Generator Size Calculator
Estimate the minimum generator class from simultaneous running watts, the largest startup surge and operating margin.
Estimate the minimum generator class from simultaneous running watts, the largest startup surge and operating margin.
Correct generator sizing starts with actual loads rather than the peak-watt number printed on the box.
Use this calculator for planning, then verify voltage, outlets, fuel-specific output and appliance labels.
Add Running Watts, Largest Surge and Margin
Enter all simultaneous running loads, then add the largest additional motor-start surge and 15–25% headroom.
Calculator Guidance
Use the result as a minimum running-watt target. Final selection must also match 120/240V requirements, outlets and fuel rating.
Generator Size Calculator
Verify voltage and outlet requirements before purchase.
How the Calculator Works
Add the running watts of loads that will operate together, then add the largest realistic startup surge and operating margin.
Example Load Plans
| Load Plan | Running | Surge | Practical Class |
|---|---|---|---|
| Refrigerator + lights + router | 1,200W | 1,200W | 3,000W |
| Furnace + refrigerator + sump | 3,200W | 2,500W | 7,000W |
| Well pump + broad essentials | 5,000W | 4,000W | 11,000W |
| RV air conditioner + appliances | 2,800W | 1,500W | 5,000W |
Important Limitations
Voltage Matters
A 120V generator cannot run 240V loads.
Fuel Changes Output
Propane and natural gas often reduce capacity.
Surges Can Overlap
Use realistic worst-case startup timing.
Central HVAC Needs Measurement
Compressor demand varies widely.
Transfer Equipment Has Limits
Cord, inlet and breaker ratings must match.
Generic Wattages Are Estimates
Use actual equipment labels.
How to Use the Result
Round Up to a Real Model
Choose the next available running-watt class.
Check Continuous Rating
Do not buy from peak watts only.
Verify 120/240V
Match pumps and panel loads.
Add Fuel Runtime
Size storage for outage duration.
Plan Safe Connection
Use cords or transfer equipment correctly.
Review Manufacturer Data
Confirm surge and outlet ratings.
How to Use the Result in the Real World
Treat the calculated result as a planning baseline rather than an exact promise. Real systems lose energy through conversion, wiring, temperature effects and internal electronics, while appliances can draw differently from their nameplate ratings.
Leave practical headroom instead of selecting equipment that exactly matches the result. Extra margin helps with startup surges, battery aging, changing weather and future loads, and it reduces the chance that normal variation pushes the system beyond its limits.
When the result affects a major purchase, verify the important inputs. Measure appliance consumption when possible, check the exact inverter or charge-controller limits, and compare the calculation with the equipment manufacturer's specifications.
Frequently Asked Questions
Should I add all starting watts?
No, add simultaneous running watts plus the largest realistic extra surge.
What margin should I use?
About 15–25% is a practical range.
Why does voltage matter?
Some pumps and appliances require 240V.
Do propane ratings differ?
Usually yes.
Can this size central air?
Only as a preliminary estimate; measure compressor startup.
Should I use peak or running watts?
Use running watts for the generator class.