A farm solar array can lose output for months without anyone noticing: a tripped inverter on a shed nobody visits, a blown string fuse, a roof coated in grain dust. Monitoring is how you find out in days rather than at the next electricity bill. This page covers what a monitoring system should show, what "good" output looks like for your roof, and how to tell a fault from normal ageing.
What a monitoring system should show
- Generation per inverter — daily and monthly kWh, so one underperforming inverter stands out from the rest.
- Alerts — an email or app notification when an inverter stops, trips on grid voltage or reports an insulation fault.
- Import and export — from the export meter or the inverter's own meter, which you also need for Smart Export Guarantee payments.
- Your own consumption — a current clamp on the incoming supply shows how much of the output the farm uses itself, which is where most of the value is.
- String or panel-level data — optional, through optimisers or microinverters. Useful on complex roofs and multi-building sites, where one shaded or damaged section is otherwise hidden in the total.
On a site with arrays on several buildings, make sure every inverter reports into the same account. A fault on a distant barn is the one most likely to go unnoticed.
Check actual output against a proper prediction
The yardstick is a prediction for your location, roof angle and system size — not a rule of thumb. PVGIS, the European Commission's free solar model, gives monthly figures. For a south-facing 30° roof near York it predicts about 960 kWh per kWp a year: roughly 118 kWh per kWp in June and 29 in December. A 50 kWp array there should therefore produce something like 5,900 kWh in an average June and 1,430 kWh in an average December.
Weather moves those numbers from year to year, so compare over whole months and against the same months in previous years. One dull week is not a fault. A month well short of prediction in good weather, or one inverter consistently behind an identical neighbour, is.
Faults versus normal ageing
Panels do lose output slowly. A widely cited review of field data (Jordan and Kurtz, Progress in Photovoltaics, 2013) found a median degradation rate of about 0.5% a year for crystalline silicon modules. Anything much faster than that points to something fixable:
- A string reading zero or far below its neighbours — often a failed connector, a blown fuse or damaged cable.
- An inverter that trips repeatedly — on rural lines this can be grid voltage rising at times of high export; the network operator and installer can investigate.
- A gradual loss that cleaning restores — soiling. Livestock sheds, grain stores and dusty yards soil panels faster than most roofs, and bird droppings shade individual cells.
- A new shadow — a grown tree, a new silo or building on the south side.
To put a value on it: a 100 kWp array on a 30° roof near York generates about 96,000 kWh a year, so every 1% of lost output is about 960 kWh — roughly £240 a year at an assumed 25p per kWh of avoided import. A fault left for a season costs far more than that.
Keep the commissioning record
IEC 62446-1 sets out the tests and documentation for grid-connected PV systems. The results recorded at commissioning — string voltages and currents, insulation resistance, the inverter settings — are the baseline that later inspections and fault-finding compare against. Keep them with your monitoring login and your installer's contact details. The farm solar maintenance guide lists what an annual service should check.
Who watches the dashboard?
You can, and on a busy farm it helps to set alerts to more than one phone. Many installers also offer operation and maintenance contracts that include remote monitoring. If you take one, ask which alerts are set up, who responds and how quickly, and whether cleaning and repairs are included or charged separately. If you are thinking about adding panels or a battery, see upgrading an existing farm system; for storage on its own, see battery storage for farms.
Monitoring — frequently asked questions
What should a farm solar monitoring system show?
At minimum: generation from each inverter, alerts when an inverter stops or trips, and the export and import readings from your meter. Better systems add string or panel-level data and, with a current clamp on the supply, your own consumption — which is what tells you how much of the output the farm is using.
How do I know if my solar panels are underperforming?
Compare monthly output with what PVGIS predicts for your location, tilt and size, allowing for the weather. Panels lose output slowly — a review of field data put the median at about 0.5% a year — so a sudden drop, one string far below the others, or a month well short of prediction points to a fault, soiling or shading.
What is the baseline for checking a farm solar system?
The commissioning tests. IEC 62446-1 sets out the tests and documentation for grid-connected PV, and the results recorded when your system was installed are what later inspections and fault-finding compare against. Keep that paperwork with the monitoring login.