Waste Heat - Heat & Thermodynamics Step by Step

Waste heat is the part of the heat input that must be rejected rather than converted to useful work.

highlighted = computed this step

Not all heat becomes work

The heat input is 30 joules, but useful work is only 12 joules.

Q_{\text{in}} = 30\ \text{J}\qquad W_{\text{out}} = 12\ \text{J}

Waste heat is the input heat minus the useful work.

Q_{\text{waste}} = Q_{\text{in}} - W_{\text{out}}

Subtract 12 joules from 30 joules. The waste heat is 18 joules.

Q_{\text{waste}} = 30\ \text{J} - 12\ \text{J} = 18\ \text{J}