In general, watts are calculated by taking the current, in amps, and multiplying by the voltage. AC currents involve additional steps, which may include multiplying by a power factor (for single-phase currents) or multiplying by a power factor and a coefficient (for three-phase currents).
Single-phase and three-phase currents are distinguished by how the power is distributed, with single-phase systems supplying all of the voltages in unison while three-phase systems supply the voltages in three sequential stages. In three-phase currents, if the voltage is taken from line to line, the coefficient is the square root of three. If the voltage is measured from the line to a neutral ground, then the coefficient is simply three. The power factor is the ratio of the measured power in the line to the theoretical power supplied to the circuit.