Generator Input Form
Select preset household loads, edit wattages, and add custom circuits for a more realistic construction estimate.
Example Data Table
Sample construction planning scenario| Load | Qty | Running W per Unit | Surge Multiplier | Total Running W | Total Starting W |
|---|---|---|---|---|---|
| Refrigerator | 1 | 700 | 3.00 | 700 | 2100 |
| Sump Pump | 1 | 1050 | 3.00 | 1050 | 3150 |
| LED Lighting Circuit | 1 | 300 | 1.00 | 300 | 300 |
| Router and Modem | 1 | 80 | 1.00 | 80 | 80 |
| Microwave | 1 | 1200 | 1.20 | 1200 | 1440 |
In this sample, base running load equals 3,330 W. After diversity, reserve, and startup allowance, the final recommendation usually lands near a small whole-home standby unit.
Formula Used
Base Running Watts = Sum of (Quantity × Running Watts)
Diversified Running Watts = Base Running Watts × Diversity Factor
Extra Startup Watts per Unit = Running Watts × (Surge Multiplier − 1)
Surge Adder = Sum of the largest simultaneous startup extras
Starting Demand = Diversified Running Watts + Surge Adder
Recommended Output = Max(Continuous, Starting) ÷ Target Loading
Required kW = Recommended Output ÷ 1000
Estimated Current = Recommended Output ÷ (Voltage × Power Factor)
This method works well for early construction planning, remodel estimates, and backup power scoping. Final selection should still consider transfer equipment, fuel supply, site temperature, altitude derating, conductor sizing, and local code requirements.
How to Use This Calculator
- Select the loads you want powered during an outage.
- Edit running watts if your equipment nameplate differs.
- Set the surge multiplier for motor-driven appliances.
- Add custom loads for circuits not listed above.
- Adjust diversity if not every load runs together.
- Set reserve margin for safer long-term capacity.
- Choose target loading to avoid oversizing or strain.
- Click the calculate button and review the suggested kW.
- Download the result summary as CSV or PDF.
Frequently Asked Questions
1. What is the difference between running watts and starting watts?
Running watts are the steady power needed during operation. Starting watts are the temporary higher demand during motor startup. Pumps, compressors, and blowers usually create the largest surge loads.
2. Why should I add a reserve margin?
Reserve margin helps cover measurement errors, seasonal demand changes, aging equipment, and future additions. It also reduces the chance of nuisance overload trips during emergency operation.
3. What does the diversity factor do?
Diversity factor reduces the total running load when all selected appliances are unlikely to operate together. It can prevent oversizing, but it should be used carefully on critical backup circuits.
4. Why is target loading important?
Target loading keeps the recommended generator from operating at its absolute limit. This leaves headroom for reliability, better performance, and small future load increases.
5. Should I size for the whole house or only essentials?
Many homes only back up essentials like refrigeration, sump pumps, lights, heating controls, internet, and select receptacles. Whole-house backup usually needs a larger and more expensive unit.
6. Why does the calculator show both kW and kVA?
kW represents real power used by the load. kVA reflects apparent power and matters when power factor is below one. Generator spec sheets often list both values.
7. Do I need to consider transfer switches and panels?
Yes. Generator size alone does not complete the design. Transfer switch rating, panel arrangement, conductor sizing, and load prioritization all affect a safe installation.
8. Is this calculator enough for final equipment purchase?
It is a strong planning tool, but final purchase decisions should be checked against equipment submittals, site conditions, fuel availability, electrical design, and local inspection requirements.