Electrical
Generator Size Calculator
Sketch a backup-power requirement from running load and one starting surge before specifying a generator system.
Result
Electrical work is life-safety work governed by the NEC or your local code, and must be designed and installed by a licensed electrician. Conductor and overcurrent sizing depends on ambient temperature, conductor bundling, termination ratings, continuous-load rules and more. This is an educational estimate and must never be used as the basis for an installation. Only the largest motor's surge is counted, since simultaneous starts are unlikely but not impossible. Standby generators also need a transfer switch, and back-feeding a panel without one is illegal and can kill utility workers.
Get a Free Business AuditWhat moves the number
- List only the loads intended to run during an outage.
- The largest-motor input should already be part of total running watts.
- Starting behavior and the assumed headroom affect the proposed capacity.
Where this one goes wrong
This model adds one motor surge. Multiple overlapping starts, fuel-specific ratings, altitude and load-management settings can change the equipment required. An apparently ample watt rating does not establish acceptable starting performance.
Questions
About this calculator
Should I add the largest motor twice?
Include its normal draw in total running load and identify it again in the motor field. The formula adds the extra starting portion, rather than treating the motor as another running load.
Can I keep a smaller generator?
Possibly, if the essential-load plan and manufacturer assessment support it. Reducing simultaneous loads is a legitimate alternative to replacing existing equipment.
Is this enough to design the connection?
No. Generator sizing is a planning check; manufacturer data, transfer equipment and locally adopted electrical rules govern the installed system.
The formula
starting watts = running watts + (largest motor × (surge multiple − 1))
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How to use this calculator
- Total running watts. Enter your own figure; the page starts at 5,500.
- Largest motor's running watts. Well pump, AC compressor, refrigerator.
- Start-up surge multiple. Enter your own figure; the page starts at 3.
- Safety headroom (%). Enter your own figure; the page starts at 20%.
- Voltage (V). Enter your own figure; the page starts at 240.
The result updates as you type, so you can try a figure, see what it does to generator size, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Total running watts | 5,500 |
| Largest motor's running watts | 1,500 |
| Start-up surge multiple | 3 |
| Safety headroom | 20% |
| Voltage (V) | 240 |
What comes out
| Result | Value |
|---|---|
| Generator size main result | 10.2 |
| Peak starting watts | 8,500 |
| Running current | 22.9 |
| Next size in this planning table | 12 kW |
Peak demand is about 8,500 watts when the largest motor starts. With headroom that needs roughly 10.2 kW, nearest standard size 12 kW.
How sensitive the answer is
Holding everything else steady and changing only total running watts, this is what happens to generator size. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Total running watts | Generator size |
|---|---|
| 2,750 | 6.9 |
| 4,125 | 8.6 |
| 5,500 example above | 10.2 |
| 8,250 | 13.5 |
| 11,000 | 16.8 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Total running watts
- A number, between 0 and 1000000, starting at 5,500.
- Largest motor's running watts
- Well pump, AC compressor, refrigerator.
- Start-up surge multiple
- A number, between 1 and 10, starting at 3.
- Safety headroom (%)
- A percentage, between 0 and 100, starting at 20%.
- Voltage (V)
- A number, between 0 and 1000, starting at 240.
What the results tell you
| Result | What it is |
|---|---|
| Generator size | Modeled peak demand including the percentage headroom you entered. |
| Peak starting watts | Total running demand plus the extra surge above the largest motor's normal draw. |
| Running current | Simple running watts divided by voltage, not a complete AC starting-current analysis. |
| Next size in this planning table | First entry in this limited comparison list at or above modeled demand; no equipment suitability is implied. |
Before you rely on this number
Conductor sizing, voltage drop and load figures are governed by code, and the code takes precedence over any calculator. Ambient temperature, conduit fill, continuous load factors and the specific installation all change the answer. Treat this as a first pass, not a design.
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