Bird Droppings on Solar Panels: How Much Production Loss?
Bird droppings cause more solar production loss than general dust, sometimes by a factor of 10. Here's the data and what to do about it.
Most homeowners think bird droppings are mostly an aesthetic problem. They're not. Droppings cause concentrated 'hard shading' on a single cell, which can disable an entire panel, and sometimes an entire string, out of proportion to how much surface they cover.
Hard shade vs. soft shade
Dust and pollen create soft shade, a small, even production hit across the whole panel. Bird droppings create hard shade: 100% blocked on one cell, normal output on the rest. Modern solar panels handle hard shade poorly because of how their bypass diodes are wired. A single dropping on the wrong cell can knock 30% off a panel's output.
Why this is worse than it sounds
Panels are wired in series. A panel running at 70% can drag down every other panel in its string. In severe cases we've measured a single dropping costing 200+ watts across an array.
What to do about it
Schedule cleaning more frequently if you have heavy bird activity. Don't try to clean droppings yourself with a garden hose, dried droppings need to be softened with deionized water and lifted with a soft brush, not blasted off. Aggressive hose pressure can micro-scratch the anti-reflective coating where the dropping was.