On the longest day of the year — Philadelphians will experience 15 hours and 35 seconds of sunlight during Thursday's solstice — solar energy was at the top of the agenda for the Philadelphia Energy Authority.

The authority says that so far this year more than 750 residents have expressed interest in Solarize Philly, its group-buying program for residential solar customers, and 17 have signed contracts to install rooftop solar systems, said Laura Rigell, the authority's solar manager.

During the program's inaugural last year, 186 homeowners signed contracts, exceeding the goal of 50. The goal this summer is 150 new installations, Rigell said. The deadline to enroll is July 31.

Under the program, several installers agreed to discount the rooftop panels if enough customers sign up. This year's program also includes an option for subsidized financing for low- and moderate-income households. Eligibility will be determined by utility bill payment history rather than credit scores, Rigell said.

Owners of solar systems produce some of their own electricity, reducing their utility bills. Residents interested in installing solar can visit www.solarizephilly.org to learn more.