The key readings to understand
DC input power (Watts or kW): the power currently being generated by the panels. On a clear sunny day, a well-positioned 4kW system should show close to its rated capacity around solar noon — typically 3.2–3.8kW after inverter losses. Significantly lower readings on a clear day (say, under 2kW for a nominally 4kW system) are worth investigating.
AC output power (Watts or kW): the power currently being delivered to your home and/or exported to the grid. This will be slightly lower than DC input due to inverter conversion losses — typically 2–5% below DC input on a well-functioning modern inverter.
Daily yield (kWh): the total energy generated since midnight. Compare this to a rule-of-thumb expectation: in summer (June/July) a south-facing UK system should generate roughly its rated kWp × 5–6 hours of peak sun per day; in winter (December/January), roughly × 1–1.5 hours. A day significantly below these ranges on a clear day warrants attention.
Grid status and export
Most modern inverters display whether the system is connected to the grid. A "grid-tied" display is normal. "Grid fault" or "grid off" on a grid-tied inverter means the system has detected a grid anomaly and has disconnected itself — this is a safety feature (required under G98/G99 regulations) and typically resolves within 90 seconds when the grid normalises.
Some inverters display a separate export reading — how much power is currently going to the grid. This figure is the gap between what is generated and what the home is consuming at that moment. Many older inverters do not display this directly; you need a separate generation meter reading and consumption monitoring to calculate it.
Error and fault codes
All inverters display fault codes when something is wrong. Common codes include: isolation fault (high resistance detected between DC circuit and earth — take this seriously, do not reset without investigation); over-temperature (inverter is too hot — check ventilation around the unit); grid frequency fault (grid frequency outside tolerance — usually transient and self-resolving).
Your inverter's manual lists all fault codes and recommended responses. Keep it accessible — or note the model number so you can find the PDF online if needed.
If your inverter is showing a fault code repeatedly and resetting but not clearing it, contact your installer. Some fault conditions — particularly isolation faults — indicate a genuine electrical problem that should not be ignored.
When to be concerned
On a clear, sunny day, if your DC input is significantly below what you would expect for the time of year, suspect soiling, shading, or a fault. Soiling is the most common cause of gradual underperformance. A quick visual check of the panels from the ground — looking for bird droppings, accumulated grime, or visible lichen — is a useful first step.
If the inverter shows zero output on a day with clear sky, check the AC isolator (typically a red rotary switch near the inverter) has not been accidentally turned off. This is a surprisingly common cause of apparent system failure.
Consistent underperformance over weeks — not a single bad weather day — is the signal that warrants a professional inspection or cleaning visit.