Poor Solar Panel Performance? Be Sure To Check Both Energy and Power.

Poor solar panel performance? What you should check

If you suspect your solar power system has poor performance – here’s what you need to check.

Although I personally can’t think of anything in life more exciting than getting a new solar power system installed, I sincerely hope you can – otherwise you’d be as weird as me.  Having said that, even those who score 10 out of 10 on the Voight-Kampff normal human test should feel excited when they get their first solar system.  Unfortunately, this excitement sometimes gives way to confusion and heartbreak if they erroneously come to the conclusion that their new solar panels are not performing as well as they should.

It’s not unusual for people with new systems to think they have a problem. They know how many kilowatts of solar panels they have and then realise that the instantaneous power their system is producing (in kilowatts) is lower even though the skies are clear and the sun is shining.

For the most part, solar panel and solar inverter capacities are maximums that are rarely reached.  There are a variety of factors that impede your system from reaching its full capacity.  Even on a sunny day, a number of these dirty1 factors can be hard at work preventing your system from ever performing at max power.

Checking your system’s power output will often be enough to allow you to be confident it is performing well.  But because solar power output is so variable it is not good at indicating if your system is performing badly.  This makes it important to not just look at instantaneous power readings, but to check how many kilowatt-hours of energy your system has produced over time.  In this article I go over how to check the performance of your solar panel system — whether it’s new or old — by looking at both its power and energy output.

New Solar Almost Always Works As It Should

First off, the good news is new solar systems almost always perform as they should — or at least they do if you use a reputable installer.  If you use a shoddy one then they might use damaged panels, forget to connect half of them, use inadequate wiring, place panels where they will suffer from shading you weren’t informed about, implement a flawed design or do something else that will reduce their output.  So far I haven’t heard of solar panels being installed upside down, but I suppose anything is possible2.

One way to be sure you are getting a good installer is to perform exhaustive research into the companies you are thinking of using.  Or you could save yourself some effort and get quotes through us.  My boss, Finn Peacock, puts a lot of effort into making sure we will only ever refer you to people who we trust to do quality installations.  He works really hard at it.  If I watch him for just a few minutes I feel tired and have to go take a nap.

While the very large majority of solar power systems work well on the day they are installed, it is still possible for their performance to decline.  This is normally a very slow process if quality components are used as there are no moving parts.  But it is still possible for problems to occur and so you may want to check for poor solar panel performance3.

Reading Your Solar Inverter

To find out how much power or energy your inverter is producing, first you’ll have to read it.  There are two ways to go about this:

  1. Use the internet.
  2. Read the display on the inverter itself.

Reading Your Inverter’s Display Screen

If your installation is old or cheap then you probably won’t be able to monitor the inverter online.  In this case you’ll need to look at the display screen on the solar inverter itself.  Almost every inverter, whether it can be monitored online or not, has a display screen — although there are exceptions.  There should be a button to press that changes the information displayed.  Exactly what is shown varies between inverters but it should be able to tell you:

  • How many watts or kilowatts of power it is currently supplying to your home and/or the grid.
  • How many kilowatt-hours of energy it has produced so far today.
  • How many kilowatt-hours it has produced in total since it was installed.

Reading Your Inverter Online

If you can monitor your inverter using your laptop, mobile phone, or whatever, then things should be easier for you.  In additional to having both power and energy information online there are likely to also be pretty coloured graphs to go along with it4.

Online solar panel performance monitoring

So pretty! (Image: ZeverCloud)

Kilowatts Of Power Vs. Kilowatt-Hours Of Energy

If you are doing things the hard way and looking at the display screen on your inverter and find a figure in either watts or kilowatts that decreases when the sun goes behind a cloud and increases when the cloud passes, then congratulations — you have found the power:

Power is measured in watts, never gallons, and one thousand watts equals one kilowatt.  It represents what your inverter is cranking out right at this moment.  But the power from a solar panel system can be quite variable.  If you watched your inverter’s power output over 24 hours you’d see considerable variation through the day and a long period of no power at night.  Also, you would be in desperate need of a more interesting hobby.  Maybe you should think about getting a TV?

solar power output graph

This graph of the power output from a solar system throughout the day shows how instantaneous power can jump up and down throughout the day. Instantaneous power readings are not a good way to gauge system performance.

Because the power output is so variable you can get a better idea of how your system is performing by looking at energy instead, which is measured in kilowatt-hours.  If your solar system provides 4,000 watts of power for one hour then that will come to 4 kilowatt-hours of electrical energy.  If your inverter provides 4,000 watts of power for 20 minutes but then the sun goes behind a cloud so your inverter only provides 1,000 watts for the next 40 minutes then the average amount of power supplied in that hour would be 2,000 watts which would come to 2 kilowatt-hours of energy.


