Generator Sizing Calculator
Screen required generator kVA from running kW, PF, and a 10–25% margin, then map to standard catalog frames. Example: 80 kW @ 0.9 PF + 10% → 100 kVA. Motor starting and altitude derate are not modeled here.
Input Parameters
Quick: running kW and PF. Advanced: safety margin encodes expansion and inrush headroom before you match vendor kVA frames.
About this calculator
Combines running kW, PF, and a planning safety margin into required generator kVA before vendor curves and transient studies. For related load-to-current tools in one place, use the power calculator hub.
Calculation Results
Engineering disclaimer
This generator sizing is intended for preliminary selection only. Final generator choice must consider starting currents, harmonics, step loading, altitude, temperature, fuel type, and manufacturer data, confirmed by a qualified engineer and supplier.
Calculation Results
Power Factor: 0.90
Base Apparent Power: 88.89 kVA
Safety Margin: 10% (× 1.10)
Required Generator kVA: 97.78 kVA
Recommended Generator Size: 100 kVA (next standard size above 97.8 kVA)
Generator Sizing Formula
Step 1: kVA = kW ÷ Power Factor
Step 2: Required Generator kVA = kVA × (1 + Safety Margin)
The resulting kVA value is then matched to the next higher standard generator rating such as 100, 150, 200, 250, 300, 350, 400, 500, 750, 1000 kVA.
Use higher margins for high inrush motor loads, future expansion, or sites with poor power factor. Always confirm the final rating with vendor performance curves and local standards.
Guides for Generator Selection
For more context on kW, kVA, and generator sizing, see:
Frequently Asked Questions
Why do I need a safety margin for generators?
A safety margin covers future load growth, motor starting currents, and uncertainties in the load estimate. Without it, the generator may be overloaded during startup or peak operation.
How does power factor affect generator size?
Lower power factor means more kVA is required for the same kW. Generators are rated in kVA, so poor PF may push you to a larger machine even if the kW is moderate.
Can I oversize the generator too much?
Yes. Very lightly loaded generators may run inefficiently, may have issues with wet stacking on diesel machines, and cost more to purchase and maintain. Try to keep typical loading in a healthy range (for example 40-80%).
Do motor starting currents affect generator sizing?
Yes. Large motors can require several times their running current when starting. You may need a larger generator, soft starters, or sequenced starting to avoid excessive voltage dips.
Is this calculator enough for final generator selection?
No. It provides a good first estimate. Final selection should include detailed load profiles, transient performance, altitude and temperature corrections, fuel type, and manufacturer data reviewed by a qualified engineer.
Advanced Result Explanation and Next Step
The output kVA is a planning estimate. For advanced sizing, evaluate motor start sequence, allowable voltage dip, harmonic content, and future expansion stages.
Related checks: continue upstream and downstream electrical validation.
Transformer sizing, kW to kVA converter, and factory load calculator.