Buying a generator of any kind –- a portable, an inverter, or a standby –- is a serious investment. There are several things that need to be assessed. These can include the type of generator that best aligns with your use case, local codes and requirements, upfront cost and maintenance, just to name a few. Chief among these is generator sizing: sizing a generator accurately goes a long way in ensuring it performs both as expected and safely. Getting this part wrong carries long-term consequences –- and that goes for both under sizing and oversizing.

A simple guiding principle in sizing a generator is the 80% rule: size the generator to run around 80% of its rated operating capacity for continuous use. This is the same philosophy used when sizing and installing a circuit and its appropriate overcurrent protection (breaker). For a standby generator, you can safely target a continuous operating range of 60% to 80%. A properly maintained generator should have no issue running at full capacity during emergencies or during a critical event, provided it’s only for a short duration. When considering your generator needs and size, it’s never a bad idea to consult a professional.

Buying a generator while overlooking the size is among the worst mistakes many people make when using a generator, and applying the 80% rule is a good way to avoid those pitfalls. The rule itself isn’t strictly a code-mandated requirement, but is considered an industry standard practice. According to the International Organization for Standardization (ISO) as outlined in ISO 8528, emergency standby power generators that are connected to the power grid and provide backup power during an outage, should not be loaded by more than 80% of their rating and should not exceed an annual operation of 200 hours.

When sizing a generator for your needs, all concurrent and non-concurrent loads –- as well as inrush current for those loads –- connected to the generator need to be assessed. Many modern generators, especially standby generators, feature some implementation of load shedding, whereby the generator and its associated equipment (controller and transfer switch) will temporarily disconnect certain loads to prevent overload. Keep this in mind when selecting a generator, or consulting with a professional.

Depending on certain environmental factors, some generators will need to be derated, meaning the capacity of the generator is intentionally reduced to reflect the real world conditions it will be operating in. Conditions affecting the derating of a generator involve climate, altitude, fuel and air quality, or the age of the generator. Diesel generators in particular require special attention to sizing and derating, as they are susceptible to “wet stacking,” a phenomenon where unburned fuel accumulates in the exhaust system, leading to serious problems.