“Should I clean them???” …it’s a question we get from customers before (and after) we install solar panels. The answer depends on the amount of lost energy and electricity savings. Let’s find out how much energy you are losing by having dirty solar panels.
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The Dirty Solar Panel Experiment
Perfect Testing Conditions
An opportunity to analyze the impact of dirty solar panels presented itself on my personal solar system. I have a low slope roof (2:12) and have managed to miss every isolated rain storm for the last two months. My house is less than a year old and I am constantly stirring up dirt from various construction projects. All of these conditions have made my solar panels filthy.
I have multiple arrays on my house including the HESOLAR Test Array where we compare solar panels. This experiment will use an isolated row of 12 x SunPower 335watt solar panels with SolarEdge DC Optimizers. DC optimizers will allow us to measure variations in Power and Energy from the solar panels.
Experiment Process:
1) Record the Solar Panel Production BEFORE CLEANING. Note that each solar panel has a slightly different power output. Premium solar panel manufacturers will guarantee a Positive Power Tolerance meaning that every solar panel performs AT or ABOVE an advertised value. The Power and Energy data recorded before cleaning allows us to identify the impact dirt has on each individual solar panel.
BEFORE CLEANING: Power Output for Each Solar Panel when compared to the Overall Average. (Percent Difference)
2) Clean the Solar Panels: We are going to clean every other solar panel as a control for the experiment. Most solar panel manufacturers recommend using water and a soft rag for cleaning. Common Sense comes in to play here: DO NOT USE chemicals, pressure washers, abrasive rags, etc…
We removed dirt and pollen from every other solar panel as a control for the experiment.
3) Record the Solar Panel Production AFTER CLEANING. We used the dirty solar panels as reference to find an interval with a similar amount of sunlight.
Module Level Monitoring allows us to display a 2.6% increase in Power Output from Solar Panels
Results – from Cleaning Solar Panels
It’s important to understand the difference between our two measurements: Power and Energy. Power is the instantaneous output (Watts) at a given time and energy is the amount of power delivered over a period of time (Watt Hours).
Power Improvement from Clean Solar Panels:
The graphic above concludes that clean solar panels will produce 2 to 3% more power then dirty solar panels.
Note that we do not have an easy way to quantify “dirty” besides the pictures above. Power output is popular for discussion but only offers data for a single moment of time. Our energy data below will give more practical results for solar owners wondering how much money they are losing from having dirty solar panels.
Energy Improvement from Clean Solar Panels:
We measured an average 3.5% higher energy yield with the clean solar panels. The table below displays the daily energy yield difference between dirty and clean solar panels.
% Energy difference from Control Average
Panels 1, 3, 5, 7, 9 and 11 are LEFT DIRTY throughout the experiment. The average of these panels is the CONTROL AVERAGE.
| BEFORE Cleaning | AFTER Cleaning | Cleaning Effect | |
| Panel 1 | -0.98 % | -1.01 % | -0.03 % |
| Panel 2 (DIRTY) | 1.71 % | ||
| Panel 2 (CLEAN) | 5.39 % | 3.67 % | |
| Panel 3 | 0.24 % | 0.25 % | 0.02 % |
| Panel 4 (DIRTY) | 0.96 % | ||
| Panel 4 (CLEAN) | 4.41 % | 3.45 % | |
| Panel 5 | -0.06 % | -0.15 % | -0.09 % |
| Panel 6 (DIRTY) | -4.06 % | ||
| Panel 6 (CLEAN) | -0.67 % | 3.39 % | |
| Panel 7 | -1.57 % | -1.57 % | 0.00 % |
| Panel 8 (DIRTY) | -1.16 % | ||
| Panel 8 (CLEAN) | 2.31 % | 3.47 % | |
| Panel 9 | 2.74 % | 2.75 % | 0.01 % |
| Panel 10 (DIRTY) | 1.69 % | ||
| Panel 10 (CLEAN) | 5.20 % | 3.51 % | |
| Panel 11 | -0.37 % | -0.26 % | 0.10 % |
| Panel 12 (DIRTY) | -0.09 % | ||
| Panel 12 (CLEAN) | 3.32 % | 3.41 % |
Conclusion
Dirty solar panels will see an average 3.5% Production boost when cleaned. In my opinion this is an insignificant loss when considering how dirty the solar panels were! Dirt accumulation (soiling) is accounted for by our design team when calculating solar production. Decide for yourself: Consumers can use the formula below to estimate lost revenue from dirty solar panels:
Cleaning Value Formula: [Monthly Production in kWh] x [0.035] x [Electricity COST per kWh]
Rain Cleaning: After writing the rough draft for this HE Article we had a heavy rain come through. The cleaned panels now have an average 1.85% Performance Boost over the dirty solar panels. This displays that a good rain shower can effectively clean dirty solar panels… just not as good as water, a rag, and some elbow grease.
% Energy difference from Control Average
Panels 1, 3, 5, 7, 9 and 11 are LEFT DIRTY throughout the experiment. The average of these panels is the CONTROL AVERAGE.
| BEFORE Cleaning | AFTER Cleaning | AFTER RAIN | Cleaning Benefit AFTER Rain |
|
| Panel 1 | -0.98 % | -1.01 % | -1.02 % | -0.04 % |
| Panel 2 (DIRTY) | 1.71 % | 2.39 % | ||
| Panel 2 (CLEAN) | 5.39 % | 4.01 % | ||
| Panel 3 | 0.24 % | 0.25 % | 0.19 % | -0.05 % |
| Panel 4 (DIRTY) | 0.96 % | 2.00 % | ||
| Panel 4 (CLEAN) | 4.41 % | 2.97 % | ||
| Panel 5 | -0.06 % | -0.15 % | -0.23 % | -0.18 % |
| Panel 6 (DIRTY) | -4.06 % | 1.51 % | ||
| Panel 6 (CLEAN) | -0.67 % | -2.55 % | ||
| Panel 7 | -1.57 % | -1.57 % | -1.60 % | -0.03 % |
| Panel 8 (DIRTY) | -1.16 % | 1.64 % | ||
| Panel 8 (CLEAN) | 2.31 % | 0.48 % | ||
| Panel 9 | 2.74 % | 2.75 % | 2.59 % | -0.14 % |
| Panel 10 (DIRTY) | 1.69 % | 1.87 % | ||
| Panel 10 (CLEAN) | 5.20 % | 3.57 % | ||
| Panel 11 | -0.37 % | -0.26 % | 0.07 % | 0.44 % |
| Panel 12 (DIRTY) | -0.09 % | 1.69 % | ||
| Panel 12 (CLEAN) | 3.32 % | 1.6 % |
Impact of Rain on Solar Panels
This experiment displays the benefits of rain on solar panels. However, rain can also cause (VERY SMALL) performance issues on solar panels at a low angles. Here’s what happens:
- Rain carries dirt and pollen
- The solar panel frame sits up higher than the glass on the solar panel and creates a small area for the rain to pond
- The rain later evaporated leaving behind the dirt and pond
- The video below displays this effect and shows off the drainage feature built into Panasonic HIT Solar Panels.