Size a commercial or industrial backup generator. Enter critical loads and demand factor to find the minimum kW and kVA generator capacity needed.
Commercial and industrial generator sizing differs from residential sizing because of scale, load diversity, and the demand factor. Business loads may total hundreds of kW on paper, but not all loads run simultaneously. The demand factor (typically 0.60–0.85) accounts for this diversity, reducing the required generator size below the simple sum of all loads.
Critical loads in a business include life-safety systems (emergency lighting, fire alarms, elevators), IT infrastructure (servers, networking), refrigeration (restaurants, groceries, medical), and essential operations equipment. Non-critical loads can be shed during an outage to reduce the generator requirement.
This calculator takes your total critical load and applies a demand factor to determine the real generator capacity needed. It provides both kW and kVA recommendations, accounting for typical commercial power factors. Use it for budget planning, RFP specifications, and preliminary electrical engineering.
Integrating this calculation into regular energy reviews ensures that conservation strategies are grounded in measured data rather than assumptions about building performance and usage patterns.
Commercial generators represent a significant investment ($20,000–$500,000+). Accurate sizing prevents overspending on oversized equipment or risking business disruption with undersized backup power. Data-driven tracking enables proactive energy management, helping organizations reduce operational costs while progressing toward environmental sustainability goals and carbon reduction targets. This quantitative approach replaces rough estimates with precise figures, enabling facility managers to identify the most cost-effective opportunities for reducing energy consumption.
kW Needed = Σ(Critical Loads) × Demand Factor kVA = kW ÷ Power Factor
Result: 150 kW / 187.5 kVA
Total critical loads: 200 kW. Demand factor: 0.75. kW needed: 200 × 0.75 = 150 kW. At PF 0.80: kVA needed = 150 ÷ 0.80 = 187.5 kVA. A 200 kVA generator would be the appropriate selection.
NEC defines three priority levels: Emergency (NEC 700): Life-safety — exit lights, fire alarms, smoke control, elevators. Must start within 10 seconds. Legally Required Standby (NEC 701): Ventilation, communications, sewage. Must start within 60 seconds. Optional Standby (NEC 702): Convenience loads the owner chooses to back up. No time requirement.
Don't apply all loads simultaneously. Step-loading (applying loads in stages 3–5 seconds apart) reduces the generator's peak demand during startup. The ATS can be programmed to connect load blocks in sequence. This can reduce the required generator size by 15–25%.
Local codes typically require 2–48 hours of fuel at full load. Hospitals often need 72–96 hours. Calculate fuel storage: Hours × gal/hr at rated load. For 200 kW diesel generator at full load (~14 gal/hr), 48 hours = 672 gallons.
The demand factor is the ratio of actual peak demand to total connected load. If a building has 200 kW of equipment but never runs more than 150 kW simultaneously, the demand factor is 0.75. It reflects real-world load diversity.
Office buildings: 0.60–0.75. Retail: 0.70–0.80. Restaurants: 0.75–0.85. Hospitals: 0.80–0.90. Data centers: 0.85–0.95. Manufacturing: 0.60–0.80 (varies widely by process). Use metered data for the most accurate factor.
Generator cost: $200–$500 per kW for the unit. Installation: 50–100% of unit cost (includes ATS, fuel tank, concrete pad, electrical connections, permitting). Total installed: 50 kW = $25,000–$60,000. 200 kW = $80,000–$200,000.
Diesel: More fuel-efficient, better for high loads and long runtimes, requires fuel storage. Natural gas: Unlimited fuel supply (if gas lines intact), lower maintenance, cleaner emissions, but less fuel-efficient. Diesel dominates for large commercial applications.
N+1 means having one more generator than needed. If your load requires 2 generators, you install 3. If one fails, the remaining two still meet full demand. This is standard for hospitals, data centers, and other mission-critical facilities.
NEC Article 700 (emergency systems) requires automatic start and load transfer within 10 seconds. Article 701 (legally required standby) allows 60 seconds. Article 702 (optional standby) has no time requirement. Each category has different sizing and fuel storage obligations.