Signs your solar system is underperforming

The single most useful test is a year-on-year comparison at the same time of year. Solar output varies enormously by season, so comparing October against July tells you nothing except that it is autumn. Comparing this October against last October tells you a great deal. If output has fallen noticeably against the equivalent period in previous years, and the weather has not been exceptional, something has changed. The next step is to work out whether it is a component fault, soiling, new shading or a settings change — most of which can be established without going near the roof.

Compare like with like

Pull up your monitoring history and set it to monthly totals. Look across years at the same month.

A small decline year on year is expected — panels degrade slowly. A step change is not. A drop that appears in a single month and then persists points at a component that failed on a specific date.

If you have no historical data, compare against the performance estimate produced at installation, remembering that estimates are modelled annual figures rather than promises about a given month.

Look at the inverter

Most faults announce themselves here, and most people never look.

Error codes or fault lights. Note the exact code and check it against the manual before calling anyone; some are transient grid conditions that clear themselves.

Zero output on a bright day. A dead inverter, a tripped isolator or a tripped protective device at the board.

String imbalance. Where the inverter reports per-string figures, two similar strings should produce similarly. One string near zero means that string is offline — a blown fuse, a failed connector, an isolator left off.

Output capped well below expectations at midday. Could be a configured export limit, a derating condition, or an inverter running hot in a poorly ventilated loft.

Check the obvious external causes

New shading. Trees grow. Neighbours build extensions, install aerials, or let a hedge run away. Shading that did not exist at installation is one of the more common causes of a gradual decline, and it is usually worse in winter when the sun tracks low.

Soiling. Dust, pollen, lichen, moss creeping up from the tile line, and bird droppings. Expect a gradual reduction, worse on shallow pitches and where rain does not wash the array clean.

Bird activity. Nesting under an array traps debris, blocks airflow and produces persistent soiling. Often the first sign is noise rather than a monitoring figure.

Debris at the bottom edge. Leaves and moss collecting along the lower row, shading the bottom cells. Disproportionately damaging because of how strings behave.

Things that look like faults and are not

Clipping. An array producing more than the inverter can convert will sit at the inverter’s limit on the brightest days. This is normal in a deliberately over-sized design and not a fault.

Hot weather. Panels are less efficient at high cell temperatures. A very hot, hazy July afternoon can produce less than a cool, clear May morning.

Battery behaviour changing. A change in tariff, a firmware update or a settings change can alter charge and discharge windows. Check settings before assuming hardware.

A tariff or app change. Sometimes the system is fine and the reporting changed.

A sensible order of investigation

1. Compare against the same period in previous years.
2. Check the inverter display or app for codes, zero output or string imbalance.
3. Check isolators and the consumer unit for anything tripped or switched off.
4. Look at the array from the ground with binoculars: visible soiling, debris, obvious damage, nesting material.
5. Check whether shading has changed since installation.
6. Review settings, especially if a battery or a tariff is involved.
7. If none of that explains it, arrange a health check.

What a health check involves

An engineer will inspect the array and mounting, test the DC strings including insulation resistance and open-circuit voltage, check connectors and cabling, inspect the inverter and its environment, verify isolation and protection, review monitoring data, and check the array for damage that is not visible from the ground.

Where module-level electronics are fitted, per-panel data makes this much quicker — an underperforming module identifies itself rather than having to be found.

Do not go up there

The most dangerous part of owning a solar system is the temptation to inspect it yourself. Roofs are unforgiving, DC voltages on a live array are substantial, and panels are damaged easily by being walked on. Diagnose from the ground and from the loft, and bring in access equipment for the rest.

Published generation estimates and comparisons are illustrative industry figures, not Solar Fx figures for your system. Actual generation, savings and payback depend on the property, roof orientation and pitch, shading, system design, your electricity usage and tariff, export rates and future energy prices. Figures shown are estimates and are not guaranteed.