Generator Sizing Calculator
Size a generator in kVA from running load, power factor and design margin, rounded to a standard rating, with the rated line current at that size.
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Estimate only. Not engineering or safety advice. Check the result against the applicable standard and the equipment data, and have a competent engineer review it before it is relied on. Terms of use.
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About this calculator
Use this for a first estimate of the generator rating needed to carry a known running load. It converts kW to kVA with the power factor, adds your design margin, rounds up to a standard size and gives the rated current of that size for the breaker and cable.
Limits of this calculator
This covers the running load only. Motor starting, non-linear loads, and altitude or temperature derating need a separate check and can raise the size. Only the required rating is cross-checked against separately coded values; the standard-size table and rated current are not yet.
Cross-check: 67 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review.
Verification status: Indicative
Indicative. Estimation only; further engineering checks are required. Basis: Sizing estimate; starting and transient loads need a full study.
What this calculator checks
- Standard kVA at or above the load, with the margin entered.
What remains unverified
- Motor starting, transient voltage and frequency dip, harmonics and altitude or temperature derating.
Verification register entry
| Calculation author | Rahul Sharma (author) |
|---|---|
| Technical review | No independent technical review has been carried out |
| Individual professional approval | Not recorded |
| Formula reference | not recorded |
| Standard and edition | ISO 8528-1, 2018 Ed 3 (published edition; Ed 4 DIS was under development at review date) |
| Standard review state | Public-scope screened; licensed clause review still pending |
| Scope-screening note | Defines generating-set application, ratings and performance. The calculator only estimates kVA from kW, power factor and a user margin; load steps, ambient derating and engine/alternator ratings are not assessed. |
| Cross-check test cases | 67 reference vectors (tests/reference_vectors.json) |
| Review date | not recorded |
| Next review date | not recorded |
| Status | Indicative |
Needed before this calculator can be marked Verified: reviewer name, qualification and review date; licensed clause-by-clause standard review; exact formula reference; formula source (textbook or standard); next review date.
Formula
Worked example
| Input | Value |
|---|---|
| Running load | 400 kW |
| Power factor | 0.8 |
| Design margin (%) | 25 |
| Line voltage (L-L) | 415 V |
- S = 400 / 0.8 × 1.25 = 625 kVA
- Next standard size = 625 kVA
- I = 625 kVA / (√3 × 415) = 869.5 A
Assumptions
- Three-phase, continuous or standby duty as rated
- Running load only
- Standard sizes (kVA): 10, 15, 20, 30, 40, 50, 62.5, 75, 100, 125, 150, 200, 250, 320, 400, 500, 625, 750, 1000, 1250, 1500, 2000, 2500
Motor starting, non-linear loads and altitude or temperature derating need a separate check and can raise the size.
Inputs and results
| Input | Unit | Allowed or shown | Note |
|---|---|---|---|
| Running load | kW | greater than 0 kW | |
| Power factor | (number) | greater than 0, up to 1 | Rough starting point only. Resistive heating and incandescent loads are 1.0; a fully loaded induction motor is typically 0.8 to 0.88 and falls sharply when lightly loaded; discharge lighting depends on its ballast or driver. For real work use the nameplate or a measured value. |
| Design margin (%) | (number) | 0 or more, up to 100 | Growth and loading headroom |
| Line voltage (L-L) | V | greater than 0 V |
| Result | Unit | Allowed or shown | Note |
|---|---|---|---|
| Required rating | kVA | Shown to 1 decimal place | |
| Next standard size | kVA | Shown to 1 decimal place | |
| Rated current at that size | A | Shown to 0 decimal places |
How it is calculated
Each input is converted to the base unit of its quantity, the formula above is applied in the order shown in the worked example, and each result is shown in the unit you choose. Results are held at full precision and rounded only for display, except where a step selects a standard size, as the worked example shows. A value outside its allowed range is refused with a message instead of giving a result.
Typical values
| Running load (kW) | Required rating (kVA) | Next standard size (kVA) | Rated current at that size (A) |
|---|---|---|---|
| 50 | 78.1 | 100 | 139 |
| 100 | 156.3 | 200 | 278 |
| 200 | 312.5 | 320 | 445 |
| 400 | 625 | 625 | 870 |
| 800 | 1,250 | 1,250 | 1,739 |
Other inputs are the same as in the worked example above.
Standards and further reading
- IEC 60909-0 (short-circuit currents)
- IEEE Std 519 (harmonic limits)
Check results against your utility's connection requirements and the equipment data sheets. These are named as background reading. Elecmetric does not embed clause text or rating tables, and the reviewer record for this calculator is separate from this list.
Frequently asked questions
What do I need to enter?
Enter running load, power factor, design margin (%) and line voltage (L-L). Change the unit of a value with its selector and the result stays the same.
Why does the generator size depend on power factor?
Generators are rated in kVA, while loads are usually known in kW. The same kW at a lower power factor needs more kVA, so the calculator divides the running load by the power factor.
Does this allow for motor starting?
No. Large motors can pull the voltage down at start and may need a larger set. Check them with the motor starting current and voltage dip calculators.
What design margin should I add?
It is a design choice that covers growth and uncertainty in the load figure. Use the figure your specification or client sets; the calculator adds it as a percentage.
How accurate are the results?
Every figure comes from the same tested engine as the Elecmetric calculators and is meant for estimating. 67 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review. Check design values against the standard and the equipment data you work to.
Formula guides and reference
Longer explanations with worked examples and tables, written for the same topic.
- Generator Formulas: kVA, Current and Fuel: Generator formulas for kVA from kW and power factor, rated current, fuel use from specific consumption and tank runtime
- Generator Droop and Load Sharing Formulas: Generator droop formulas: the droop percentage, the load from the frequency and the load shared by parallel generators…
- Motor Starting Formulas: DOL, Star-Delta: Motor starting formulas for direct-on-line, star-delta, autotransformer and soft starter current and torque
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