The fire affected two transmission lines and caused the collapse of the electricity system across the island, officials say.
Supply is expected to be restored to most areas on Thursday.
Puerto Rico's Electric Power Authority has been undergoing restructuring and is seeking funds to update what it says is outdated equipment.
The fire department said it had extinguished the blaze at the power plant in the south of the island, which serves most of the island's 3.5 million people.
The cause of the fire is still unclear.
There was traffic chaos as the outage forced schools and businesses to close earlier, reports say.
The blackout also caused 15 fires across Puerto Rico as a result of malfunctioning generators.
A big fire at a power plant has left 1.5 million people without electricity in the US territory of Puerto Rico.