Plan backup power for offices, sites, and camps. Model mixed loads with growth and derating. Get a clear kVA target and ATS amperage guidance.
Use this as a quick reference for typical site loads. Adjust PF, demand, and starting multipliers to match your equipment and operating sequence.
| Load | Type | Qty | Input | Unit | PF | Demand | Start mult | Notes |
|---|---|---|---|---|---|---|---|---|
| Site lighting | Lighting | 1 | 8 | kW | 0.95 | 1.00 | 1.0 | Low surge |
| Water pump | Motor | 1 | 15 | HP | 0.85 | 1.00 | 3.5 | DOL start |
| Welding outlets | General | 1 | 12 | kW | 0.80 | 0.70 | 1.2 | Intermittent |
| Office / IT | UPS/IT | 1 | 6 | kW | 0.90 | 1.00 | 1.3 | Rectifier inrush |
This tool sizes a generator using running load, starting surge, and adjustment factors:
Notes: Derating is a rule-of-thumb approximation. Always verify with manufacturer derating curves, alternator limits, and transient performance requirements.
A dependable standby selection begins with a complete schedule of connected loads, quantities, and realistic operating patterns. For construction sites, lighting, offices, dewatering pumps, welding outlets, and compressors commonly dominate demand. Use demand factors to reflect diversity; for example, intermittent welding may be modeled at 0.6–0.8 rather than 1.0.
This calculator converts motor horsepower to electrical input using kW = HP × 0.746 ÷ efficiency. It then converts kW to kVA using kVA = kW ÷ PF. Typical PF values are 0.80–0.90 for mixed site loads, 0.95 for lighting circuits with correction, and near 1.00 for resistive heaters.
Motor starting can require several times the running kVA for a short duration. The tool models starting with a multiplier (often 2–6 for DOL starting), and estimates a worst case where all loads are running and the largest motor starts. If your sequence allows two large motors to start together, increase multipliers or add a dedicated row.
Growth covers future additions and uncertainty; 10–25% is common for temporary works with evolving scope. A safety margin helps absorb transient impacts and measurement error. Derating accounts for reduced air density and cooling at high altitude and temperature; the auto mode applies rule‑of‑thumb reductions. Always verify with manufacturer curves for the chosen model and enclosure.
After selecting a standard kVA size, confirm full‑load current at site voltage and phase. The calculator suggests an ATS rating using a conservative step above 125% of full‑load current. Final equipment selection should also check short‑circuit withstand, generator transient response, harmonics from UPS loads, and any code requirements for emergency or legally required standby systems.
Generators are commonly rated in kVA, while many loads are listed in kW. Power factor links them: kVA = kW ÷ PF. Low PF increases kVA demand and can drive the generator size.
For direct-on-line starts, 3–6 is typical. Soft starters or VFDs can reduce starting demand substantially. Use manufacturer data when available and model worst-case starting sequences for critical motors.
Use demand to represent diversity. If a load runs intermittently, set 0.5–0.8 based on duty. For life-safety or continuous loads, keep demand near 1.0 and avoid optimistic assumptions.
It includes a starting multiplier option for UPS/IT inrush, but it does not calculate harmonic distortion. For large UPS systems, confirm generator compatibility, alternator sizing, and any K-factor or THD limits.
High ambient temperature and altitude reduce available engine power and cooling capacity. If you ignore derating, a generator may not deliver its nameplate rating in real site conditions, risking overload and voltage dip.
It is a planning estimate based on 125% of full-load current, rounded up to a standard size. Final ATS selection should verify continuous rating, short-circuit withstand, and coordination with protection devices.
Underestimating motor starting and future growth. A generator that meets running kW may still fail during starts due to kVA surge and voltage drop. Model the start sequence and choose a practical margin.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.