If the load is inductive with Ia lags Va by less than 30 degree, say there, then wattmeter
W1 = Ia . Vac cos (30 - theta)
And if Ia lags Va by more than 30 degree then
W1 = Ia . Vac cos(theta - 30)
You can do the same for capacitive load to get the equation.
Hope this helps.