Every grid-connected solar system in the UK has a small meter of its own, separate from the one your supplier reads. It is the generation meter, and it counts every kilowatt-hour the panels have ever made. It is the most honest instrument in the house, and almost nobody looks at it.
Finding it
It is usually within a metre or two of the inverter, or next to the consumer unit: a small grey or white box with a digital display and a red light that flashes faster on a sunny day. The display shows a running total in kWh, sometimes cycling with other figures. The total is the one you want. On a system fitted under the Feed-in Tariff you will have read it every quarter to claim; on a newer system you may never have been shown it.
If you genuinely have no generation meter, the inverter’s own display or its app will give a lifetime total. That is acceptable, with one caution below.
Taking the readings
Photograph the display with your phone. The photo carries the date, which saves writing it down and settles any later argument. Do it today, and put a reminder in your calendar for the same date next year. That is the whole method. If you already have an old reading, from a FiT claim or a previous photo, you have the year already.
Subtract the older reading from the newer one. That is what the system produced in the year between them. A full year matters: comparing April to October tells you about the seasons, not about the system.
What to compare it with
A well-placed system in the UK makes roughly 800 to 1,000 kWh a year for every kWp installed: more in the south-west, less in the north, less again on a roof that faces east or west rather than south, and much less with shading. Your system’s size in kWp is on the MCS certificate, the handover pack or the quote.
So a 4 kWp system facing south in the Midlands should show somewhere around 3,400 to 3,800 kWh between the two photos. Panels also age: allow about half a per cent less for every year since installation, so a ten-year-old system is entitled to be five per cent down on new.
If your figure is within ten per cent of that expectation, the system is doing its job, and the weather accounts for the rest. If it is fifteen to twenty per cent below, something is worth a look. If it is thirty per cent or more below, something is wrong, and it has probably been wrong for a while.
Why the app is not enough
Monitoring apps show you that the system is generating now, and a total that goes up. They rarely show you what the total should be, which is the only comparison that matters. Some apps also count differently from the meter, because the inverter measures on one side of the wiring losses and the meter on the other, so an app figure can read a few per cent higher than the meter. For year-on-year comparison either is fine, as long as you use the same one both times. For a warranty claim the meter wins.
When one year is not enough
A single bad year can be the weather. Two bad years in a row are not. Keep the photos, and the pattern tells you more each year. A sudden step down between two readings, rather than a slow slide, usually means a fault: a string that has stopped, an inverter that has been tripping, or a sensor that was never right.
What to do next
Take the photo today. If you already have an older reading, do the subtraction and the comparison above. The free yield check on this site does the comparison properly, from real weather data for your postcode rather than a rule of thumb: enter the size, the orientation, the year it was installed and your twelve-month figure, and it tells you whether the number is in the normal range. If it is not, the Health Check is the step that finds out why and what fixing it would be worth.