Never gallons! (Image: SMBC)

If you want to read an article that bangs on about the difference between kilowatts and kilowatt-hours at a basic level go here or buy this book.  Otherwise, just remember kilowatts of power is a snapshot of what your inverter is doing at one instant in time while kilowatt-hours of energy is a movie that can cover an hour, a year, or your inverter’s entire life5.

7 Things That Reduce Your Solar Power Output

Looking at the power output of your solar system under good conditions can let you know if it is working well.  But it is difficult to be certain if your system is performing poorly just from looking at power alone.  This is because there are so many things that can reduce its power output without there actually being anything wrong with it.  These include:

  • The angle of the sun:  When the sun is low in the sky, whether due to the time of day or the season, less power will be produced.
  • Solar panel orientation: Panels facing east or west will generate less power than those that face north.
  • Clouds and haze: Less sunlight reaching the panels means lower power output.
  • Heat:  High temperatures reduce panel efficiency.  In a heatwave, solar panels can reach 65 degrees and this can reduce power by 20% compared to panels that are at 25 degrees.
  • Wind: While sunshine heats panels a nice breeze helps cool them down.
  • Dirt: Bird poo, leaves, and grime in general can greatly reduce output.
  • System losses: Wiring resistance results in about 2% of power being lost while modern inverters often have losses of 3-4% as they change the DC power from the solar panels into the AC power homes use.

An Example Of Low Power Output

Let’s play pretend and say you have just had a brand new solar power system with 6.5 kilowatts of north facing solar panels and a 5 kilowatt inverter installed.  It’s 3:30 in the afternoon on a sunny clear day when the installer switches on the inverter to show you it’s working.  But despite being surrounded by brilliant sunshine you are disappointed to see your brand new, perfectly clean, 6.5 kilowatts of solar panels are only producing  2.4 kilowatts of power.  You might think you’ve wasted your money on a bum system but fortunately this is almost certainly not the case.

  •  If it’s 3:30 in the afternoon then depending on the time of year the afternoon could be more than half over.  This will cut the output of north facing panels by more than half.  If it’s winter then the sun will be lower in the sky, further reducing output.
  • Even if the air temperature isn’t high panels can still get hot in the sun and this will reduce their output.  The effect will be worse if there is no wind and on a mild day may cause losses of 8%.
  • Unavoidable wiring and inverter losses may reduce power output by 5% or more.
  • While the skies may look clear they could be hazier than usual, cutting output by a few more percent.

When all these losses are added together they can easily reduce the output of 6.5 kilowatts of solar panels down to 2.4 kilowatts or less.  Because there are so many factors influencing power it is difficult to use it to determine if things are performing properly.  But you can still give it a shot.

Diagnosing Your System Health Using Power

In order to use the power output of your solar system to see if it is performing well you will want to measure it under good conditions.  If your inverter information is available online then it should be easy.  If it’s not online then you’re going to have to read the inverter display screen yourself.

You will want to check the power output when the sun is directly over the panels.  Or at least as directly over them as it will get on that day.  This will be at solar noon for north facing panels.  It is best to check when this is at your location as it usually won’t be at 12 o’clock on the dot.  But half an hour on either side of solar noon will make little difference.

If your panels face west the sun will be directly over them around 1 hour and 20 minutes after solar noon and around that long before if they face east.  If your panels face in more than one direction then split the difference.

If your panels face in a northerly direction from northeast through to northwest and the power output is roughly 80% of the total panel capacity then your system is probably working fine.

If your panels face east or west and the power output is around 70% of the total panel capacity then your system is also probably performing fine.  If you have panels facing in more than one direction, then split the difference again.

Note that the power output can’t go above the capacity of the solar inverter.  So if there are 6.5 kilowatts of north facing panels 80% of that would be 5.2 kilowatts.  But if they are attached to a 5 kilowatt inverter the power output won’t go above that amount.

You can be confident your system is working well if the power output is high enough, but because there are so many factors that can reduce it a low result doesn’t necessarily mean there is anything wrong.  It could simply be bad luck.  Even just one bird poop can seriously reduce power output performance.  While taking measurements over multiple days can help, to be certain rather than look at kilowatts of power you will need to look at kilowatt-hours of energy.

Diagnosing Your System Health Using Energy

PVWatts Is Our American Pal

The best way to be sure if your system is operating correctly is to measure how many kilowatt-hours of energy it has produced over a period of time.  The longer the better.  The trouble with this approach is you actually need time to gather the performance information.  A year is good because, while there is variation between years, solar energy output tend to be reasonably constant over that length of time.  But over a month it’s easy for unusually sunny or cloudy weather to throw off your comparison.

