A generator that starts but cannot carry the building is not a backup-power solution. It is an expensive delay during an outage. Knowing how to size commercial generators means looking beyond a building’s utility bill and identifying exactly what must stay operational, what those loads demand at startup, and how the system will perform under real site conditions.
For a clinic, that may mean refrigeration, lighting, IT, and essential medical equipment. For a warehouse, it may be dock doors, security, communications, and selected HVAC. The right answer is rarely a generator sized to the entire connected load. It is a generator engineered around the loads that protect safety, operations, revenue, and recovery.
How to Size Commercial Generators: Start With the Critical Load
The first step is defining the generator’s purpose. Is it supplying life-safety systems only? Is it keeping a facility operational through a multi-day utility outage? Or is it supporting a process that cannot tolerate even a brief interruption?
Create a load schedule from panel schedules, equipment nameplates, electrical drawings, and actual operating data where available. Separate every load into one of three groups: loads that must run immediately, loads that can start after a delay, and loads that can remain off during an outage. This distinction prevents unnecessary generator cost while protecting the functions that matter.
Do not assume the utility service size is the generator size. A 2,000-amp service may support a facility with far less actual demand, while a smaller service may include motors or process equipment with significant starting requirements. Demand data is useful, but it must be reviewed alongside the operating plan for an outage.
For existing facilities, a power study can reveal what the building actually uses at peak conditions. For new construction, the electrical design load calculation is the starting point, then the emergency or standby load schedule is refined around the owner’s continuity goals.
Convert Loads Into Generator Demand
Commercial generators are commonly rated in kilowatts (kW) and kilovolt-amperes (kVA). Real power is measured in kW. Apparent power is measured in kVA. The relationship depends on power factor:
kW = kVA × power factor
A generator’s published rating may be expressed in both values at a specified power factor, often 0.8. That does not mean every facility load behaves at 0.8. Modern buildings often include variable frequency drives, UPS systems, LED lighting, computer equipment, and other electronic loads that can affect power factor and harmonic performance.
Add the running kW of all loads expected to operate at the same time. Then evaluate kVA and power factor, especially for large motor loads and electronic equipment. This is where a simple spreadsheet can become misleading. Two systems with the same total kW can place very different demands on a generator.
A properly developed load calculation also accounts for diversity. Not every connected load runs simultaneously, and not every system needs to be on during an outage. However, diversity should be based on a documented operating sequence, not optimism. If production, HVAC, or refrigeration equipment may run together during a recovery event, size for that condition.
Account for Motor Starting and Step Loads
Motor startup is one of the most common reasons an apparently adequate generator performs poorly. A motor can draw several times its normal running current when starting. Across-the-line motor starts create the largest impact, while soft starters and variable frequency drives can reduce starting demand but introduce their own electrical considerations.
The generator must maintain acceptable voltage and frequency when the largest motor starts or when a major block of load transfers onto the system. If voltage dips too far, contactors may drop out, controls can fault, and the building may enter a cycle of failed restarts.
Sequence large loads whenever possible. Rather than starting several rooftop units, pumps, compressors, or conveyors at once, an automatic transfer switch and control strategy can bring them online in stages. Load sequencing often allows a facility to meet its uptime requirements with a more cost-effective generator than a design based on every load starting at once.
Ask these questions early: What is the largest motor? How does it start? What equipment is already running when it starts? Can that start be delayed? The answers affect generator size as much as the total connected kW.
Match the Generator to the Load Type
A generator is not just an engine with an alternator. Its alternator, voltage regulator, engine response, controls, and fuel system must suit the application.
Nonlinear loads deserve close attention. UPS systems, battery chargers, servers, imaging equipment, VFDs, and switch-mode power supplies can introduce harmonics. Depending on the equipment and the percentage of nonlinear load, the system may need an alternator designed to handle harmonic currents, a larger generator, input filtering, or coordinated UPS settings. This is particularly relevant in clinics, data-heavy offices, public safety facilities, and sites with substantial power electronics.
