Every July the phone rings with the same worry. The app shows 10 kWh for the day, and back in January it was showing 35. Nothing is broken. In Melbourne, a healthy system makes roughly a third as much in the middle of winter as it does at the height of summer.
Shorter days and a lower sun
Two things happen in winter. The days are shorter, so there are about nine and a half hours of daylight in late June against more than fourteen in late December. And the sun sits much lower in the sky, so its light arrives at a shallow angle and passes through more air on the way. Each panel simply receives less energy. Add Melbourne's grey winter weeks and the daily total falls a long way.
As a rough guide, each kW of panels makes about 3.6 kWh a day averaged over the year. In January that is closer to 5.5. In June and July it is nearer 1.8. For a 6.6 kW system that is about 36 kWh a day in midsummer and about 12 in midwinter. These are estimates, and your roof will differ.
Why shade gets worse in winter
A low sun throws long shadows. The neighbour's second storey, the chimney or the plane tree out the front may not touch your panels in December but can cover half of them at 3 pm in June. On the narrow blocks around Northcote and Thornbury this is the most common reason winter output looks worse than the guide. It is also why we fit microinverters or optimisers on shaded roofs, so one shaded panel does not hold back the rest.
What is not normal
A winter drop is gradual and follows the season. These signs point to a fault instead:
- Output falls off suddenly from one week to the next
- The app shows zero on a clear day
- The inverter shows a red or orange light or an error code
- This July is far below last July in similar weather
That last check is the most useful one. Compare the same month across years, not winter against summer.
What you can do about it
Not much, and that is fine. Run the dishwasher, washing machine and hot water in the middle of the day, when winter solar is at its best. Do not climb up to clean or inspect the panels. If the numbers look wrong for the season, ask us for a health check.