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Generator Sizing

When you select a generator it is important to make sure that the size and capabilities of the generator set are matched to the intended load. A generator that is undersized for a load will not last as long and may damage sensitive equipment on the load. A generator that is oversized provides a margin of safety and allows for future expansion. Bigger isn't always better, however, since oversized generators are more expensive to maintain and operate. Diesel engines may also develop maintenance problems by being under loaded for extended periods of time.

The primary operators of generator systems should be trained to identify the symptoms of generator overload. A careful operator will be able to identify symptoms of overload and correct the problem before permanent damage can be done to the system. The following list is not intended to be comprehensive:

Low Frequency: As the load increases it will cause the engine RPM's to slow, thus reducing the frequency.

Low Voltage: Inherently regulated generator designs have a loss of voltage output during an overload.

Circuit Breaker: If the circuit breaker is tripped it may indicate that you are overloading the system. Thermal breakers build heat over time from the excess current until they reach the trip point. Thermal breakers will take time to trip during an overload condition and are affected by ambient temperatures.

Digital Control Warning: Most WINCO liquid-cooled emergency standby generators come equipped with an digital genset controller that monitors the actual load and will shut down the system in case of an overload. This is the most reliable method for ensuring a generator system is properly protected.

Many appliances like refrigerators, freezers, air-conditioners, sump pumps, well pumps and other electric motors require substantially more power to start than to run. A generator must be sized large enough to handle the starting surge of all the appliances to be powered. It may be unlikely that all of these appliances would be starting simultaneously under normal conditions, but it may be very likely to occur when an emergency generator first starts and accepts the initial load.

Some guidance will be given on different ways to size your generator system. Do not use this information to substitute for the competent advice of a licensed electrician, electrical engineer or other appropriate professionals. Each method of sizing has varying degrees of accuracy that should be accounted for in calculations. Worksheet links are available below for help in properly sizing a generator system.

     Average Wattage Guide: Lists common appliances and gives an estimate of starting and running watts.

     Sizing Worksheet: This WINCO worksheet allows you to list the powered loads and calculate power required.

Field Test: It is often quickest and easiest to go on site to directly measure the load. By monitoring the incoming power lines it is possible to get an accurate picture of how much electrical power the building will need. Consideration should also be given to motor starting requirements and worse case power scenario need when testing or there is risk of under sizing the generator.

Utility Bill/Max Usage: Most utility companies can provide you with historical energy consumption data. Some utilities may measure the maximum powered used over a period of time. If the maximum period is averaged over 15 minutes the power company data may not properly account for instantaneous consumption spikes causing the generator to be undersized.

Calculate by Circuit, Appliance or Motor: This method can be the most practical method and is normally preferred. The load that will be powered by the generator is divided into circuits, appliances or motors. The starting and running load for each circuit is calculated or measured and then the total potential starting and running loads are calculated. The generator is then sized to be able to handle the total running load.

Match Service: This technique normally results in oversized generators. It is common for homes in the US to have 200 amp services. Most households will never come close to using this much energy at any one time.