How do I know if my PV system is working properly? This is a very good question and something that many customers of solar energy systems wonder. The answer is to spend some time and sometimes some money in monitoring the performance of your system over an extended period of time.
Perhaps the first point to mention is that a good installer should do this for you. They should care deeply whether or not their installations perform as well as they will have predicted. Before choosing your installer try to determine what they offer in terms of after sales support and care.
If you like to take matters into your own hands there are a number of ways to monitor your system. If you are not inclined to pay extra for monitoring hardware, the simplest solution is to use information from the displays of the generation meter or the inverter (or both). Both these instruments will be present in all PV systems installed in the UK by a microgeneration certification scheme installer and will be able to tell you the number units of electricity generated since the system was put in. Get into the habit of checking this number at the end of each day, along with a record of what the weather was like – clear sky, partial cloud, heavy overcast etc …
Your installer will have provided you with an estimate of the annual energy production in kWh – in southern England this should be in the region of 850 kWh per kWp). This means that over the course of the year you should expect an average of two and a half kWh of energy each day for every kWp you have installed. This is heavily dependent on the weather of course. On a clear day you could expect 8 kWh, and on a very dark day as little as 0.2 kWh. After several days of checking you should be able to have an idea of whether your system is significantly under-performing.
Over time, your measurements will become more reliable. Each month of the year has an expected solar energy output. Ask your installer for a chart showing the average monthly variation of solar energy for your location. After one month you can compare the energy you got with what you would expect. Be warned however, monthly solar energy output can vary widely, some months can be particularly good or bad for solar compared to normal, so comparing with the average is not necessarily accurate. Still, you should be able to tell if you are getting 30% less energy than you should be.
Getting a more accurate picture of your system's performance is challenging. For instance, if your system is underperforming by 10%, how do you prove this?
A good method is to compare it to a near reference system which is known to perform well, and compare daily production to that. This can be challenging to find however, especially in the UK where there are still very few PV installations.
Another technique is to try to determine if there is something wrong with your system. Your inverter should also tell you the DC voltage and current coming from your solar panels. Under a clear sky, you can check if these values are in line with what they should be from the datasheet. Whether the voltage or current is lower than expected can provide information on what might be wrong.
If your system is underperforming, it could well be because of shading. If shadows are passing across the solar panels during the course of the day that were not accounted for in the system design, then this can really contribute to underperformance. Try to get in a position where you can see the solar panels at different times of the day. Any shadows on the module during the middle of the day (when they should be producing the most energy) can be serious. Many systems will be shadowed in the morning or evening, but this is generally less severe. Make sure to also check for dirt or muck on the panels, even small markings can cause big performance losses for solar systems.
It is also worth looking out for long term degradation. Whilst rare, it has been known for systems to get significantly worse over time.
If you would like to buy a monitoring solution yourself then the simplest product is a power meter. Examples are the 'OWL' meter or the 'Wattson' made by DIY Kyoto. These are both simple meters that can easily be installed by clipping a sensor onto the AC output cable of your inverter. What's useful is that the data can be transmitted wirelessly and viewed in real time and even stored to show you how energy production varies over the course of the day.
Depending on your installer, many will offer to monitor your system for you. This can be advantageous since they will have access to data from a large number of systems on which to benchmark performance. Make sure to ask them exactly how they plan to do this however.
On the upside, most systems should perform fine and are illegally to go wrong but when you're investing such a large amount of money in a PV system, its nice to know how to check its working.