Use purified water under 10 ppm TDS to clean solar panels — never soap, and never a high-pressure washer. Tap water dries into a mineral film that scatters light and costs yield; detergents leave a residue that attracts dust faster. Deionised or RO-filtered water dries off panels completely clean.
Why tap water undermines the clean
South African tap water typically carries 50–300 ppm of dissolved minerals. On a window that means visible spots; on a panel it means a translucent film across the very surface that must transmit light. You can rinse a panel with tap water and make it look clean while leaving a yield-costing layer behind — measured output, not appearance, is the test.
The three rules of panel-safe washing
Purity: below 10 ppm, verified with a TDS meter. Softness: soft bristles and water pressure no stronger than a garden flow — high-pressure washers can force water past seals and stress cells, and several panel warranties exclude pressure-washing damage outright. Temperature: clean cool panels, morning or late afternoon, so water neither shocks hot glass nor flash-dries into film.
What about soap, and what about rain?
Soap trades one film for another; its residue is sticky and re-soils the panel faster than plain dirt would. Rain is nature’s tap water — 5–30 ppm plus whatever it washes off the air and the frame above the cell. Rain keeps panels from getting worse; it does not make them clean. The general water science is in why pure water dries spot-free, and the full comparison in best water for cleaning windows.
Getting purified water at panel scale
A few panels: DI-purified water from a small vessel is enough. Full rooftops and repeat work: an RO stage in front of the resin drops the running cost to near zero per litre — the decision maths is in RO vs DI for window cleaning.
The purity hardware
TDS meter R260 to test what you have; reverse osmosis system R3,900 or the mobile RO water trolley R15,210 to make what you need. Delivery systems — brushes to machines — are under solar panel cleaning machines.