Efficiencies of photovoltaic system components

Supplemental material for the Feature Article "Home photovoltaic systems for physicists" PHYSICS TODAY, July 2008, page 42.

Thomas W. Murphy Jr

Nothing is 100% efficient. In addition to imperfect photovoltaic panels, a PV system is subject to inefficiencies in transmission, in the charge controller and inverter, and in charging the battery. As the numbers presented here will show, however, the components of a PV system perform rather well. Photovoltaic panels made of single-junction crystalline silicon typically achieve efficiencies of about 16%, which compares favorably with the theoretical maximum of 23% under conditions of unconcentrated solar illumination. Properly sized transmission wire should give better than 96% transmission efficiency. Charge controllers and inverters typically achieve efficiencies of about 95% and 90%, respectively. Lead–acid batteries have charge efficiencies (amp-hours out compared with amp-hours in) of around 90–95%. The energy efficiencies are not as good because charging takes place at a higher voltage than discharging. The batteries I use in my system achieve roughly 80% energy efficiency. The number improves if batteries are more deeply discharged but in that case the battery won't live as long.