Generator Sizing Calculator
Total continuous appliance demand, account for motor-starting surge, reserve headroom and environmental derating, then select the next practical generator size.
Generator Sizing Calculator inputs
Every material assumption is visible and editable. Invalid or internally inconsistent entries are stopped before a result is shown.
Generator Sizing Calculator results
How the generator sizing calculator works
The generator sizing calculator is designed for one specific search intent. Its formulas, controls, validation, outputs and explanation are tailored to that task rather than borrowed from another calculator.
Running watts and starting watts
A generator sizing calculator must distinguish continuous demand from temporary motor inrush. Enter each load as a name, running watts, starting watts and quantity. The normal design mode adds the largest additional starting surge to the total running load, representing one major motor starting while the other selected loads are operating.
Simultaneous-start alternative
The all-start option sums every entered starting value. That can be deliberately conservative, but real sequencing, soft starters, variable-frequency drives and automatic load management may reduce the required peak. Conversely, nonlinear electronics, compressors and pump motors can have behavior not captured by a simple nameplate list.
Headroom and derating
Capacity headroom is applied before selecting a standard size. The generator sizing calculator also displays a simple editable planning derating based on altitude above 1,000 feet and temperature above 77°F. Manufacturer curves, fuel type, enclosure, duty rating and site ventilation control the actual available output and must supersede this estimate.
Electrical and installation checks
Approximate rated current is calculated from the selected voltage and phase. Real installation requires correct transfer equipment, neutral and grounding design, conductor and overcurrent sizing, fuel supply, exhaust clearance, permits and load classification. Verify appliance nameplates and obtain manufacturer and qualified-electrician approval before purchase or connection.
Load-inventory example
List only loads intended to run during an outage, using appliance or equipment nameplate values when available. A refrigerator, sump pump and furnace blower may not start together, so the largest-surge mode is often a clearer planning scenario than summing every peak. Critical medical, life-safety or business-continuity loads require professional design, redundancy and manufacturer-approved transfer equipment beyond this worksheet.
Common sizing mistakes
Do not use only average utility-bill consumption; generator capacity is based on simultaneous power demand and starting behavior. Running watts and starting watts must use the same quantity logic. Electric resistance heat, well pumps, air conditioning and shop motors can dominate a residential load. Fuel choice and altitude can reduce output. The added generator fuel usage and run-time section uses editable manufacturer no-load, half-load and full-load rates rather than pretending one universal fuel curve applies. Never backfeed a building through a receptacle; use approved transfer equipment and professional installation.
Generator Sizing Calculator questions
Do I add all starting watts to all running watts?
Usually the largest extra starting surge is added to the running total; use all-start only when that scenario is justified.
Why is the recommended size larger than the raw total?
Headroom and environmental derating increase the required nameplate capacity.
Can I size from breaker amperage alone?
Nameplate running and starting watts provide a better load inventory; breaker size may not equal actual demand.
Is this a substitute for an electrician?
No. Generator and transfer-system installation is safety-critical electrical work.