Endoreversible heat engine efficiency is 1-sqrt(Tc/Th) Units are absolute degrees. For 120F heat and 70F ambient: 1-sqrt(294/322) = 4.4% So, for every 100 wat

Endoreversible heat engine efficiency is 1-sqrt(Tc/Th) Units are absolute degrees. For 120F heat and 70F ambient: 1-sqrt(294/322) = 4.4% So, for every 100 watts of heat, you could recover 4.4 watts of power.