Transformers & motors
Generator Sizing & Load Calculator
Generator sizing compares the running total, the largest single-start surge, and an entered reserve. The existing generator-load route now serves this sizing intent to avoid a duplicate page.
Size a generator estimate from running and starting loads, then apply your own reserve and power-factor assumptions.
Calculated result
Generator sizing totals
Calculating…
Calculated locally in your browser
Planning math only. Verify equipment specifications and installation requirements separately.
What this calculator returns
Compare both continuous and surge ratings, duration, derating, fuel, and manufacturer documentation.
Formula and variables
The calculation runs entirely in your browser. Static formulas, definitions, examples, and tables remain readable without JavaScript.
Running total = Σ(run W × qty); peak = running total + largest(start W - run W); reserve result = running × (1 + reserve).
- P
- Real or mechanical power, as identified by the calculator.
- S
- Apparent power in VA or kVA.
- V
- Winding, line, or load voltage.
- I
- Winding, line, or estimated input current.
- PF
- Entered power factor where real and apparent power differ.
- η
- Entered conversion or motor efficiency.
Starting watts are total starting demand per unit, not only the extra watts.
Worked example
| Input | Value |
|---|---|
| Loads | See the example load list in the calculator |
A 700/2,200 W refrigerator and 1,050/2,150 W pump total 1,750 W running; the refrigerator creates the larger added surge, producing a 3,250 W single-start peak.
Reference table
| Apparent rating | 240 V single-phase | 480 V three-phase |
|---|---|---|
| 1 kVA | 4.17 A at 240 V 1φ | 1.20 A at 480 V 3φ |
| 5 kVA | 20.83 A at 240 V 1φ | 6.01 A at 480 V 3φ |
| 10 kVA | 41.67 A at 240 V 1φ | 12.03 A at 480 V 3φ |
| 25 kVA | 104.17 A at 240 V 1φ | 30.07 A at 480 V 3φ |
Frequently asked questions
How does the calculator handle starting loads?
It assumes the single largest entered surge occurs while the other loads remain at running power.
Can two motors start at the same time?
The model does not stack multiple simultaneous surges. Enter a more conservative scenario separately if loads may start together.
Assumptions and limitations
- Assumes one largest surge starts at a time.
- No transfer switch, feeder, breaker, conductor, ventilation, or fuel-system sizing.
Method and sources
Review the electrical formulas, calculation methodology, and standards status for the references most relevant to this calculation. The broader technical sources index records source scope and verification. Last reviewed .