This study presents a comparison of field measurements of energy delivered by five integral collector storage (ICS) passive solar water heater systems installed at various geographic locations in Virginia to predicted values calculated using Annual Performance Methodology (APM). APM is a prediction method developed by Alan Zollner that offers quick and easy comparisons of design option for ICS systems. Several different methods exist today that are meant to predict the performance of this type of system and that might be used as a design tool to help a designer make appropriate design decisions. Some of these methods are quite complicated and do not lend themselves to quick and easy comparisons of various design options. This study measured the amount of energy delivered by five ICS systems over a six month period. The amount of water drawn out of these systems daily was also recorded. This data was compared to predicted values calculated using APM to determine if APM could predict the performance of these systems within plus or minus 15 percent of the measured values. This study demonstrates that APM was able to predict long term performance of ICS systems within plus or minus 15 percent eighty percent of the time. Short term performance however demonstrated variation that in some cases were quite large and could not be considered reliable predictions. / M. Arch.