Generator Sizing Inputs
Example Data Table
| Equipment | Watts/Unit | Qty | Demand Factor | Start Multiplier |
|---|---|---|---|---|
| LED Lighting | 800 | 1 | 1.00 | 1.00 |
| Compressor Motor | 2200 | 1 | 1.00 | 3.00 |
| Water Pump | 1500 | 1 | 0.90 | 2.50 |
| Socket Circuit | 1200 | 1 | 0.75 | 1.20 |
Formula Used
1. Connected load per item: Connected Load = Watts per Unit × Quantity
2. Running load per item: Running Load = Connected Load × Demand Factor
3. Starting load per item: Starting Load = Running Load × Starting Multiplier
4. Diversified running kW: Total Running Load × Diversity Factor ÷ 1000
5. Required continuous kW: Diversified Running kW × Continuous Load Factor
6. Future-adjusted kW: Required Continuous kW × (1 + Future Margin)
7. Recommended kW before derating: Max(Future-Adjusted kW, Future-Adjusted kW + Starting Reserve)
8. Final recommended kW: Recommended kW Before Derating ÷ (1 − Total Derating)
9. Generator kVA: Recommended kW ÷ Power Factor
10. Full load current: Single phase: I = kW × 1000 ÷ (V × PF), Three phase: I = kW × 1000 ÷ (√3 × V × PF)
How to Use This Calculator
- Choose single-phase or three-phase operation.
- Enter voltage and expected overall power factor.
- Set diversity, continuous load factor, and future margin.
- Enter altitude and temperature derating if needed.
- Add each load with watts, quantity, demand factor, and starting multiplier.
- Press the calculate button to view the recommended size.
- Review kW, kVA, current, and the comparison graph.
- Download the table as CSV or export the page as PDF.
Frequently Asked Questions
1. Why is generator size larger than running load?
Generators must handle motor starting currents, future growth, and site derating. A unit sized only for running load may stall or trip during heavy startup.
2. What does the demand factor represent?
Demand factor estimates how much of each connected load operates simultaneously. It helps avoid oversizing when circuits are not always fully active.
3. What is a starting multiplier?
It models startup surge, especially for motors and compressors. A multiplier above one increases the temporary power requirement during starting.
4. Why does power factor matter?
Generator alternators are commonly rated in kVA. Lower power factor needs more kVA for the same kW output, affecting final size selection.
5. Should I include future margin?
Yes, when expansion is likely. A modest reserve can reduce replacement cost later and improve operating flexibility during changing load profiles.
6. How do altitude and temperature affect sizing?
High altitude and hot ambient conditions reduce engine and alternator performance. Derating compensates by increasing the recommended nameplate size.
7. Is this calculator suitable for all projects?
It is a solid planning tool, but final sizing should confirm motor codes, transient performance, harmonic content, and manufacturer application limits.
8. What if I have many nonlinear electronic loads?
Consider harmonic distortion, UPS charging currents, and crest factor. Sensitive loads may require additional alternator sizing or better voltage regulation.