Keith Dysart wrote:
. . .
By substitution, it is easy to show that the rules used to
derive Vf and Vr results in
P(t)=Pf(t)-Pr(t)

So, while superposing powers, does not work in general, it does
for this case. Directional wattmeters take advantage of this
ideosyncracy to let the user compute the power being delivered
to the load.
. . .

And it works only if Z0 is purely real. When it's not, P(t) also
includes Vf*Ir and Vr*If terms which are neither "forward" nor "reverse"
power. It's unfortunate in a way that Z0 is close enough to being real
for decent lines at Amateur frequencies that these terms are small.
Otherwise they'd have to be confronted and some mistaken assumptions
abandoned.

Roy Lewallen, W7EL