How do you size a generator for a backup power system?

1 Step 1: Identify the critical loads

The first step is to identify the critical loads that need to be powered by the generator in case of a power failure. Critical loads are those that are essential for the operation, safety, or security of your facility, such as lighting, fire alarms, refrigeration, medical equipment, or communication systems. You need to list all the critical loads and their ratings in terms of power (kW or kVA), voltage, frequency, and power factor. You also need to determine the starting and running characteristics of each load, as some loads may require more power to start than to run, such as motors, compressors, or pumps.

2 Step 2: Calculate the total load

The second step is to calculate the total load that the generator will have to supply. This can be done by adding up the power ratings of all the critical loads, taking into account the power factor and the diversity factor. The power factor is the ratio of the real power (kW) to the apparent power (kVA), and it indicates how efficiently the load uses the electrical energy. The diversity factor is the ratio of the maximum demand of a group of loads to the sum of their individual maximum demands, and it reflects the probability that not all loads will operate at the same time. For example, if you have 10 lights with a power rating of 100 W each, and a power factor of 0.8, the apparent power of each light is 100 W / 0.8 = 125 VA. The sum of the individual maximum demands is 10 x 125 VA = 1250 VA. However, if you assume that only 80% of the lights will be on at any given time, the diversity factor is 0.8, and the maximum demand of the group is 1250 VA x 0.8 = 1000 VA.

3 Step 3: Select the generator size

The third step is to select the generator size that can meet or exceed the total load calculated in the previous step. The generator size is usually expressed in terms of its rated power output (kW or kVA), rated voltage, rated frequency, and rated power factor. The generator size should be able to provide enough power to start and run all the critical loads simultaneously, or in sequence if a transfer switch is used. The generator size should also take into account the derating factors that may reduce its performance, such as altitude, temperature, humidity, fuel quality, and maintenance. A general rule of thumb is to choose a generator size that is 10% to 20% larger than the total load, to account for the derating factors and the future expansion of the load.

4 Step 4: Verify the compatibility and compliance

The fourth and final step is to verify the compatibility and compliance of the generator with the load and the utility. The generator should be compatible with the load in terms of voltage, frequency, power factor, waveform, and harmonics. The generator should also be compliant with the utility in terms of synchronization, interconnection, protection, and metering. The generator should meet the relevant codes, standards, and regulations that apply to the installation, operation, and maintenance of backup power systems, such as the National Electrical Code (NEC), the National Fire Protection Association (NFPA), the Occupational Safety and Health Administration (OSHA), and the Environmental Protection Agency (EPA).