Six fire engines and around 40 firefighters tackled a flat fire on New Park Road in Clapham Park.
Part of the roof of a residential block was damaged by fire. Nine people, including a child, were led to safety. There were no reports of any injuries.
A London Fire Brigade spokesperson said: "On arrival, firefighters saw smoke coming from the fifth floor and entered the property to tackle the fire, which was burning beneath the roof lining.
"Firefighters used PVStop on solar panels. PVStop, is a black liquid polymer coating designed to cover solar panels like a liquid tarpaulin. Stored in an extinguisher, it’s sprayed onto solar panels when crews respond to fires. PVStop works by blocking the sunlight that powers solar panels, so the process of converting light into electricity is stopped, allowing crews to safely respond."
The Brigade was called at 1303 and the fire was under control by 1451. Fire crews from Clapham, Brixton, West Norwood, Battersea and surrounding fire stations attended the scene.
The fire is believed to have been accidental and caused by resistive heating with the solar panel terminals.