Hi
For simple circuits you find the circuit impedance Z, then appy very know formulas for power calculataion P= V^2/Z, where V is the supply voltage and P is the power consumption. Now, if the circuits are complex, with CIs and other components, we use the Z parameters where a circuit is seen as a black box and the Z parameters are obtained.
About the guard current, it pretty depends on the circuit nature, it is different for a circuit which makes some filtering to other which may control a motor. Cost and extra power are traded off in this matter.
Sal