add additional panels to counteract the expected losses mentioned in your blog … Say an extra 10-20% worth of panels 6kw of panels with a 5kw inverter ?
    And would this be acceptable to Energex ?

  5. I have noticed that most Inverters are rated at their relevant size e.g. (3kw inverter rated at 3000, 4kw @ 4000,) BUT the 5kw inverters seem to only be rated @ 4600 Why is this ?
    why are the 5kw inverters rated at a reduced Ac power rating ?

  6. Davyd Lewis says:

    I have had a 3 kW solar system in Melbourne since early 2010 and it has been a complete failure. The solar power credits have averaged $30 – $50 per quarter with no noticeable drop in usage from the grid. The installer, Modern Solar, cannot explain the poor performance of the system and completely refuses to do anything to improve it. Is there any reputable tester I can get to assess why the system is not working?

  7. phase.verocity says:

    Well i’m running a baby one,, basically I have 6x250w mono panels hooked up to a 60amp regulator that only goes off grid. I’ve turned my power off because I don’t believe electricity should be worth 24.4 cents a kWh. At the moment its approaching summer and I’m getting around 1200 watts in the best hours of the day or 8-9 kWh’s a day. I know the panels are not at the right angle and that is usually the biggest mistake people make but its near impossible for me to change them because they’re on the roof of a very large building. Resistance also plays a big part in off-grid. The fuller my batteries become the less I can use to charge the 34kwhs of battery power I’m using. Before hooking up to solar I used roughly 12kWh’s a day but with a lot of changes such as instant hot water and a laptop instead of a big computer i’m using around 5-6kWh’s a day. I’ve heard lots of people having problem hooking up to the grid and I don’t trust electricity companies, They’re out for as much as they can get like every other business these days. Would be nice if business picked up some of that “for the people” power thing that was fashionable in the early years.

  8. I had Moderns in and they tried really hard to sell me a 3 kW system on the south west facing side of the roof because that was the only spot they could put 16 panels. Had to sign the contract just to get the salesman out of the house at 7pm. Followed up the next day with a letter to cancel the contract. Eventually went with 1.5 kW facing north west with another vendor – 2 k$ vs 8 k$.

    • That does not surprise me at all about Modern Solar.

    • phase.verocity says:

      Being stand alone, I installed mine mostly myself although with a little help from a solar guy who charged me $150 and made sure they were ok. for the panels and the cable and roof rails etc 6 panels cost me around $1250 plus cable $60 plus rails $250 plus a breaker box $40 = $1750. Thats not installing anything to do with a new electricity panel or grid tie inverter either. So $2k is pretty dam good because the grid tie inverter is likely to cost $2-300. My off grid one that is 5kw cost $490 for a cheap powerjack from china. I know grid tie ones can get very expensive too so you’ve done very well.

  9. I am in the process of finding a solar system and so far the one that strikes me so far is 13 sunpower panels rated at 327w and a sma sunnyboy 4000tl inverter.(11170 fully installed) 6 panels on east and 7 panels on north .If I understand correctly this amount of panels exceeds the CEC guidelines of 2013 for this inverter. Is this correct?

  10. So how many panels can I have hooked up to a 3 kW inverter ,I previously have 12 x250 watt panels now,my other question is I want to go a little bigger and put a 5 KW growatt inverter and another 10 panels where is the best company or best electrician to get these done,I previously had true value solar put this last lot up ,good price but don’t know if good choice any advice grateful thanks…….Eddie.

    • Hi Eddie,

      If you want to upgrade your solar system then the first thing to check is how it will affect any Feed In Tariff you are currently on. Some states will let you upgrade without penalty,in other states you have to be careful how you do the upgrade.

      I explain the things you need to know in great detail here:


      Hope That Helps,


  11. Fin,

    I had an Eversolar 5kW inverter installed with 20x250w TNS Black a Max solar panels. We live on rural property in Wedderburn NSW 2560 which uses the usual electricity and our water (water pump), flushing and septic tank all work through electricity. We have an average daily usage of 29kWh. All our 20 panels are facing West, but at around 2.30pm I have notice that around 5 panels have a fair bit of shade on them due to the fact that we are on acreage with a lot of very tall tree’s.
    We have received two quarterly bills since having the solar system and on both bills we have only had a saving of around $100. The inverter shows that it generates around 17kW on average per day.
    Should I have had a bigger inverter and more panels installed to accommodate for our needs. As the solar panel company has advised me that the system is working to near capacity according to the Clean Energy Council. At the point of sale I was told by the sales rep that with a 5kW inverter I should have around a 75-80% saving per quarter on our electricity bills providing we use our electricity the same as we usually do.


  12. Hi there i have just installed a 3.16kw system seems to work okay but it has a real time peak of only 2000w has never got higher than 2.1kw, LIve in melbourne and was expecting about a 20 % loss but 30% seems a bit high. 12x260w Jinko panels with a ABB one 3kw inverter.

    Any thoughts,

  13. John Barwick says:

    Hi Finn,
    First of all I would like to say thank you for making a complex subject understandable to non professionals like all us.
    My Question is I have 16 Phono poly 500w in a true North alignment at about 15 degrees angle.
    I am running them into a grid tied 3kw Fronius inverter about 4 years old.
    My difficulty is that for about 4 months in winter I have varying degrees of shadow across some of the panels from very high Camphor Laurel trees situated a little West of North for about 3 hours per day. I am considering having Tigo optimisers fitted to the system to try to lessen the damage. I have been quoted $2000.00 for the job. The cost does not bother me but I would like to have your opinion concerning optimisers. I live in South East Queensland.
    Many thanks

    • Tigo optimisers are good by all accounts and can be used with any central inverter. They will reduce the effect of the shading. How much by is hard to quantify – so it is hard for you to work out the payback of the optimisers.

