Question

If 240J of energy is stored in a battery and only 4J is transferred today the surroundings to light it up, what is the efficiency of the energy ?

Answer 1

The efficiency defines how much useful work can be done compared to the total energy content. Therefore, the efficiency of the energy is

$$\eta=\frac{W}{E}\cdot100\%=\frac{4}{240}\cdot100\%=1.67\%.$$