Calculator Inputs
Use the form below to estimate the generator size required for your electrical load profile.
Example Data Table
Use this sample table to understand how different inputs influence calculated generator capacity.
| Scenario | Phase | Voltage (V) | Current (A) | Power Factor | Efficiency (%) | Demand Factor (%) | Starting Multiplier | Safety Margin (%) |
|---|---|---|---|---|---|---|---|---|
| Workshop backup set | Three | 415 | 120 | 0.80 | 95 | 90 | 1.50 | 20 |
| Small office standby set | Single | 230 | 85 | 0.92 | 94 | 80 | 1.25 | 15 |
| Pump-driven site load | Three | 400 | 160 | 0.78 | 93 | 95 | 2.20 | 25 |
Formula Used
1) Apparent Power
Single phase: kVA = (V × I) / 1000
Three phase: kVA = (√3 × V × I) / 1000
2) Real Power
kW = kVA × Power Factor
3) Efficiency-Adjusted Real Power
Adjusted kW = Real Power / Efficiency Fraction
4) Demand-Adjusted Running Power
Running kVA = (Adjusted kW / Power Factor) × Demand Factor
5) Starting Requirement
Starting kVA = Running kVA × Starting Multiplier
6) Recommended Generator Size
Recommended kVA = max(Running kVA, Starting kVA) × (1 + Safety Margin)
How to Use This Calculator
- Choose single-phase or three-phase supply according to the connected load.
- Select the intended generator duty: standby, prime, or continuous service.
- Enter system voltage and full-load current from drawings, nameplates, or design calculations.
- Provide a realistic power factor for the connected equipment mix.
- Enter efficiency, demand factor, starting multiplier, and safety margin values.
- Press the calculate button to view the results above the form.
- Review the recommended size, nearest standard size, and the plotted comparison chart.
- Export the results as CSV or PDF for documentation or design review.
Frequently Asked Questions
1) What does generator kVA represent?
Generator kVA is the apparent power capacity a set can supply. It combines voltage and current and is useful because generators are usually selected by apparent power, not only by real power.
2) Why is power factor included?
Power factor converts apparent power into real power. Loads with lower power factor demand more current for the same real work, so generator size must reflect that electrical behavior.
3) Why use a demand factor?
Demand factor reflects the likelihood that all connected loads will run together at full level. It helps produce a more realistic generator estimate for diversified systems.
4) What is the starting multiplier for?
Many motors and compressors draw high inrush current during startup. The starting multiplier increases the running requirement so the generator can better support these short, heavy surges.
5) Should I always add a safety margin?
A safety margin is usually wise. It allows for temperature effects, aging, load growth, voltage regulation limits, and small uncertainties in the initial load estimate.
6) Is standby rating the same as prime rating?
No. Standby rating supports emergency operation for limited durations. Prime rating supports variable daily use for longer periods. Continuous rating assumes relatively steady output conditions.
7) Can I use line current from equipment nameplates?
Yes, provided the current reflects the intended operating condition. For groups of loads, use a calculated total or a measured current representative of the combined demand.
8) Does this replace a full engineering study?
No. This tool is excellent for preliminary sizing and comparison. Final selection should still consider harmonics, voltage dip, altitude, ambient temperature, protections, and manufacturer data.