A fire department spokesman said the blast tore a hole in the roof of a family-owned alternative-energy business and shattered windows of nearby buildings in the complex, the Los Angeles Times reported.
Sources told the newspaper that firefighter Timothy Larson underwent surgery after suffering major injuries to his arms and legs. Another employee of the energy company also suffered serious injuries.
Fire officials are investigating the cause of the explosion that occurred late Tuesday.
Sylmar is a district in the San Fernando Valley region of Los Angeles.