British Themal Unit = 1 lb of H2O x 1 degree F or 1 wooden match

Therm = 100,000 Btu = 100 ccf natural gas

Therm = 29.2 kwh

F = Solar Fraction: percent of total heat load provided by solar

DHW: Domestic Hot Water

Collector Efficiency:  Percent of total solar radiation absorbed by collector.  Solar thermal is about 50% - 70% efficient.