Generator size and wattage calculator
How big a generator needs to be for the loads it will carry: the running watts, the extra surge when the biggest motor starts, and the kW and kVA to ask for.
Your numbers
The loadsMargin See your resultResult
Generator running output 15.0 kW12,000 W + 25% headroom Starting (surge) watts 18,000 Wwhile the largest motor starts In kVA 18.8 kVAat power factor 0.80 Stop working this out by handFleetZap Rentals tracks which generator is out, with whom and until when, and bills each rental per day, week or month.
See FleetZap Maintenance · Try the demoThe formula
Running kW = Running watts × (1 + Headroom) ÷ 1,000 · Surge = Running watts + Largest motor × (Surge factor − 1) · kVA = kW ÷ Power factor
Size for the worst moment: everything running while the biggest motor starts. With the example: 12,000 W plus 25% is 15 kW, 18.75 kVA at 0.8, and the set must handle an 18,000 W start. Start motors one at a time where you can. This is a first estimate; for building standby systems or large motors, have an electrician or the generator dealer size it.
Using the numberWhat to do with it
- An oversized diesel generator that runs at light load for long periods can wet-stack; size close to the real load.
- Match voltage and phase to the job, not just the kW.
- On rentals, note the load and hours at return; they set the next service.
Guides behind this calculator
Generator maintenance Generator maintenance cost and checklist Generator service companiesMore free calculators
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The heart of FleetZap, and everything a maintenance team needs in one place: work orders and preventive maintenance, inspections and the field app, plus a parts store with purchasing, compliance and certificate tracking, multi-location assets with a full lifecycle, and the KPIs that prove it’s working.
- Work orders
- Preventive maintenance
- Procedures, inspections & field app
- Parts & purchasing
- Compliance
- Assets, reports & KPIs
Good to know
How do I size a generator?Add the running watts of everything on at once, add headroom (often around 25%), and check the set can handle the surge when the largest motor starts.
What is the difference between kW and kVA?kW is real power; kVA is apparent power. kVA = kW ÷ power factor, and 3-phase generators are usually rated at a power factor of 0.8.
How much extra power does a motor need to start?Often two to three times its running watts for a moment, and more for some motors. The motor's nameplate or maker gives the starting figure.
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