A good source for information on how many kilowatt-hours you can expect your solar power system to produce in your location is the U.S. site PVWatts.  While it’s not perfect, if you live near a major population centre you can enter information about your system and it will tell you how many kilowatt-hours your can expect it to produce over a year.  It also gives monthly totals.  Here’s what it says one kilowatt of north facing panels will produce in Sydney:

Solar electricity output in Sydney

Image: PVWatts

Over one year they can be expected to produce roughly 1,400 kilowatt-hours for an average of around 3.8 kilowatt-hours a day.  The worst month on average is May while the best is December, but there will be a fair amount of variation in the monthly totals from year to year.

Annual Kilowatt-Hours Per Kilowatt Of Solar Panels By Capital

To try to save you the time and effort of working out how to use the PVWatts site I have put the information into the graph below for you.  If you know how many kilowatt-hours your solar power system generated over a year you can divide that by how many kilowatts of solar panels you have and compare it to a figure on the graph if you are in or near a capital city.

Annual kilowatt hours per kilowatt of solar panels by capital

If your result isn’t as high as the figure the graph gives, don’t panic — yet.  The graphs shows how much one kilowatt of solar panel capacity will generate over a year if it faces directly north and suffers from little to no shading.  If your panels face northeast or northwest reduce the figure from the graph by 4%.  If your panels face east or west reduce it by 14%.  And because PVWatts tends to be a little optimistic you might want to reduce it by another 5% to reflect the fact that life isn’t always as awesome as Americans think it is.

If I had a northwest facing system in Canberra that produced 1,400 kilowatt-hours per kilowatt of solar panel capacity over the past year, I would take the 1,553 kilowatt-hour figure from the chart and reduce it by 4%, which would give me 1,491.  That’s above what my panels produced so I’d then reduce it by 5% to allow for Yankee optimism and get 1,416.  This is still a little above what my panels produced but it’s close enough.  While I’d like its output to be higher I would not suspect there is anything wrong with the performance of my solar power system.

What To Do If There Is Something Wrong

If the energy output of your system is considerably lower than what it should be, then your installer is your first port of call.  But you might first want to check the performance shortfall isn’t due to dirt or shading.

Dirt and grime on solar panels can have a major effect on output.  A bird only has to eat one bad curry to cause a major reduction.  But over the course of a year rain should keep the panels relatively clean so things such as bird droppings should only have a temporary effect.  However, it could still be worth checking as your location could be especially dirty.

Shading can result from trees growing, new construction, TV aerials, roof pipes, and other obstructions.  Your installer should inform you if anything is going to cause a major amount of shading before your system is installed so you can decide if you want to go ahead.  It can be a good idea to check your documentation because sometimes installers inform customers about shade issues but over time this is forgotten.

If your installer is no longer in business and your system is clearly under-performing you will need to find an installer willing to inspect your it and perform any needed repairs.  This can be difficult as many installers don’t want to work on systems that aren’t their own.  When my parents needed someone they didn’t have a problem as the first installer they contacted was able to refer them to someone willing to do the job.  But some people have had a lot of difficulty.  If you’ve tried and can’t find anyone you can contact us and we’ll find someone for you.


  1. One of the factors is, literally, dirt.
  2. While it sounds odd, being installed upside down has probably happened with double glass panels.
  3. Yes – I know it is actually the solar power *system* performance you are checking but we have to throw some crumbs to Google’s search algorithm – and Australians search ‘poor solar panel performance’ a lot more than ‘poor solar system performance’
  4. Or if you have ‘Solar Analytics‘ or similar 3rd party monitoring you should be looking at that. But then you are unlikely to be reading this article – because a good 3rd party monitoring system will tell you instantly whether your system performance is up to scratch – you don’t need to do any of this manual diagnosis.
  5. While this would be a very boring movie, at least it would be better than The English Patient.
About Ronald Brakels

Ronald was born more years ago than he can remember. He first became interested in environmental matters when he was four years old after the environment tried to kill him by smashing fist sized hailstones through the roof of his parents’ Toowoomba home. Swearing revenge, he began his lifelong quest to reduce the harm the environment could cause. By the time he was eight, he was already focused on using the power of the sun to stop fossil fuel emissions destabilizing the climate. But it took him about another ten years to focus on it in a way that wasn’t really stupid


  1. Thanks. I do weekly and monthly readings but don’t really track yearly output, which sounds like the best way to identify problems.
    Slightly off topic but those in SA might want to check out the SAPN portal for viewing import/export stats on their system.

