Volts (measured) x Current (measured) equals Indicated Power(volt-amps).

Volts x Current x cosine(phi) equals Real Power(Watts).

Volts x Current x sine(phi) equals Reactive Power (vars).

Phi is the phase difference between current and volts, referenced to volts.

Cosine(phi) is also called the power factor.

Potential and current must be measured if the load has an inductive or capacitive component.

Let's not talk about unbalanced loads.

Woodward Governor sells a device called a Digital Synchronizer and Load Control (DSLC) which performs these measurements. Encorp sells a similar device. Multiple DSLCs can be connected to exchange information about the power supplied by the generator each is controlling to perform load sharing amongst the generators.

Try your hand at sampling the V and I waveforms several times a cycle and computing the power components.