PS, I wrote:

"...you need the current variation due to
the signal. You will find this will give the same answer as your
voltage calculation as long as you use rms current..."

which is ambiguous. You can't use the supply current variation
because this is not directly related to the signal. Use the ac
current through each half of the primary and the transformed load
resistance or, more simply, the secondary current and load
resistance, to derive power output.

cheers, Ian