Original: In thermodynamics , the thermal efficiency ( eta _ { th } , ) is a dimensionless performance measure of a device that uses thermal energy , such as an internal combustion engine , a boiler , a furnace , or a refrigerator for example .
Simplified: So , for a boiler that produces 210 kW ( or 700,000 BTU\/h ) output for each 300 kW ( or 1,000,000 BTU\/h ) heat-equivalent input , its thermal efficiency is 210\/300 = 0.70 , or 70 % .