linear model of a power load at the end of a transmission line
First technique : Model the load as a disturbance that tries to prevent you from reaching your goal, i.e. regulating the voltage. That way, you will have 2 transfer functions. The first one will be

G(s) = y(s)/u(s) where y is your output, u your input (setpoint)

H(s) = y(s)/d(s) where d is your disturbance (i.e load). d(s) might a be a step of a certain number of amps for example. 

You simply have to balance the 2 transfer functions. For example, you could aim for a 60-degree phase margin for both.

Second technique,

Model 2 transfer functions and try to optimize both of them (phase margin, gain margin, sensitivity, etc.)

Gempty(s) = y(s)/u(s) at no load.

Gfull(s) = y(s)/u(s) at full load.

I don't know exactly what kind of power systems you're modeling, but for buck and boost converter for example, the transfer function in continuous-conduction mode is barely affected by load variations, while in discontinuous-conduction mode, a load variation can make you unstable.