  14. Hi Finn, I 6kw system installed, (5kw inverter) with two different different strings of panels of 3kw one NE and one NW, my production has been quite low compared to pv output figures in my area. My peak power maxes out at 2.4 to 2.5kw. I am concerned that all my panels aren’t working, any tips or tricks to work out if this is the case?

    • Hi Jay,

      If your panels are clean and unshaded, then it certainly sounds like there is a problem with your system.

      If it is in warranty – then get the installer back to do a system test.

      Otherwise – expect to pay about about $200 for ann independent installer to test the system for you and find out what the cause is.

      Hope That Helps,


  15. Colin Makin says:

    This has been bothering me for quite some time. The day after our 3 kW system was installed we got an output of 3kW. It has never got past 2.5kW since. I thought there must be sometime wrong with system. Your article clears things. Thanks.

  16. Hi, I recently had a 5kw Fronius with 5.6kw of panels put in. I expected it to go gangbusters yesterday as it was a cloudless, sunny day, but the best it could do was 4.9kw. Is this normal?

    • Ronald Brakels says:

      Hi David, yes that’s quite normal. An output of 4.9 kilowatts is 87.5% of your panels’ 5.6 kilowatt capacity and heat, dust, sub-optimal orientation and other factors can easily account for the shortfall. Note that with a 5 kilowatt inverter you will never be able to get more than 5 kilowatts out of your system, but as you can see, you are very unlikely to be losing any significant amount of power due to this.

      • Thanks for that. I had someone tell me that a 5kw inverter will never put out 5kw, is this true? Pus my panels are Risens so I’m not sure if they are that great?

        • Ronald Brakels says:

          I’m afraid a 5 kilowatt inverter will never put out more than 5 kilowatts of AC power. But because panels so very rarely produce their rated output, having panel capacity greater than your inverter capacity is not a bad idea.

          I don’t think Risen panels are a bad choice. They are tier one and have a 12 year product warranty which is better than the usual 10. Risen is also apparently the most financially sound of the large Chinese panel manufacturers, so hopefully they will be around for a long time to come.

  17. Love to get your opinion on my panels,
    I installed a 9.2KW system 3 years ago. Over that time the best they generated was 7.2KW which occurred only for one day. Accounting for system losses, I have suspected that the system is not generated what I was sold. The installer has not returned my calls or emails asking for their advice. I would say on average, they generate about 5 to 6 KW.

    • Ronald Brakels says:

      Hello Keith, Ronald here. It is possible for a 9.2 kilowatt system to only produce a maximum of 7.2 kilowatts, particularly if it is not facing north. One reason for the low maximum output is the system might be inverter limited. This occurs when the inverter capacity is smaller than the panel capacity. I don’t know where you are, but the general rule of thumb for Australia is a system should produce an average of around 4 kilowatt-hours a day per kilowatt of panels. Less in Melbourne or Tasmania and more in Central or Northern Australia. Panels that face east or west instead of north produce about 14% less. So I would expect a 9.2 kilowatt north facing system to produce around 36 kilowatt-hours a day, if there is no shade falling on the panels through the day.

  18. Hello Dear
    Few days ago we install a solar system with 5 kw inverter and use 8 panels 250watts .but the issue is that inverter shown the total 1.2kw. whats the issue. whey he not give 2000 whatts?
    Detail we use 4 panels 250w in series make it 64 Ampere and other 4 are same.total panels are 8 in 250w. And inverter is mppt 5kw.
    plz tell me whats the reason.

    and 1 moor plz on a 5kw inverter how does panals install ?

    • Ronald Brakels says:

      Hello. There are several reasons why your panels may only be producing 60% of their rated output. They are only likely to produce close their maximum output when the sun is directly above them on a very clear day and the panels aren’t too hot. The panels will all need to be facing the same direction. There may be a 5% or greater inverter loss and a much smaller wiring loss. Because the system is new dirt should not be a problem, but you can still check the panels are clean. When connecting panels to an inverter it is important not to exceed its maximum DC input voltage. This will depend on the inverter. It may be 750 volts or it may be less. If your inverter has 2 MPPT (Maximum Power Point Trackers) it can accept two separate strings, or sets, of panels and these two sets can face in different directions if desired. There are some tools available on the internet that can help with design. This one, by SMA inverters may be useful:


  19. It’s so funny to me that you point your panels North! Great website. If two panels are placed on top of each other panels 1 produces 100 percent and panel 2 produces 0 percent. But as the distance between them is increased the shadow on panel 2 decreases and the output increases. So at what point is the output of both panels equal? Also is the relationship linear or expoental? It could be very far maybe a mile… I mean kilometer but I’m very curious! Thanks mate.

    • Ronald Brakels says:

      Well, it takes roughly 100 meters per meter of size for an object’s umbra to disappear when in sunlight. The umbra is the dark, central part of a shadow. Since a typical solar panel is about 1.65 meters in length it should no longer cast an umbra after about 165 meters. It’s penumbra would still be falling on the panel on the ground, but at that height it would be really diffuse and I would think the output of the two panels would be basically equal. But this is just speculation on my part and if you want solid facts you should get them straight from the horse’s mouth:



  1. […] the other options as defaults. The “derate” factor is an estimate of the solar system losses and 77% is a little pessimistic – but will do for this calculation. Enter your system size, […]

  2. […] (If you want to learn why panels don’t get to a 100% performance ratio I go in to great detail as to why here) […]

  3. […] A typical solar inverter sold as “a 3kW inverter” can usually have more than 3kW of solar panels safely connected to it, because 3kW of panels never actually produces 3kW of power for the reasons outlined here. […]

  4. […] 3kW of panels will generally only produce 80% (2.4kW) of their rated peak power due to losses. […]

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