  2. Colin Palmer says

    What about inverters tripping on overvoltage and reducing the overall output..?what can be done other than trying artificially applying load..?

  3. Great information, thanks.
    I was recently horrified t see a neigbour watching the installers put a system on a roof facing WNW (in Sydney) and about 80% shaded at 4pm.
    I will be very surprised if he is not very disappointed with the performance of his system, even to the extent of removing it and selling it for whatever he can get unless he can persuade his neighbours to substantially lop the offending trees..

  4. A good article. Pity some people, even some that are active on this site do not understand the difference between power and energy… even refusing to accept a term like kWh.

  5. Hi there
    Is there any way that I can subscribe to this blog?
    Been mooching around this page and can’t seem to find a way to do so….
    Makes such a great change to get a laugh around such a dry topic…
    Reluctantly Self Educating Off Grid Owner

  6. Can confirm by personal experience that the figure of 1371kws for Melbourne is close as my system last week notched up 14000kws since installation in September 2011. I live in a area that receives more rain, hence it is overall cloudier to some degree than other parts of Melbourne. There has been no noticeable lessening of production since install.

  7. Rod, you probably mean 1371 kWh and 14000kWh.

    Or otherwise enlighten me what does kws mean?

  8. Peter Seligman says

    As has been explained many times, energy is the product of power and time. An example of power, is horsepower HP, which happens to be 746 watts. An example of time is the fortnight. So energy could be expressed in HPfortnight, a term about as convenient as the Btu (British Thermal Unit), which the Yanks still use.
    By the way a HPfortnight is 251 kWh for future reference.

  9. Martha Bodon says

    Hi everyone l has Solar Panel takes 5yrs before you start getting your money back…To make money…..in business you have put money…

    • I don’t understand what you are trying to say. In Radelaide a solar system will pay itself off in 4 – 5 years.
      That is a return on investment of 25%. If you can suggest a better investment, I’ve got a lot of cash at the moment looking for a home.

  10. I found the PVWatts link very useful. I had noticed my system’s maximum kW output was way down in winter even on sunny days, but according to that site it is still well above what it should be. Now I can sleep at night. Thank you.

  11. Bob Hughes says

    Your earlier blog mentioning the PVWATTS website was very helpful. I agree with John (July 27 2018 at 3.09 pm). I satisfied myself that my eastwards facing panels were giving me as good a performance as I could expect for my often cloudy locality (Victor Harbor SA 5211), and that hypothetically moving the panels to face westwards would make little difference to annual power output. North facing was not an option on my roof. Thanks.

  12. John Flanagan says

    Hi Finn, I have found this article very interesting and quite informative. We have been considering solar for quite some time, and the more I learn (from you), the more inclined I am to get it done. Just knowing what to expect and some of the pitfalls is a great help. Thank you for your efforts in educating the community.
    Best regards, John

  13. Well done Finn & Team & Thanks- Have a 3kw inverter with 12 X 200 w panels installed Oct 2012 on an eastern facing roof which has provided 22219 kwh – Paid for itself in 5 years.

    In Sept 2017 had installed an extra 2.43 kw inverter & 9 X 270 w panels on a western roof which has produced 3408 kwh in 10 months – Thnx for your help Finn

    Now the state gov is threatening our FIT (7.135 cents at the moment)

    Michael of Warnbro (WA)

  14. Chris Davidge says

    As the web is world wide, someone like me could be reading this advice, and live in the northern hemisphere. All you advice is valid for the whole planet except of course panels should be south facing in the northern hemisphere.

  15. Looking for irradiation and temp measurement system I can add to my solar system to more accurately and instantly measure performance. Any suggestions?

  16. How to clean the panels if they are dirty?
    Can they be cleaned without turning them off?
    Can I spray water during a heatwave to cool them down?

    • Ronald Brakels says

      Most of the time rain is enough to clean panels:


      If they are dirty you can hose them down from the ground, but don’t spray them when they are hot. Also, don’t hit them directly with high pressure water. It should fall down on them like rain. It’s not necessary to turn the panels off if you are spraying water onto them from the ground. Spraying panels to cool them in a heatwave may damage them due to the sudden change in temperature. I don’t recommend going up on the roof to clean them because of the risk of falling.


      • Thank you for your answer on how to clean them so it it is about 90 degrees outside they can be clean right? I live in Palmdale CA and it starts the day with about 90 in the morning. And goes up to 110 at midday in the summer.

        • Ronald Brakels says

          It’s the sudden temperature change that is bad for them, so avoid putting cold water on them when they are hot. It’s best to clean them early in the morning, in the evening after they have cooled off, or when it is heavily overcast.

