A class-A transformer coupled, transistor power amplifier

A class-A transformer coupled, transistor power amplifier is required to deliver a power output of 10 watts. The maximum power rating of the transistor should be less than:

Given P_{o (ac)} = 10 Watts, P_{D (max)} =?
efficiency of transformer coupled amplifier = 50%

so, η = 50% =	P_o(ac)
	P_i(dc)

.5 =	P_o(ac)
	P_D(max) + P_o(ac)

.5 P_{D (max)} = .5 P_{o (ac)}
or P_{D (max)} = P_{o (ac)} = 10 watts.
This is the required maximum rating of the transistor.