What "prime" and "standby" actually mean
These aren't marketing terms — they're formal duty ratings that determine how a generator is engineered and how it should legally and practically be used.
Standby power is designed for emergency backup only — running when the mains supply fails, for the duration of that outage. Standby-rated generators are typically permitted to run at their full rated output for a limited number of hours per year (often around 200–500, depending on the manufacturer's rating), with no sustained overload capacity built in. They're built to start reliably and deliver full power immediately, not to run economically for weeks on end.
Prime power is designed to be a site's main source of electricity — running for unlimited hours, with a variable load that changes throughout the day, and typically an allowance for a short overload (often around 10% for one hour in twelve) to cope with load spikes. Prime-rated gen sets are built with heavier-duty components because they're expected to run far more hours over their life, under continuous duty rather than occasional bursts.
Learn more: Standby vs Prime Power: Key Differences
Why this matters more than kVA size
A generator sized correctly in kVA but rated for the wrong duty will fail in one of two predictable ways:
- A standby unit used as a prime power source will wear out fast. Running full-time on a duty cycle it wasn't engineered for shortens engine life significantly and can void the manufacturer's warranty.
- A prime-rated unit bought "just in case" for occasional backup costs considerably more upfront than necessary, and running it lightly loaded for short bursts (which is exactly what standby use looks like) causes wet stacking — carbon build-up from incomplete combustion at low load — reducing efficiency and engine health over time.
The right choice depends entirely on how the generator will actually be used, not on wanting "the bigger, more capable option to be safe."
How to work out which one you need
Ask these questions before looking at kVA figures at all:
- Do you have a reliable mains connection already? If yes, and the generator is purely a fallback for outages, you need standby.
- Are you off-grid, on a site with no permanent mains supply, or regularly running for extended hours as your main power source? That's prime power territory — common on construction sites, remote sites, and some agricultural operations.
- How many hours per year will it realistically run? If you're expecting more than a few hundred hours annually, or genuinely continuous operation, standby-rated equipment is the wrong tool even if it's cheaper to buy.
- Does your load fluctuate significantly through the day? Prime-rated gen sets are built with the overload margin and control systems to handle that variability; standby units generally aren't.
Sizing still matters — just do it second
Once duty rating is settled, size the unit to your actual load, not your total connected load:
- List everything that runs simultaneously during normal operation, plus the largest motor-starting surge in your load list.
- Add roughly 20–25% headroom for safety margin and future growth.
- For three-phase commercial supplies, check the true power factor of your equipment mix (see our kW vs kVA efficiency guide) rather than assuming a flat 0.8 — sites with heavy motor loads often need more capacity than a simple kW conversion suggests.
A quick example
A café with a reliable mains connection that occasionally loses power during storms needs a standby generator sized to its peak simultaneous load — fridges, coffee machine, lighting, POS systems — plus starting surge and headroom. It will run rarely, for short periods, so standby duty and modest sizing is the right call.
A construction site with no mains connection running power tools, site lighting, and welfare cabins for months at a time needs a prime-rated gen set, sized with more care since it's running continuously and the load will vary hour to hour as different equipment switches on and off.
The bottom line
Don't start the buying conversation with "what size do I need?" Start with "how will this generator actually be used — as an occasional backup, or as my main power source?" That answer determines the duty rating, which determines the build quality, warranty terms, and realistic running hours you can expect — and getting it right the first time avoids paying twice.
Not sure whether your business needs prime or standby power? Talk to our team about your site's actual usage pattern — we'll help you spec the right Cummins gen set for the job, not just the biggest number on the spec sheet.