1. Generation that doesn't match the forecast
Every solar installation comes with a generation estimate — either from the installer's proposal or from post-installation modelling. If your measured generation is consistently below that estimate for the same time of year, something is wrong. Common causes include shading that wasn't accounted for, panel soiling, degraded string connections, or inverter faults. The key word is 'consistently' — a single cloudy week isn't a signal. A rolling 30-day underperformance against modelled generation is.
2. High self-consumption that doesn't move the bill
A solar system that's generating well should reduce your grid import. If your monitoring shows high self-consumption but your bills aren't going down, the metering configuration may be wrong — you could be seeing gross generation counted as self-consumption when the export is actually higher than it appears. Check that your monitoring separates import, export, and generation correctly. A common misconfiguration is treating load minus generation as self-consumption without verifying against actual import data.
3. Inverter clipping at mid-day
Inverter clipping happens when a solar array generates more DC power than the inverter can convert to AC. The inverter flattens its output at its rated capacity, and the excess generation is lost. A small amount of clipping on peak summer days is normal. Persistent clipping for hours at a stretch suggests the system was sized with too small an inverter, or that the inverter's capacity has degraded. You can see this clearly in generation data — the output plateaus sharply rather than following a natural bell curve.
4. Single-string failure in a multi-string system
Most commercial systems have multiple strings of panels feeding into the inverter. If one string develops a fault — a failed panel, a corroded connection, a blown fuse — the rest of the system continues operating. The total output drops, but it doesn't stop. Without string-level monitoring, this fault can go undetected indefinitely. Typical indicators are generation that's running at a consistent fraction of expected output — for example, a four-string system running at 75% on clear days suggests one string has failed entirely.
5. Night-time draw from the inverter
Most inverters enter a low-power standby mode when solar generation stops. A small standby load (a few watts) is normal. An inverter drawing 50W or more overnight typically indicates an inverter fault or a failed bypass relay. On an annual basis, an inverter drawing 100W overnight costs roughly $150–$200 in additional grid import — and it's a sign that the inverter may be approaching the end of its operational life. This is easy to see in 5-minute interval data: look for a consistent non-zero load on the solar circuit during dark hours.