Generator Sizing Calculator
Generator sizing is a multi-constraint optimization problem balancing steady-state running load, motor starting inrush capability, harmonic distortion limits for VFD and UPS loads,...
Formula
Source: NEC Articles 220, 700, 701, 702; IEEE 446 (Emergency and Standby Power); NFPA 110 | Last reviewed: July 26, 2026
Examples
200 kW
= 380.4 kVA
- largest_motor_hp = 50
- pf = 0.85
- growth_margin = 20
200 kW running load + 50 HP starting -> ~380 kW generator. kVA = 380 / 0.85 = 447 kVA -> select 500 kW/625 kVA
50 kW
= 79.7 kVA
- largest_motor_hp = 10
- pf = 0.9
- growth_margin = 10
50 kW load + 10 HP motor -> ~80 kW generator -> select 100 kW
1500 kW
= 2541 kVA
- largest_motor_hp = 300
- pf = 0.85
- growth_margin = 25
Large commercial facility -> ~2,500 kW generator (multiple units typically)
Quick Reference Table
| Application | Typical kW Range | Common Fuel | NEC Article |
|---|---|---|---|
| Small Residential (essential loads) | 7-10 kW | Propane / NG | 702 |
| Whole-House Residential | 20-26 kW | Propane / NG | 702 |
| Large Luxury Home | 30-48 kW | NG / Diesel | 702 |
| Small Commercial / Retail | 50-150 kW | Diesel / NG | 700/701/702 |
| Mid-Size Office Building | 200-500 kW | Diesel | 700/701/702 |
| Hospital (per unit) | 1,000-2,000 kW | Diesel | 517/700 |
| Data Center (per unit) | 2,000-3,000 kW | Diesel | 702 |
| Industrial Plant | 1,000-5,000+ kW | Diesel / NG | 701/702 |
| Code Letter | kVA/HP Range | Typical Motor Types |
|---|---|---|
| A | 0.00-3.14 | Small fractional HP, high-efficiency |
| F | 5.0-5.6 | Standard efficiency TEFC 10-50 HP |
| G | 5.6-6.3 | Standard efficiency open drip-proof |
| H | 6.3-7.1 | High-efficiency TEFC motors |
| J | 7.1-8.0 | Premium efficiency, high locked-rotor |
| K | 8.0-9.0 | High-slip, high starting torque |
Where is this used?
(1) Life safety loads (NEC 700): Fire pump (25-300 HP electric motor-driven -- a 100 HP fire pump at 460V draws 124A FLA and 745-870A LRA, requiring the generator to be sized primarily for the fire pump starting surge rather than the building's other loads.
NFPA 20 requires the generator to start and the fire pump to be online within 10 seconds).
Emergency lighting (0.5-2W/sq ft).
Fire alarm system (500W-2kW continuous).
Elevator recall and smoke control fans powering stairwell pressurization.
(2) Healthcare essential electrical systems (NEC 517): A 200-bed hospital typically requires 1.5-3 MW of generator capacity deployed as 2-3 paralleled generators for N+1 redundancy.
Life safety branch: egress lighting, fire alarm, nurse call.
Critical branch: operating room receptacles, ICU power.
Equipment branch: medical gas compressors, HVAC for critical areas.
(3) Data center backup power (Uptime Institute Tiers): A Tier III data center requires N+1 generator redundancy with minimum 12-hour on-site fuel.
A 5 MW IT load at PUE 1.4 draws 7 MW total, requiring 8-10 MW of generator capacity.
UPS input harmonics (8-12% THD) and battery recharge current post-discharge (10-25% added load for 2-8 hours) must be factored into generator sizing.
(4) Industrial process continuity: Food processing plants with refrigeration compressors (50-200 HP screw type) and conveyor systems -- worst-case is all compressors restarting simultaneously after a momentary utility interruption.
Staggered restart using PLC sequencing substantially reduces required generator size.
(5) Water/wastewater lift stations: Remote unmanned sites with 10-100 HP submersible pumps -- a single generator sized to start the largest pump with all other pumps already running.
Redundancy comes from a spare pump, not a second generator.
(6) Prime power for off-grid: Mining camps and remote communities use generators as primary power (not backup).
Generators are sized for 70-80% of standby rating for continuous duty, with multiple units for load sharing and fuel efficiency optimization.
(7) Utility demand response: Facilities deploy generators to reduce grid demand during peak events, earning capacity payments of $5-20/kW-month plus energy payments.
These generators are exercised monthly regardless of grid conditions.
