To determine how much power your generator set needs, you must know not only the nominal power of the equipment that will operate at the same time, but also the peak power consumption, especially at start-up. For example, an electric motor can consume up to three times its nominal power at start-up. The generator set must be able to provide the peak power consumed by all the equipment that needs to start at the same time. For resistive equipment (lighting, television, small electrical appliances, etc.), a safety factor of 30% must be added when determining the power of the generator set. For inductive equipment (i.e. electric motors), their nominal power must be multiplied by 3 to take into account peak consumption.
For generator sets that deliver a three-phase power supply, the power is indicated in kilo-volt-amperes (kVA): this is referred to as the apparent power. For the ones that supply direct current or single-phase power, the power is indicated in kW: this is referred to as active power.
The difference between kVA and kW comes from the phase shift between each phase. This phase shift is called cos φ (cosine phi). To obtain the active power of a generator set whose apparent power and cos φ you know, multiply these two values, for example: 1 kVA x 0.8 = 800 W (as a general rule, the cos φ of generator sets is 0.8).
Generally, generator set manufacturers indicate the continuous power and the maximum power that corresponds with an overload of about 20% of the continuous power for a limited time.