An investigation into the cause of the blaze was opened on Thursday.
"Firefighters safely extinguished the fire on the roof and no other part of the hospital was affected by the fire," a spokesperson for the service said.
Avon Fire and Rescue Service station manager Ben Thorpe confirmed the cause was "accidental" and "due to a fault in the solar panels".
Mr Thorpe said there had been an increase in fires caused by solar panels.
"I think the statistics are showing that they are on the increase although of course solar panel installations are on the increase," he explained.
"Solar panels are inherently safe but I would say the advice given is that people should ensure they use a reputable company, they are qualified to install solar panels, with a regular maintenance schedule put in place."
