{"id":12360,"date":"2026-08-28T07:09:57","date_gmt":"2026-08-28T05:09:57","guid":{"rendered":"https:\/\/solar-assistance.be\/calculating-yield-loss-due-to-soiling\/"},"modified":"2026-08-28T07:09:57","modified_gmt":"2026-08-28T05:09:57","slug":"calculating-yield-loss-due-to-soiling","status":"publish","type":"post","link":"https:\/\/solar-assistance.be\/en\/calculating-yield-loss-due-to-soiling\/","title":{"rendered":"Calculating yield loss due to soiling"},"content":{"rendered":"

A system that produces noticeably less on a sunny day does not automatically indicate defective solar panels. Dirt, pollen, dust, bird droppings, and soot can restrict light intake. Anyone who wants to calculate yield loss due to soiling<\/strong> must look further than just the difference compared to last year. Solar radiation, temperature, shade, and technical malfunctions also influence production.<\/p>\n

A reliable calculation helps you make the right decision. Sometimes cleaning is profitable. Sometimes the cause lies elsewhere, and a technical check prevents unnecessary costs or longer downtime.<\/p>\n

Why soiling costs yield<\/h2>\n

Solar panels work best when sunlight reaches the entire panel surface. An even layer of dust or pollen usually reduces light intake only slightly, but spread over a large surface, it can still add up. Stubborn soiling often has a greater effect. Think of bird droppings, leaves, moss edges, agricultural dust, soot, or deposits that remain after long dry periods.<\/p>\n

Not all soiling causes the same loss. A thin layer of dust on a sloped roof sometimes washes away with sufficient rain<\/a>. On a flat roof, at a low pitch, or under trees, dirt tends to stay put. The environment also plays a role. Systems near heavy traffic, industry, fields, or coastal areas generally need more attention.<\/p>\n

Localized soiling is extra relevant. A small part of a panel that is heavily covered can affect the production of a larger part of the string. This depends on the electrical structure of the panels and the bypass diodes. Therefore, a visual impression alone is not enough. You want to know exactly how much energy the system is missing.<\/p>\n

Calculating yield loss due to soiling with comparable data<\/h2>\n

The simplest basic formula is:<\/p>\n

Yield loss in percent = (expected yield – actual yield) \/ expected yield x 100<\/strong><\/p>\n

Suppose your system is expected to produce 1,000 kWh in a comparable period. The inverter registers 930 kWh. The difference is then 70 kWh. The calculated loss amounts to 7 percent.<\/p>\n

The crucial question then is: how much of that 7 percent is actually caused by soiling? To answer this, you must substantiate the expected yield as well as possible. Do not compare blindly with the same month last year. April this year could be much sunnier or gloomier than last year. An expansion of the system, new shade, or a changed inverter setting also makes such a comparison less useful.<\/p>\n

Preferably use production figures per day or per week, along with solar radiation data. If you do not have a radiation meter, you can work with the yield of a comparable, well-functioning system nearby. This remains an indication. For an accurate picture, system data and monitoring are much more reliable.<\/p>\n

Correct for weather and temperature first<\/h3>\n

More sun generally yields more power, but panels perform less well when they get very hot. A clear, cold day can therefore yield relatively more than an equally sunny, hot day. Therefore, take radiation and temperature into account before identifying soiling as the cause.<\/p>\n

For systems for individuals, a practical comparison is often sufficient: choose two periods with similar weather, the same month, and a similar day length. For larger systems, a performance analysis based on measured radiation is the better approach. This allows you to see if the loss is structural and if it is limited to certain strings, inverters, or roof surfaces.<\/p>\n

Compare before and after a professional cleaning<\/h3>\n

The most practical method is a controlled before-and-after measurement. Record the production for several suitable days before cleaning. Then have the panels professionally cleaned and compare again under similar weather conditions.<\/p>\n

The formula remains the same:<\/p>\n

Cleaning effect in percent = (yield after cleaning – corrected yield before cleaning) \/ corrected yield before cleaning x 100<\/strong><\/p>\n

An example: your corrected production before cleaning averages 24 kWh per day. After cleaning, the same system produces 26 kWh per day with similar radiation. The difference is 2 kWh, or approximately 8.3 percent. This is an indication that soiling had a real impact.<\/p>\n

Note: never compare a single isolated day. Clouds, hazy clouds, temperature, and shade can distort the result. Include multiple days and preferably choose a stable measurement period. For larger systems, an analysis per string can provide a lot of extra clarity.<\/p>\n

