Thermal efficiency (ηth) of any heat engine is a dimensionless performance measure of a device that use thermal energy, such as internal combustion engine, boiler, furnace etc, and can be defined as the ratio of the work it does (W) and the heat input at the heigh temperature (QH)
Thermal Efficiency is the ratio of work done (W) and the heat at high temperature(QH). For Carnot engine efficiency is found by the difference between the highest temperature(Tₕ) and lowest temperature(Tc) to the highest temperature(Tₕ). For refregirator efficiency is found by the heat absorbed(Qₕ) to the difference between heat released(Qc) and heat absorbed(Qₕ).