A transformer is used to light a 100 W and 110 V lamp from a 220 V mains. If the main current is 0.5 amp, the efficiency of the transformer is approximately

Options

(a) 50%
(b) 90%
(c) 10%
(d) 30%

Correct Answer:

90%

Explanation:

? = P output / P input x 100
= 100 / 220 x 0.5 = 90.9%