waste energy – energy that isn't transferred in a useful way

To remember the meaning of waste energy, use the following mnemonic:

It was a waste of energy (waste energy) because heat wasn't transferred in a useful way.

Waste energy refers to energy that is not usefully transferred or used for the intended purpose during an energy transfer or transformation. Whenever energy is converted from one form to another, some of it is always dissipated to the surroundings, usually as heat or sound, which cannot be used productively.

For example, in a light bulb, electrical energy is transferred into light energy (useful) and heat energy (waste), and in a car engine, chemical energy from fuel is converted into kinetic energy to move the car, but a large amount is wated as heat and sound. This wasted energy spreads out into the surroundings and becomes less useful because it is difficult or impossible to collect and use again.

The concept of waste energy is closely linked to efficiency - devices that waste less energy are more efficient. 

Reducing waste energy is important for conserving resources and improving the sustainability of energy use, which is why scientists and engineers work to design more efficient energy systems.