Comprising inline flowmeter/integral BTU computer combination, System-30 measures thermal energy in chilled water, hot water, and condenser water systems. NIST-traceable meter has maximum operating pressure of 400 psi and liquid temperature range of 32-200°F. Semiconductor-based temperature sensors provide delta T accuracy within 0.15°F. Optional communications interface delivers energy, flow, and temperature data to control system through single network connection.