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

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.

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?

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.

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:

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.

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.