Real-World Usage Scenarios
Commercial Office Building Generator Retrofit
A 75,000 sq ft, 5-story office building has a 250 kW running load consisting of lighting at 1.0W/sq ft, a 40 HP elevator, a server room at 75 kW including UPS, two 30 HP HVAC chilled water pumps, plus building automation and fire alarm. The largest motor starting is the 40 HP elevator on across-the-line starting: 40 x 0.7457 x 6 = 179 kVA starting surge. Generator sizing: (250 kW running + 40 x 0.7457 x 3 = 89.5 kW starting allowance) x 1.15 growth margin = 390 kW. At PF 0.85: 390 / 0.85 = 459 kVA. The engineer selects a 450 kW/563 kVA standby-rated diesel generator at $95,000 installed including a 48-hour belly tank, weatherproof enclosure, automatic transfer switch rated 800A, and a permanent load bank connection. Monthly exercise testing with a 100 kW load bank prevents wet stacking since the building normally loads the generator at only 60-70% during actual outages.
Hospital N+1 Generator System Design
A 150-bed community hospital has an essential electrical system load of 1,800 kW distributed across three branches: life safety at 150 kW, critical at 400 kW, equipment at 1,250 kW including a 150 HP chiller. The largest motor is the 150 HP centrifugal chiller equipped with a soft starter limiting inrush to 3x FLA: 150 x 0.7457 x 3 = 335 kW starting allowance. Total sizing: (1,800 + 335) x 1.15 growth = 2,455 kW. NFPA 110 Level 1 and NEC 517 require emergency power within 10 seconds. The design uses three 1,250 kW diesel generators in parallel providing N+1 redundancy -- any single unit can be down for maintenance while the remaining two carry the full load. Each generator has a 1,500-gallon sub-base tank for 72 hours of operation at 80% load. Installed cost: $1.2 million including paralleling switchgear and automatic transfer switches for each branch.
Remote Wastewater Lift Station Prime Power Sizing
A regional wastewater utility operates a remote lift station with three 60 HP submersible pumps (two duty, one standby) at 480V three-phase. Each pump has FLA of 77A and LRA of 460A (NEMA Code G, 5.6-6.3 kVA/HP). Running load with two pumps: 2 x 60 x 0.7457 = 89.5 kW. Starting allowance for the third pump starting with two running: 60 x 0.7457 x 5.6 = 250.6 kVA starting surge. At PF 0.82 typical for submersible motors: 250.6 kVA x 0.82 = 205.5 kW momentary starting load. Total during starting: 89.5 + 205.5 = 295 kW. With 15% growth: 339 kW. Selected: 350 kW standby / 320 kW prime-rated diesel with 500-gallon fuel tank for 5 days continuous operation at 50% average load. SCADA-connected via cellular modem for remote monitoring. Installed cost: $85,000.
Common Mistakes to Avoid
Sizing by simply summing connected kW
The most pervasive and expensive generator sizing mistake is adding up all equipment kW nameplate ratings and selecting a generator of that size. This ignores two critical constraints: (1) motor starting inrush, which typically requires the generator to be 2-3x the running load of the largest motor; and (2) generator alternator kVA capability -- if the facility has a poor power factor (many lightly loaded motors at PF 0.75), the alternator kVA rating may be the limiting constraint, not the prime mover kW. A 500 kW generator at PF 0.80 delivers only 625 kVA. If the connected load is 500 kW at PF 0.75 = 667 kVA required, the alternator is undersized even though the prime mover kW is adequate. Always verify both kW and kVA constraints.
Neglecting UPS and VFD harmonic effects on the generator
Data centers, hospitals, and industrial plants contain substantial UPS and VFD loads that draw non-sinusoidal current rich in harmonic frequencies (5th, 7th, 11th, 13th harmonics). These harmonic currents cause additional heating in generator alternator windings and can destabilize the automatic voltage regulator (AVR). Generators are typically rated for a maximum of 8-12% total harmonic voltage distortion (THVD). If harmonic load exceeds this, the generator must be de-rated -- typically 10-25% for UPS loads and 5-15% for VFD loads. Generator manufacturers publish de-rating curves based on the percentage of non-linear load relative to total generator kVA. For facilities with more than 25% non-linear load, specify a generator with a permanent magnet generator (PMG) exciter or oversized alternator for stable voltage regulation.
