What is the energy input for a device that provides 20 units of useful energy and has an efficiency of 0.25?

Efficiency is the measure of how efficient a process is. It is used to assess the ability of a process in avoiding waste energy, materials, money and time in doing a desirable output. It is calculated as;

Efficiency = useful energy ouput / total energy input
0.25 = 20 / total energy input

total energy input = 80

It’s the energy source of stars

Outermost (valence) electrons