Enter Commercial Load Details
Add normal loads, motor loads, starting demand, reserve margin, and site derating values.
Example Data Table
| Facility Type | Running Load | Starting Load | Reserve | Suggested Size |
|---|---|---|---|---|
| Small Office | 42 kW | 68 kVA | 20% | 100 kVA |
| Retail Building | 95 kW | 155 kVA | 20% | 200 kVA |
| Medical Clinic | 140 kW | 240 kVA | 25% | 350 kVA |
| Light Industrial Site | 310 kW | 520 kVA | 25% | 750 kVA |
Formula Used
The calculator first converts motor horsepower into kilowatts using: Motor kW = HP × 0.746 ÷ efficiency.
Total connected load is then adjusted by diversity: Running kW = Total connected kW × diversity factor.
Apparent power is calculated as: Running kVA = Running kW ÷ power factor.
Starting demand is estimated with the selected motor starting multiplier: Peak kVA = Running kVA + extra motor starting kVA.
The final required size is: Required kVA = Peak kVA × reserve × growth × harmonic allowance ÷ site derating factor.
How to Use This Calculator
- Enter all known commercial loads in kilowatts.
- Add HVAC tons and the expected kW per ton.
- Enter motor horsepower, motor count, and starting method.
- Set power factor, diversity, reserve, growth, and derating values.
- Choose phase, voltage, fuel type, and backup runtime.
- Press the calculate button to view the suggested generator size.
- Download the result as a CSV file or PDF report.
Commercial Generator Sizing Guide
Why Proper Sizing Matters
Commercial generator sizing is not only about adding loads. It also includes starting surge, site conditions, reserve capacity, and future expansion. A generator that is too small can stall during motor startup. It can also cause voltage dips. Sensitive equipment may shut down during those dips.
Know Your Running Load
Start with the equipment that must stay on during an outage. Include lighting, HVAC, refrigeration, IT systems, pumps, elevators, security, and process machines. Some facilities do not need every load at once. That is why a diversity factor is useful. It reduces the connected load to a realistic operating load.
Plan for Starting Surge
Motors can need much more power when they start. Direct online starting may create a large surge. Soft starters and drives can reduce that demand. The largest motor often controls the generator size. Staged starting can also help. It lets one major load start before another begins.
Apply Safety Margins
A reserve margin helps the generator run below its maximum rating. This improves reliability. Future growth is also important. Many buildings add more equipment later. Harmonic allowance can help when nonlinear loads are present. These include drives, chargers, and computer power supplies.
Consider Site Conditions
Heat and altitude can reduce available engine output. The calculator uses derating values to account for those losses. Fuel planning is also essential. Diesel, gas, and propane systems have different storage and supply needs. Use the result as a planning estimate. Always verify the final selection with local codes, utility rules, and a licensed professional.
FAQs
1. What is a commercial generator sizing calculator?
It estimates the generator capacity needed for business loads. It considers running demand, starting surge, reserve margin, growth, and site derating.
2. What does kVA mean?
kVA means apparent power. It includes real power and power factor effects. Generators are often rated in kVA.
3. Why is power factor important?
Power factor connects kW and kVA. A lower power factor requires a larger generator for the same useful load.
4. Why do motors affect generator size?
Motors can draw high current during startup. This starting surge may be much larger than normal running demand.
5. What is diversity factor?
Diversity factor estimates how much connected load runs at the same time. It helps avoid oversizing when loads do not operate together.
6. What reserve margin should I use?
Many planning estimates use 15% to 25%. Critical facilities may need more. Final values depend on risk and design standards.
7. Why is derating included?
High temperature and altitude can reduce generator output. Derating adjusts the required capacity for those site conditions.
8. Is this calculator enough for final design?
No. It is a planning tool. A qualified engineer should verify load data, code rules, transfer equipment, and installation details.