Calculator
Example Data Table
| Load | Quantity | Running Watts | Starting Watts | Daily Hours |
|---|---|---|---|---|
| Refrigerator | 1 | 700 | 2200 | 8 |
| Well Pump | 1 | 1000 | 3000 | 1 |
| LED Lighting | 10 | 12 | 12 | 6 |
| Laptop Charger | 2 | 90 | 90 | 5 |
Formula Used
Line running watts: Quantity × running watts each.
Total running watts: Sum of all line running watts.
Extra surge: Starting watts − running watts.
Peak starting watts: Total running watts + largest extra surge.
Recommended watts: Peak starting watts × safety factor ÷ derating factor.
Single phase current: Watts ÷ (voltage × power factor).
Three phase current: Watts ÷ (1.732 × voltage × power factor).
kVA: Watts ÷ (1000 × power factor).
How to Use This Calculator
Enter each appliance, tool, motor, charger, light, or device.
Add quantity, running watts, starting watts, and daily hours.
Use the surge multiplier when starting watts are unknown.
Set voltage, phase, power factor, and safety margin.
Add altitude and temperature for derated engine output.
Press the calculate button to view the generator size.
Use CSV or PDF export for records and comparisons.
Power Generator Wattage Guide
Why Generator Wattage Matters
A power generator wattage calculator helps you choose a unit that can start and run your selected equipment without overload. The key point is simple. Motors, pumps, refrigerators, compressors, saws, and air conditioners often need a short starting surge. Lights, chargers, heaters, and electronics usually need only running watts. This calculator separates both values.
Running Watts and Starting Watts
Running watts are the steady watts needed after equipment is already operating. Starting watts are the short peak watts needed when one load begins. A generator must handle the steady total and the largest likely starting event. The tool adds all running watts. Then it adds the biggest extra surge above running watts. This gives a practical peak load estimate.
Safety Margin and Power Factor
Safety margin is important. A generator should not run at full output for long periods. A margin helps with voltage dip, warm weather, aging, cable losses, and future appliances. Many users choose 20% to 30%. Critical backup systems may use a larger margin.
Power factor also matters. Real power is measured in watts. Apparent power is measured in kVA. Motors and inductive loads may use more current than their watt rating suggests. This page converts watts into kVA and line current. It supports single phase and three phase estimates.
Derating and Fuel Planning
Derating is another advanced factor. Engines produce less power at high altitude and high temperature. The calculator can increase the recommended generator size when altitude or temperature reduces usable output. This helps avoid buying a unit that seems correct on paper but struggles outside.
Fuel planning is included for rough operating estimates. Enter tank size and rated fuel use. The calculator estimates load percentage, hourly fuel use, runtime, and daily fuel need from the entered run hours. Actual use can vary with engine design, maintenance, fuel type, and changing loads.
Practical Use
Use the example table to understand typical entries. Replace the sample values with your own equipment labels, quantities, running watts, starting watts, and daily hours. When starting watts are unknown, select a surge multiplier. The export buttons let you save results for job files, emergency plans, client notes, or equipment comparisons.
Review nameplate labels first. Use measured wattage when available. Keep essential loads grouped. Start large motors one at a time when possible during backup operation safely.
FAQs
What size generator do I need?
Add all running watts. Then add the largest extra starting surge. Apply a safety margin. The final value is the recommended generator wattage for your entered load list.
What are running watts?
Running watts are the steady watts used after a device starts. They are usually lower than starting watts for motors, pumps, compressors, and cooling equipment.
What are starting watts?
Starting watts are short surge watts needed during motor startup. Refrigerators, air conditioners, pumps, and saws often need much higher starting watts than running watts.
Why add a safety margin?
A safety margin helps cover voltage drops, hot weather, aging equipment, cable losses, and future loads. It also keeps the generator from working at full output continuously.
What power factor should I use?
Use 1.0 for mostly resistive loads. Use about 0.8 to 0.9 for mixed household, motor, and workshop loads unless equipment data gives a better value.
Does altitude affect generator output?
Yes. Engines often lose output at higher elevations. This calculator estimates altitude derating and increases the recommended wattage to help offset that loss.
Can this calculator estimate fuel use?
Yes. Enter tank size and rated fuel use. The fuel result is a rough estimate because real consumption changes with engine type, load cycling, and maintenance condition.
Should all motors start together?
Usually no. Start large motors one at a time when possible. This lowers surge demand and may allow a smaller generator to run the same equipment safely.