Generator Output Ratings

Generators can have up to three power output ratings: standby, continuous, and prime. Standby is the least conservative rating, the most power the generator can deliver. The prime is the most conservative. The prime rating of a generator is used where utility power is considered unreliable or not available. Manufacturers have technical papers defining how the ratings are determined and what they mean. Different manufacturers may have slightly different definitions.

Caution, the term "standby rating" of a generator is not the same as a "standby generator" defined in NEC 700, 701, and 702. There the term differentiates standby from emergency by function, that is from an emergency generator which supports equipment, areas, or systems essential to the safety of human life. Legally mandated standby generators specified in 701 usually relate to rescue equipment such as fire pumps. They are defined by the AHJ. Optional standby generators are defined in 702. Testing procedures for emergency generators are specified in NFPA 99 and I think 109 or 110. I haven't checked recently. Generators can serve multiple functions but there are additional requirements defined in NEC when they do.

Alternate energy sources like solar arrays, wind turbines, and fuel cells are typically paralleled to the utility grid. I think you can usually do the same with conventional rotary generators but generally the cost to operate them is higher than to just buy power from the grid. It also uses up their life. Requirements for protection to the grid are usually defined by the utility and can be obtained from the utility's engineering department, sometimes their web sites. I haven't bought one recently but I assume these are built into paralleling units. It is possible to sell power from this equipment back to the utility. In this case your utility meter will run backwards.


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