The best time is usually cool panels + low rain risk + enough time to dry. In the UK that often means late morning or early afternoon on a mild day, or early evening in summer (if the panels have cooled and rain risk stays low).

What matters most

  • Rain in the next 1–2 hours: even light showers can undo your drying.
  • Panel temperature: hot paint makes shampoo and rinse water flash-dry (spots).
  • Wind + humidity: wind helps drying; very humid air slows evaporation.

A simple rule of thumb

If it's not raining, the next hour looks low risk, and the car will dry steadily (a bit of breeze, not too muggy), it's a good time.