Do dirty solar panels really cost you money? A Sacramento guide

Why this matters more in the valley

In a lot of the country, spring and summer rain rinses solar panels on a regular basis. In Sacramento, we go from May to October with almost no rain at all. That is also when panels produce the most power, so a six-month layer of dust, pollen and bird droppings sits on the glass during the exact months you most need it clean.

Studies of soiling losses in dry, agricultural regions of California have measured output reductions ranging from a few percent to well over ten percent for panels left uncleaned through a dry season. Results depend on tilt angle, nearby dust sources and bird activity, but the pattern is consistent: dry climates lose more.

How to tell if your panels need cleaning

You do not need to climb on the roof. Try these instead:

• Compare your monitoring app's production on a clear day in June to the same period last year.
• Look at the panels from an upstairs window or the street with binoculars for a visible film or white spots.
• Check for bird droppings, which block individual cells and can reduce output across a whole string.
• Look for a line of dirt along the bottom edge of each panel, where dust collects as dew runs down.

Why you should not use a pressure washer

High-pressure water can force moisture past seals, damage anti-reflective coatings and even crack cells. Many panel manufacturers specifically warn against pressure washing, and doing so can put your warranty at risk.

Household detergents are another problem. Soap leaves a residue that attracts dust faster, and some cleaners can damage coatings. Hard tap water leaves the same mineral spots on panels that it leaves on windows.

The method we use

Our crew cleans panels with soft-bristle brushes fed with deionized pure water. Because pure water has no minerals, it dries without spots and leaves no residue. We work early in the morning while panels are cool, since spraying cold water on hot glass in the afternoon can cause thermal stress.

Most arrays can be cleaned from the ground or the roof edge with extension poles, so we avoid walking on shingles or tile. While we are up there we photograph the array and flag anything your installer should look at, like cracked glass, loose wiring clips or nesting birds underneath.

How often to clean

For most Sacramento homes, once a year in late May or June is enough, right before peak production. Homes near farmland, highways, construction or with heavy bird activity often benefit from a second cleaning in late summer.

If your home also needs window cleaning, bundling the two saves the trip charge and gets both done in a single morning.

Is it worth the cost?

It depends on the size of your system and how dirty it gets. For a typical 20-panel home system, professional cleaning runs around $160 to $200. If soiling is costing you even a few percent of summer production, a single cleaning can come close to paying for itself over the peak months, and you get an inspection of the array at the same time.

The simplest approach is to check your production data this year. If the numbers dipped more than you expected, a cleaning is a low-risk way to find out how much you are leaving on the roof.