Solar or battery system [Generating less than expected]what to check

A system generating well below its rated output is usually reporting the weather and the laws of physics rather than a fault, and the reason people are surprised is that the headline number is measured under conditions this country does not often provide. A panel’s rated output is established at a standard test condition — a specific irradiance and a specific cell temperature — and a real roof in Britain rarely meets both at once. Bright and cold beats bright and hot, because panel output falls as cells heat up, which is why a clear March day can outperform a hazy July one.

Two published non-faults account for a large share of the remaining confusion, and both are about darkness. SolarEdge publish a Night Mode status that appears when the input voltage is below six volts direct current, which is simply the inverter recognising that there is nothing to convert and standing down; their own troubleshooting note is to test the DC side only if the status persists in daylight. An inverter that goes dark in the evening and stays dark until morning has not failed to restart. Enphase publish the related point for their microinverter systems: if there is insufficient sunlight to power up the microinverters, they cannot communicate with the IQ Gateway, so the gateway’s communications light can sit solid red at dusk and dawn and the app can look as though it has lost half its panels while the system is entirely healthy.

Beyond that, the practical causes are the mundane ones and they are worth checking in order of how much they cost to fix. Shading is the big one and it changes through the year, so a system commissioned in June can be shaded by a neighbour’s tree in October when the sun is lower. Soiling matters more than people expect on shallow-pitched roofs, where rain does not run off effectively. Export limiting caps what the system is allowed to push out, which is a setting rather than a fault: Solis show it as the status LimByEPM, inverter under external control, and Fox ESS describe a soft limit that holds export below the set value within fifteen seconds. And a single underperforming string or microinverter will reduce the total without stopping it, which is the case worth chasing because it does indicate something real; Enphase’s own production-fault logic is that none producing points to the grid or the wiring, all on one branch points to that branch, and one or a scattered few point to the individual connections. The way to tell a fault from the weather is to compare like with like. One clear day this year against one clear day last year, at the same time of year, is far more informative than any absolute figure.

1. 1

   Compare against the same period last year rather than against the rated figure. A clear day this March against a clear day last March tells you far more than any absolute number.

2. 2

   Look at per-string or per-panel data if your monitoring provides it. A single underperforming section is the finding that actually indicates a fault.

3. 3

   Go outside around midday and look for shade on the array. Shading changes with the season and with anything that has grown or been built since the system went in.

4. 4

   Check whether export limiting is set. It caps what the system is allowed to send out and is a configuration rather than a fault; on a Solis inverter it shows as LimByEPM.

5. 5

   Ignore an inverter that goes dark overnight. SolarEdge publish a Night Mode status for input voltage below six volts, and Enphase publish that their microinverters cannot communicate with the gateway without sufficient sunlight — both are normal.

6. 6

   If one section is genuinely down, or output has stepped rather than drifted, contact your installer with the monitoring data. A step change is much easier to diagnose than a gradual one.