Page 314 -

Appendix C
Comparison of Different Power-Generation Methods 279

Off-grid PV systems have traditionally used rechargeable batteries to
store excess electricity. With grid-tied systems, excess electricity can be
sent to the transmission grid. Net metering programs give these systems
a credit for the electricity they deliver to the grid. This credit offsets
electricity provided from the grid when the system cannot meet
demand, effectively using the grid as a storage mechanism.
Pumped-storage hydroelectricity stores energy in the form of water
pumped when energy is available from a lower-elevation reservoir to a
higher-elevation one. The energy is recovered when demand is high by
releasing the water to run through a hydroelectric power generator. See
the large list of references at http://en.wikipedia.org/wiki/Solar_energy.

ptg