  17. I have a system, about 5 yrs old. One panel consistently puts out about 10% of what the other panels put out. What is most likely the cause?
    Thanks Gary

  18. Dave Atkins says

    I just received my power bill and noted that the solar system put 1598 kw back into the grid, my neighbour put 2005 kw back for the same period. We live in a rural area with no shade and clean panels. Your thoughts on why the difference?

    • Ronald Brakels says

      If the solar systems are the same size and facing the same direction then — unless one of them has a fault — the main difference in how much electricity will be exported to the grid is the amount of electricity the home consumes during the day. Only the surplus electricity is sent into the grid for a feed-in tariff.

    • Wow. Those are big systems!
      When most of us are limited to 6.5kW in town.
      How many hectares do you both cover?

  19. I’m interested to hear your thoughts on if and how I can cool my panels on REALLY hot days. I have noticed one of my systems had dropped by 75% when the temperature outside got to around 43c.
    I hose them off (in the mornings) when they start to look dusty in so they were clean at the time. They have a decent gap between the roof tiles for air flow. It’s obviously just the weather. I did spray some water around the panels, hoping the mist would blow over them. Didn’t want to crack them by hosing them directly. That seemed to help. But it would be a temporary fix, and water is precious so not the best solution.

    Any ideas?

    • Ronald Brakels says

      Hi Kylie

      Heatwave temperatures will cause solar panels to drop in output, but not by 75%. What is probably causing that large drop is the inverter overheating and reducing its output (derating) due to the heat. Normally solar inverters won’t derate at 43 degree air temperature but if they are in the sun or a hot shed or garage it can easily happen. If the inverter is derating due to heat it should display a message or code saying that’s the problem. If it is in the sun you can set up a shade for it or if it’s inside you can try improving the ventilation.

      • Thanks for your reply. Our inverter is shaded. We did that a couple of years ago and it did make a difference.

  20. My solar portal (real time) say’s that in 2020 my system produced 16,000kWh……. my home used 9,000 kWh…. my system sent 7,000 kWh back to the GRID (PG&E). But PG&E say’s I owe them $520.00 true-up and climbing… true-up due in April every year. How can I be charged hundreds of dollars in 2020 when my system sent over 7,000 kWh back to the Grid? I have a 25 year lease from SunPower Solar. ($130.00 per month). Sunpower say’s my system is working great (over producing)… PG&E say’s I’m using more power than I’m producing and to just pay the true-up bill. SunPower blames PG&E… and PG&E blames SunPower and I’m stuck paying for a solar lease AND paying PG&E. as it stand right now.. I am paying aprox. $200.00 a month to watch TV and turn on a couple of lights at night. Question is, is it my system malfunctioning or is PG&E malfunctioning? Any suggestions?

  21. Today I got permission to operate and turned the system on. We have a 22kW DC system with 2 10kW inverters and 4 sets of panels on 3 different angled roofs, but the way the sun moves over our house, it moves almost directly over the point where all 3 locations “connect”, and they are in the perfect location to all 3 get sun nearly all day, no hard shadows most of the day either.

    I flipped the switches at about 1 pm, and it registered 20 kW in my solar monitoring app on the first reading, then fell immediately to 14 kW at the next 15 minute reading and has been around there for several hours, at the peak of the curve. That initial reading may have been a fluke, but 14 seems low. That’s 63% of the max DC. Each panel has an MPPT controller on it. Could it be that the inverters lowered the output due to heat? It’s nearly 38 deg outside but fairly windy. We’re getting ~14 kW AC from the inverters. I was concerned at first, but the installer said that is a good reading for instantaneous power and that I should just look at the kWh instead. I really hope I’m not going to regret this. It was a top rated solar installer, and they’ve done a great job, and used quality panels. I have no complaints, but do want to be informed.

    • Ronald Brakels says

      Hi Chris

      It is the total energy output that matter, rather than power. A useful place to get an estimate of how much energy your system should have generated is this Solcast page:


      It will give an estimated kilowatt-hour output for your system over the past week based on weather conditions. Because your system is so large, I suggest dividing it into three sections, getting an estimate for each, and then adding them together.

      While this tool isn’t perfect, if you get a result around what your system is producing, it’s likely to be fine.

  22. William Donald says

    On a new 30 panel quality system that can show the cumulative output of each panel in the monitor software over any period (day, month, year, total) almost due north facing, no shade, cleaned regularly – how much variance is acceptable between panels?

    Over 13 months I have a lowest panel of 609 & highest of 634 kWh. A 4% difference. At what point might you consider there a problem with the lower performing panels?

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