An electric water heater is tested before and after thermal insulation is fitted around its tank. Each test ends when the same useful energy has been transferred to the water. Which statement describes the effect of fitting the insulation?
A larger share of the energy supplied heats the water, so the efficiency increases.
The same useful energy reaches the water in each test, so the efficiency is unchanged.
The insulation absorbs some of the energy supplied, so the efficiency decreases.
The insulation adds extra energy to the water, so the efficiency increases.