How often should you clean a utility-scale solar plant? A cost-based cleaning-interval method

Every clean costs labour, water, fuel and equipment time. Every day between cleans costs energy. The best schedule balances the two, and the balance point differs from plant to plant.

Practice shows how far apart the answers are. LONGi’s O&M manual asks for cleaning at least every 20–30 days in dry areas and every 40–50 days in rainy ones. What South African plants actually do is in soiling losses in South Africa.

Returns also fall with each extra clean. In southern Spain, one well-timed summer clean raised a 1 MW plant’s profit by up to 3.6%, while extra cleans paid less (Micheli et al. 2021). Both the number of cleans and their timing matter.

How do you set the cleaning interval by cost? Clean when the energy you win back is worth more than the clean. Research on global soiling costs (Ilse et al., Joule, 2019) shows the cost-optimal number of cleans rises with yield, soiling rate and tariff, falls with the cost of a clean, and responds gently: doubling the tariff doesn’t double the cleans. For your plant’s numbers, use the calculator.

A cost-based interval assumes dust builds up without a break. Rain breaks that assumption. South African rain findings, including Kalkbult near De Aar, are in soiling losses in South Africa.

In practice:

• Wet months: rain may do much of the cleaning for loose dust. Skip a scheduled clean when the soiling sensor says the glass is still clean.
• Dry months: the cost-based interval applies, because nothing else resets the loss.
• Bonded soiling: rain doesn’t remove cemented dust or lichen, and neither does a shorter interval. These need removal chemistry (IEA PVPS 2022; Ilse et al. 2019). See cement dust on solar panels.

Working plants show the same pattern. A 330 MW plant in Vietnam was cleaned by robot about four times in the dry season and not at all in the rainy season, and coated strings gained more over water-cleaned strings in the dry months than in the wet ones. On the Santiago Solar tracker plant in Chile, where frequent sandstorms drive the number of cleans, the O&M team cleaned four to six times a year, alternating one wet clean with two dry ones.

Timing of each clean matters as much as the average interval. Both need a daily soiling measurement: see how to measure PV soiling.

How do South African plants decide when to clean today? Survey figures for how often South African IPPs wash, and with what kit, are in soiling losses in South Africa.

A rule of thumb reported in South Africa is to clean when a constant gap of more than 50 W/m² appears between a cleaned and an uncleaned reference cell. A trigger like that is better than the calendar, but it ignores the tariff and the cost of a clean. Two plants with the same dust but different tariffs should not clean on the same trigger.

A coating changes the maths only if it works on your site. Long-term field results are typically 20–50% less soiling, and some coatings make soiling worse (Ilse et al. 2019). In a Northern Cape trial, a hydrophobic coating increased dust soiling (du Plessis et al. 2020). Prove it on your own strings before you stretch the interval.

1. Measure the soiling rate with a soiling station or sensor. How to fit that rate is in soiling ratio, rate and loss explained.
2. Cost a full clean honestly: labour, water and its treatment, fuel, equipment time and chemistry.
3. Use the calculator for your tariff and yield.
4. Set a dry-season interval, and let measured soiling decide whether to skip a clean after rain.
5. Review each season, and after any change of method, water or coating.

RenewChem designs cleaning strategies this way, with the O&M team that runs the plant: method, interval, water treatment, equipment and crew procedure. See cleaning and O&M strategy and working with your O&M team.