The project is being implemented with no upfront capital investment from Septa, being financed through a 20-year battery services agreement with Constellation.

The ESS will provide 8.75MW of frequency regulation support on the PJM electrical grid and recover energy from regenerative dynamic braking.

Septa signed an agreement in June 2010 with Philadelphia-based smart grid company Viridity Energy and the partnership subsequently secured a $US 900,000 grant from Pennsylvania Energy Development Authority to install an energy storage system at Letterly substation.

The Letterly installation was commissioned in 2012 and is built around a Saft 800kW lithium-ion battery with ABB Envitech power equipment and Viridity VPower energy management software, capturing and storing regenerative braking energy discharged by trains on a five-station stretch of the line.

At the end of 2013 Septa awarded ABB a contract to supply a second Enviline energy storage and recovery system, which is being installed at Griscom substation on the Market - Frankford Line. This installation, which has been funded with the aid of a $US 1.44m grant from the Federal Transit Administration, differs from the Letterly system in that it uses a hybrid storage system, combining supercapacitors with batteries. According to ABB this enables the recovery of more braking energy, generates higher revenues from frequency regulation, and extends the life of the batteries.