Generator Inputs
Use the responsive calculator grid below. It shows three columns on large screens, two on tablets, and one on mobile.
Example Data Table
| Appliance | Qty | Running Watts Each | Starting Watts Each | Running Total | Starting Total |
|---|---|---|---|---|---|
| Refrigerator | 1 | 700 | 2200 | 700 | 2200 |
| Sump Pump | 1 | 800 | 2000 | 800 | 2000 |
| LED Lights | 10 | 12 | 12 | 120 | 120 |
| Microwave | 1 | 1200 | 1200 | 1200 | 1200 |
This sample mix shows how motor loads often need much higher startup capacity than simple electronic or resistive loads.
Formula Used
1) Total Running Watts
Total Running Watts = Σ (Quantity × Running Watts Each)
2) Total Starting Watts
Total Starting Watts = Σ (Quantity × Starting Watts Each)
3) Extra Startup Surge
Extra Surge = Starting Total − Running Total
4) Peak Watts Before Margin
Largest Start Mode = Total Running Watts + Largest Extra Surge
Coincident Mode = Total Running Watts + (Total Extra Surge × Coincidence Factor)
All Start Mode = Total Starting Watts
5) Recommended Generator Watts
Recommended Watts = Peak Watts Before Margin × (1 + Reserve Margin ÷ 100)
6) Apparent Power
kVA = Recommended Watts ÷ (Power Factor × 1000)
7) Output Current
Single Phase Amps = Watts ÷ (Voltage × Power Factor)
Three Phase Amps = Watts ÷ (√3 × Voltage × Power Factor)
These formulas help estimate generator size for practical planning. Final equipment selection should still follow manufacturer guidance and local electrical rules.
How to Use This Calculator
- Choose single-phase or three-phase output.
- Enter generator voltage and expected power factor.
- Set a reserve margin for future growth and better stability.
- Select the startup assumption that best matches your load behavior.
- Add each appliance, tool, or circuit as a separate row.
- Enter quantity, running watts, and startup watts for every row.
- Press the calculate button to see the result above the form.
- Review suggested kW, kVA, amps, standard size, and the graph.
- Use CSV or PDF export for sharing or documentation.
FAQs
1. What size portable generator do I need for home backup?
Add the running watts of all loads you want active together, then cover motor startup surge and reserve margin. This calculator helps estimate a practical generator size for that mix.
2. Why are starting watts higher than running watts?
Motors, compressors, and pumps often draw a brief inrush current at startup. That short surge can be much higher than steady operation and often determines the minimum generator size.
3. Should I size by kW or kVA?
Use kW for real load demand and kVA when power factor matters. Many generator specifications list both, so checking both values gives a more complete sizing picture.
4. What reserve margin should I use?
A 15% to 25% reserve margin is common for general planning. Heavier motor loads, long extension runs, and future expansion may justify a larger margin.
5. What does coincidence factor mean here?
It represents the share of total startup surge you expect to happen at the same time. Lower values assume staggered starts, while higher values assume more overlap.
6. Can I use this for tools and jobsite equipment?
Yes. Enter each tool or device with realistic running and startup watt values. Portable saws, compressors, and pumps often need special attention because of startup demand.
7. Does this replace manufacturer recommendations?
No. It is a planning calculator. Always compare the result with generator ratings, appliance nameplates, and manufacturer instructions before buying or connecting equipment.
8. Why is the suggested standard generator larger than the exact result?
Portable generators are sold in standard sizes. The calculator rounds up so you choose a commonly available unit that still provides the needed capacity safely.