Is there a free EPC API?

Yes. The government EPC register provides a free API for raw certificates in England and Wales at epc.opendatacommunities.org. Homedata’s EPC API joins EPC data to a UPRN-keyed property record and costs 1 token, equal to 1p, for a served response. You can run five sample properties free in the API playground. Failed requests and properties with no data are not charged.