UPS systems also change the sizing conversation. A UPS bridges the gap between a utility failure and generator pickup, protecting sensitive equipment from the transfer event. But its battery recharge demand can become a meaningful load after the generator starts. The generator and UPS must be evaluated as a system, including rectifier characteristics, bypass operation, recharge limits, and the facility’s planned load profile.
For facilities with solar, battery energy storage, or inverter systems, verify how each system behaves during a utility outage. Many grid-tied solar systems shut down when the grid is unavailable unless they are designed and controlled for islanded operation. Battery storage may reduce generator runtime or manage peak loads, but it does not eliminate the need for a coordinated design.
Apply Derating Before Selecting a Nameplate Size
Generator ratings are based on defined conditions, not every condition a commercial site may experience. High ambient temperature, elevation, enclosure configuration, and fuel type can reduce available output. A unit that meets the load calculation at standard conditions may be undersized on a hot rooftop, in an equipment yard with limited airflow, or at a higher-elevation location.
Review the manufacturer’s derating data for the exact generator configuration. Consider whether the generator is installed indoors or outdoors, whether exhaust heat can recirculate, and whether the enclosure has adequate clearance for cooling and service. Sound attenuation requirements, weather protection, and available footprint can also affect the equipment selection.
Fuel planning belongs in sizing, too. Diesel generators are often selected for their response to large loads and practical on-site fuel storage. Natural gas generators can provide extended operation where utility gas service is reliable and adequately sized, but gas pressure and available fuel capacity must be verified under outage conditions. The generator’s load level affects fuel consumption, so runtime must be calculated at the expected operating load, not only at a brochure’s full-load figure.
Design the Transfer and Distribution System With It
The generator is only one part of the emergency power system. Automatic transfer switches, switchgear, distribution panels, feeders, breakers, and controls must carry and coordinate with the intended loads.
A single transfer switch may be appropriate for a small facility with one defined emergency panel. Larger sites often benefit from multiple transfer switches serving different priorities, such as life safety, legally required standby, optional standby, and critical operations. This arrangement supports staged pickup and gives operators more control when fuel conservation or load shedding becomes necessary.
Selective coordination, available fault current, grounding, neutral switching, and overcurrent protection should be addressed during engineering, not after equipment arrives. Requirements vary by occupancy, jurisdiction, and the function of the system. Facilities serving the public, housing critical care functions, or supporting government operations may have specific code and authority requirements that shape the design.
Leave Room for Growth, But Avoid Guesswork
A reasonable growth allowance protects a project from becoming obsolete too soon. The right allowance depends on the facility. A stable administrative building may need little excess capacity, while a growing warehouse, school campus, manufacturing line, or clinic may have known expansion plans that should be included now.
Oversizing has consequences. A larger generator costs more to purchase, install, permit, test, maintain, and fuel. Diesel units that run at very light loads for extended periods can also face operating issues. When future growth is uncertain, alternatives such as load shedding, paralleling generators, provisions for a second unit, or battery storage may provide a better path than purchasing far more capacity than the site needs.
Validate the Design With Real-World Testing
The final generator size should be validated before procurement through an engineering review that considers load calculations, motor starting, one-line diagrams, equipment data, code requirements, and outage procedures. Once installed, the system needs commissioning and load-bank testing to confirm it starts, transfers, carries the intended load, and performs as designed.
Facility staff should also know what happens during an outage. A written sequence for load priorities, fuel checks, alarm response, and testing turns installed equipment into an operational continuity plan. For Southern California facilities, that preparation is especially valuable when outages, heat events, public safety shutoffs, or construction-related interruptions place pressure on already busy operations.
Unlimited Power Solutions helps organizations evaluate the full power system – generator, transfer equipment, UPS, batteries, and distribution – so the equipment selected supports the work that cannot stop. The best time to resolve a sizing question is while the lights are still on, with enough time to verify every critical load and build a system ready to perform when it is needed.