What the Wisconsin State Capitol Job Taught Me About Solar Panel Soiling

In May 2026, Solar Alchemist cleaned the Wisconsin State Capitol rooftop solar array, a 17-year-old installation that had never received a professional mechanical cleaning. What I measured changed how I talk about commercial soiling.

The site

• Installed: 2009 (17 years of accumulated soiling)
• Type: Government rooftop solar array, downtown Madison, WI
• Cleaning method: Pole-fed 100% deionized water applied through manufacturer-matched contact tooling (ground-based)
• Foot traffic on glass: Zero
• Chemicals used: Zero

The measurement

The State of Wisconsin's facilities team owns the production telemetry for the array. I worked from their data, not mine. The cleaning was performed on May 14, 2026, with a near-identical cleaning-day baseline and post-cleaning peak-hour measurement.

The result: 16.7% more sunlight reaching the glass, measured before and after a single cleaning — a same-panel spot reading, not a system-wide production claim. The instantaneous peak-power reading immediately after cleaning hit 933.9 W/m² on the tested panel (up from a cleaning-day baseline of 799.7 W/m²), the cleanest single-panel measurement the facilities team had recorded since 2017.

What 16.7% means in real terms

For an array of this size, a lift in the range documented on this specific site translates to thousands of kilowatt-hours a year of avoided utility offset. At mid-2026 Wisconsin commercial rates (EIA average ~$0.140/kWh), each percentage point of recovered production on a 1 MW array is worth roughly $1,750/yr. Results vary by site and by weather, and I don't promise a percentage.

Why the number was so high

NREL's field range for annual soiling loss varies widely by site, with dry regions and farm-adjacent installs at the top. The Capitol's lift landed above that typical band for three reasons:

1. 17 years of accumulated baseline. Most modern commercial arrays are cleaned on a 1–2 year cadence. This one never had been.
2. Downtown Madison air quality. The Capitol is surrounded by Highway 12/18/151 traffic, occasional construction particulate, and Lake Mendota airborne minerals.
3. Hard-water spotting. 17 years of rain-only "cleaning" left a measurable calcium-carbonate haze that only DI water could lift.

What it teaches the rest of us

Most commercial array owners have never measured their soiling loss. They assume rain handles it. The Capitol case study is a worst-case but representative data point for what 17 years of "doing nothing" actually costs.

If your facility is over 5 years old and has never had a professional cleaning, the highest probability outcome is that you're losing 8–15% of your production right now and don't know it. Request a custom bid within 24 hours and I'll quantify what your array is leaving on the roof.