Shade on Solar Panels: How Tanks, Trees and Walls Cut Your Output

The roof looked perfect in June when the system was installed. By December, a shadow from the water tank or the neighbour’s new floor creeps across two panels every afternoon, and units drop more than anyone expected. Shade is one of the most common, and most avoidable, solar problems.

Why a small shadow matters so much

Panels in a string are connected in series. Current flowing through the string is limited by the weakest panel. So shading one panel can reduce the output of the whole string, not just that one panel.

Modern panels have bypass diodes, usually three per panel, that let current skip around shaded sections. This limits the damage, but you still lose more than just the shaded area.

Winter shadows are longer

In winter, the sun is lower in the sky, so shadows stretch much further north. A water tank that casts no shadow on your panels in summer may cover a row in December. Always check shade in winter conditions when planning.

How to find shading problems

1. Look at the roof at 9 am, 12 pm and 3 pm on a clear winter day. Photograph the shadows.
2. Check your inverter app. A sharp dip at the same time every day is a shade signature.
3. If you have two MPPTs, compare them. One string much lower than the other at certain hours points to shade.

Fixes, from simple to involved

• Trim trees or ask neighbours politely, if it’s a branch.
• Move a few panels to an unshaded spot, or raise them on a higher structure above the tank.
• Re-string so shaded panels are grouped on their own string or MPPT input.
• Optimisers on individual panels can help on complicated roofs, at extra cost.
• Raise the whole array above the tank and stair room, which also frees up roof space below.

Bird droppings and dust count too

A single bird dropping sitting on a cell for weeks is a small shade that can create a hot spot. Regular cleaning prevents it. See our cleaning guide.

Planning a new layout? Read series vs parallel wiring and roof space for solar.