From percentage loss to euro loss<\/h2>\n

A percentage makes the problem visible, but the financial effect often determines whether action is needed. First, calculate how many kWh you lose annually:<\/p>\n

Annual loss in kWh = expected annual yield x loss percentage<\/strong><\/p>\n

Does your system normally produce 5,000 kWh per year and does the analysis indicate a 6 percent loss due to soiling? Then it involves about 300 kWh per year.<\/p>\n

The value of those 300 kWh depends on what you would do with it. If you normally use the solar power yourself, multiply the lost kWh by your avoided purchase price for electricity. If you feed part of it back into the grid, the lower feed-in tariff applies to that part. For companies, the effect on the energy profile and peak load can also play a role.<\/p>\n

Example: of the 300 lost kWh, you would normally use 70 percent yourself. At an avoided electricity cost of 0.30 euros per kWh, that is 63 euros. The remaining 90 kWh yield another 4.50 euros at a rate of 0.05 euros per kWh. The estimated annual loss is then 67.50 euros. If the soiling is temporary and rain washes it away quickly, cleaning might not be necessary. If the problem returns regularly, preventive maintenance becomes more interesting.<\/p>\n

When dirt is not the real cause<\/h2>\n

Soiling is visible and therefore quickly suspected. Yet, a lower yield can also indicate a technical problem<\/a>. A defective panel, a loose connector, degradation, shade growth, an error in the inverter, or a string shutdown can give a similar pattern. Grid problems<\/a> can also cause the inverter to temporarily throttle or shut down.<\/p>\n

That is why monitoring is so valuable. With monitoring of solar panels<\/a>, you can see more quickly when production deviates from the expected pattern. You don’t have to wait months until the annual statement makes it clear that energy was lost. A deviation per inverter or string also helps to distinguish dirt from a malfunction.<\/p>\n

Do you see a sudden drop, error message, or large difference between two comparable roof surfaces? Then first have a targeted check to see what is going on. Cleaning does not solve a cable break, defective optimizers, or inverter problems. Conversely, timely cleaning prevents stubborn deposits from staying longer and unnecessarily depressing the yield.<\/p>\n

When is cleaning profitable?<\/h2>\n

Cleaning is not automatically necessary every year. The correct frequency depends on the location, pitch, source of soiling, and measurable yield drop. A system on a detached, sloped roof in a quiet environment often needs less maintenance than panels under trees, near agricultural activities, or on a flat commercial roof.<\/p>\n

The best decision combines three things: a visual inspection, production data, and the cost of the work. If the estimated annual energy value is higher than the cleaning cost, or if dirt poses a risk of permanent deposits and localized problems, taking action is logical. When in doubt, a professional cleaning of solar panels<\/a> provides clarity without the risk of damage from wrong products, excessive pressure, or unsafe work at heights.<\/p>\n

Bird droppings deserve special attention. They are often localized, stubborn, and return when birds nest under or around the panels. In that case, cleaning only has a temporary effect. Installing bird protection<\/a> addresses the cause and limits new soiling, damage to cabling, and nuisance under the panels.<\/p>\n

Make measuring a regular habit<\/h2>\n

Don’t just look at the total yield at the end of the year. Regularly check whether production evolves logically with the season and the weather. Note when you see visible soiling, when cleaning has taken place, and whether the yield changes afterwards. This way, you build a reliable reference point for your own system.<\/p>\n

For those who don’t have time for that, periodic maintenance of solar panels<\/a> offers peace of mind and an overview. A check looks not only at dirt but also at mountings, cabling, inverter operation, and visible wear. This limits unexpected downtime and protects your investment in the long term.<\/p>\n

A calculation doesn’t have to be perfect to be useful. If you compare with suitable data, take the weather into account, and exclude technical causes, you will see much faster whether soiling is actually costing money. This allows you to make a decision based not on feeling, but on what your system actually performs.<\/p>\n","protected":false},"excerpt":{"rendered":"

Calculating yield loss due to soiling? Compare measurement data, correct for weather, and determine if cleaning can make your solar panels profitable again.<\/p>\n","protected":false},"author":0,"featured_media":12361,"comment_status":"","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[],"class_list":["post-12360","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-niet-gecategoriseerd"],"acf":[],"yoast_head":"\nCalculating yield loss due to soiling | Solar Assistance<\/title>\n<meta name=\"description\" content=\"Calculating yield loss due to soiling? 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