Btu (heat) meters are widely utilized as a utility metering device and as a low cost instrument for evaluating performance of solar energy systems. Approximately 35 Btu meters were systematically tested prior to installation in active solar DWH systems under evaluation. The tests indicated wide variations in meter accuracy generally ranging from one to thirty percent for totalized Btu's and one to twenty percent for totalized flow at flow rates and differential temperatures ranging from 0.03 l/s (0.5 g/m) to 0.19 l/s (3.0 g/m) and 5.6°C (10°F) to 55.6°C (100°F). Measurement errors were found to be due to non-responsive temperature sensors, flow sensors not sensitive to low flow rates, quality control and various electronic problems. It was concluded that an acceptance test of a device was essential prior to installation. On high priority projects, periodic functional checks and/or calibrations are desirable to assure quality data.