Reaffirmation with minor editorial changes. Provides test methods for determining the thermal performance of flat-plate solar energy collectors that use boiling fluids for thermal energy transfer. Applies to flat-plate solar collectors in which some of the fluid entering the collector boils and some fraction leaves as a saturated vapor. Collector heat transfer fluid may be any fluid that boils during its passage through the collector. Methods for conducting tests outdoors under natural solar irradiance and tests indoors under simulated solar irradiance.