First, compare like with like
Solar output changes through the day and the year. Compare a clear midday today with a clear midday from earlier, not with a cloudy evening. Most inverters or their phone apps show daily production, so look at that over a few days before deciding something is wrong.
Common reasons you can spot from the ground
- Shade: a new building, tree, water tank or wall now throws a shadow on part of a panel. Even a small shadow can cut the output of the whole string.
- Dust, bird droppings and leaves on the glass. Dust builds up faster after dry spells and construction nearby.
- Heat: panels give a little less power on very hot days even when the sun is strong. This is normal.
- Haze, smog, clouds and short winter days. See our guide on winter output.
What not to do, and when to call a technician
- Don't climb onto the roof if it is wet, dusty, windy or very hot, and never walk on the panels.
- Don't touch or open the panel wires, connectors or the DC box. Panels make electricity whenever there is light, and the DC side can carry a high voltage.
- Call a trained technician if output stays low on clear days after the panels are clean, if the inverter shows an error, or if you see burn marks, melted plastic or cracked glass.