Selecting standby-rated generators for continuous duty applications
Generator ratings are application-specific and not interchangeable. A standby-rated generator is designed for emergency operation during utility outages only -- limited to approximately 200 hours per year with a maximum of 25 hours per outage, and delivers 100% of nameplate kW for intermittent use. If operated as a prime power source (unlimited hours, variable load), it must be de-rated to approximately 70-80% of its standby rating. Continuous-rated generators handle 100% load for unlimited hours. Installing a standby-rated generator in a prime power application (off-grid site, mining camp, remote community) results in premature engine failure, often within 1-2 years versus a 20+ year expected service life. Always verify the rating type matches the application per ISO 8528-1.
Industry Standards Referenced
Frequently Asked Questions
What size generator do I need for my house?
Essential loads only (refrigerator, lights, furnace blower, well pump, sump pump, internet/router): 7-10 kW. Whole-house with central AC up to 5-ton, electric range, and electric dryer: 20-26 kW. For whole-house, a 22-26 kW air-cooled standby generator (Generac Guardian, Kohler 20RESC, Cummins QuietConnect) is the most common residential selection. The generator installer must perform a load calculation per NEC Article 220 to confirm the exact requirement. A soft start kit on the AC compressor ($300-500 installed) can reduce the required generator size by 5-10 kW by limiting the starting inrush. Homes with multiple AC units, electric tankless water heaters, pool heat pumps, or EV chargers may require 30-48 kW liquid-cooled generators.
Can a generator be too big?
Yes -- oversizing creates significant reliability and maintenance problems, particularly for diesel generators. Diesel engines operated below 30% load for extended periods suffer from 'wet stacking' -- unburned fuel condenses in the exhaust system, causing carbon buildup, reduced efficiency, and increased maintenance frequency. Natural gas generators are more tolerant of light loading. The minimum recommended continuous load is 30% of rated kW for diesel and 20% for natural gas. If your essential load is 50 kW and you install a 200 kW diesel generator, it operates at only 25% load during most outages, risking wet stacking. A load bank can provide supplemental load during monthly exercise testing, but this adds $5,000-15,000 to the installation cost and requires manual connection. Size the generator for the actual load -- not 'bigger is better.'
What is the difference between standby, prime, and continuous generator ratings?
Standby (Emergency Standby): Rated for operation during utility outages only -- typically 70-80% of the prime rating, with a maximum of approximately 200 hours per year and 25 hours per outage. This is the rating for backup generators at buildings with reliable utility service. Prime Power: Rated for unlimited hours of operation under variable load, with a 10% overload capability for 1 hour in any 12-hour period. Used for remote sites, construction, and mining where the generator is the primary power source. Continuous: Rated for 100% load for unlimited hours with no overload capability. Used for base-load power generation. A 100 kW standby generator (equivalent to approximately 80 kW continuous capability) should not be used for prime power -- it will fail prematurely. Per ISO 8528-1, always match the rating type to the application.
How does generator and motor voltage (208V vs 480V) affect sizing?
Generator sizing is primarily a kW and kVA exercise independent of voltage. However, lower voltage systems (208V) require higher current for the same kW, which affects conductor sizing, transfer switch ampacity, and generator breaker selection. A 500 kW generator at 480V delivers approximately 752A full-load, while at 208V it delivers approximately 1,735A -- requiring much larger conductors and a more expensive transfer switch. For commercial and industrial facilities, 480V generators are standard because they match building distribution voltage. For residential, 120/240V single-phase generators are standard because residential service is single-phase. When stepping down from a 480V generator to serve 208V loads via a transformer, account for transformer losses (1-3% additional load) and inrush current when the transformer energizes (up to 10-12x rated current for 0.1 seconds).
What fuel options are available for generators and how do they affect sizing?
Diesel: Highest power density, most common for commercial/industrial generators 150 kW and above. Fuel storage (on-site tank) required. Diesel fuel degrades over 6-12 months without treatment, requiring fuel polishing programs. Natural Gas: No on-site fuel storage required -- connects to utility gas line. However, natural gas may be unavailable during earthquakes or widespread disasters when pipelines are shut off. Natural gas engines produce approximately 10-15% less power than diesel engines of the same displacement due to lower energy density, so a natural gas generator of the same kW rating has a physically larger engine. Propane (LPG): Similar to natural gas with on-site storage (tank). Common for residential generators 7-26 kW. Propane stores indefinitely without degradation -- a significant advantage for generators that may sit unused for years. Bi-fuel (diesel + natural gas): Operates primarily on natural gas with diesel pilot injection, providing fuel redundancy.
Reviewed for accuracy
Reviewed against NEC 2023 Articles 220, 700, 701, 702, and 517; NFPA 110-2022; and IEEE 446 recommended practice · Last reviewed: July 26, 2026
All calculations are for reference only. Always verify with manufacturer data and a qualified engineer for critical applications. Learn about our editorial process.