A commercial generator should keep critical operations running without creating new problems for the electrical system. That means the right unit is not chosen by square footage alone, and the largest model is not automatically the safest choice. A sound design accounts for the building’s essential loads, motor starting demands, voltage and power factor, fuel availability, environmental conditions, and applicable code requirements.

This process is especially important for facilities with HVAC equipment, refrigeration, pumps, elevators, medical equipment, or other motor-driven loads. A careful assessment helps avoid nuisance shutdowns, voltage dips, wasted fuel, and an oversized system that never reaches an efficient operating load. The first practical step is to identify what the building must support during an outage and measure the demand those systems create. For a next step, commercial property teams can request a generator sizing quote to begin the load assessment with a qualified installer.

Request a commercial generator sizing consultation to start your load assessment today.

How to Size a Generator for a Commercial Building Around Its Essential Loads

The first step in learning how to size a generator for a commercial building is to define what the building must keep operating when utility power fails. Generator selection should begin with a documented power assessment, not a quick estimate based on the building’s square footage or the size of its service entrance. The assessment identifies essential equipment, measures its electrical requirements, and establishes which systems must receive backup power.

Build an accurate baseline from equipment and utility data

Start by inventorying every system that may need to operate during an outage. Depending on the facility, that list may include life-safety equipment, fire pumps, emergency lighting, security systems, refrigeration, servers, access controls, elevators, HVAC controls, pumps, and production equipment. The U.S. Environmental Protection Agency recommends conducting a power assessment to understand essential equipment energy needs before selecting a generator. EPA power resilience guidance supports this assessment-based approach.

Historical utility usage adds another layer of accuracy. Review interval data, demand charges, utility bills, and operating schedules to establish the facility’s baseline consumption. Historical usage does not replace an equipment-level assessment because whole-building averages can hide short demand spikes or loads that are not always active. It does, however, help the design team compare the proposed backup load with how the building actually operates across seasons, shifts, and production cycles. An EPA-backed analysis notes that historical utility usage data helps establish baseline energy consumption for sizing purposes.

Separate running load from starting load

For each essential item, record both its running load and its starting requirement. Running load is the power needed to keep equipment operating after it has started. Starting load, also called inrush current, is the temporary spike required to start electric motors. HVAC compressors, refrigeration equipment, pumps, and fans can demand substantially more power during startup than they use once running. The EPA reciprocating engine technology guide identifies starting loads as a critical commercial generator sizing factor.

Map the startup sequence as well as the steady-state operation. If several large motors start at the same time, their combined inrush can create the highest demand of the outage. A generator that covers the normal running load may still struggle to start the equipment, causing voltage instability or a failed transfer.

Prioritize simultaneous essential loads and peak demand

Next, identify which essential loads must operate simultaneously during a power failure. Categorize them by criticality. Life-safety and code-required systems typically take priority, followed by equipment needed to protect people, inventory, data, and core business operations. Lower-priority loads may be shed or sequenced if the total demand exceeds the available capacity. This priority schedule gives the electrical contractor a practical basis for automatic load management and future expansion planning.

Finally, size the system around peak demand, not average consumption. The design must account for the highest credible combination of running loads and startup events, along with the selected equipment’s kW and kVA requirements. A properly documented load schedule gives a commercial facility a defensible starting point for comparing Cummins or Kohler generator options and designing a system that supports the building’s actual outage priorities.

Why Running Load and Starting Load Are Different

A commercial generator does more than supply enough power for equipment after it is operating. It must also deliver the brief surge required to start motors and bring critical systems online. That distinction matters when evaluating HVAC compressors, refrigeration equipment, pumps, air handlers, and other motor-driven loads. The U.S. Environmental Protection Agency identifies motor starting requirements as a critical factor in commercial generator sizing because starting loads are often significantly higher than running loads (EPA guidance).

Running load keeps equipment operating

Running load is the steady power required to keep essential equipment operating after it has started. For example, an HVAC system may draw a relatively consistent amount of power while maintaining indoor temperature. And a refrigeration system may settle into a predictable operating range after its compressor reaches speed. Lighting, controls, network equipment, and other non-motor loads also contribute to the ongoing demand.

This is the number most people think of first when they ask how to size a generator for a commercial building. It is important, but it is only part of the calculation. A generator that appears adequate based on steady-state wattage may still struggle when one or more motors start.

Starting load creates a temporary demand spike

Starting load, also called inrush current, is the temporary spike of power required to start an electric motor. A compressor or pump must overcome inertia and reach operating speed before its demand falls toward its running level. If several systems start together after a utility outage, their starting demands can overlap. The generator must be selected and configured to withstand that peak event, not simply the average demand once everything is running.

That is why the sequence and timing of automatic transfer and load start-up deserve attention. Facility teams should identify which HVAC, refrigeration, pump, and life-safety systems must operate simultaneously, then account for the largest starting events in the design. Controls or staged starting may help reduce the instantaneous burden. But they should be evaluated as part of a complete electrical design rather than assumed as a substitute for adequate capacity.

An undersized system can experience an excessive voltage dip during motor starting. That dip may cause sensitive computers, control boards, security equipment, or communication systems to reset even when the generator eventually recovers. Proper sizing therefore considers both continuous running load and the highest expected starting current, helping a Cummins or Kohler system bring equipment online without compromising connected electronics.

Power Factor, Voltage, and Environmental De-Rating

A generator can show enough kilowatts on paper and still be the wrong fit for a commercial facility. The sizing review must also account for power factor, voltage, and the conditions in which the equipment will operate. These details determine whether a Cummins or Kohler generator can deliver stable power to the connected load without exceeding its electrical limits.

Why kW and kVA are both important

Electrical loads consume working power, measured in kilowatts (kW), but the generator also supplies apparent power, measured in kilovolt-amperes (kVA). Power factor is the ratio of working power to apparent power. A lower power factor means the generator must provide more kVA to deliver the same useful kW. The combined kVA of essential equipment, including its power factor, must remain within the generator’s available capacity. This is a key part of how to size a generator for a commercial building, especially where motors, compressors, transformers, and electronically controlled equipment operate together.

How kW, kVA, and generator rating relate

Item	What it represents	Sizing implication
kW	Useful or working power consumed by equipment.	Shows the energy the load uses to perform work.
kVA	Apparent power, reflecting kW and power factor.	Combined essential-load kVA must fit within generator capacity.
Generator rating	Nameplate maximum output under stated conditions.	Total connected kVA must not exceed the nameplate rating.

Voltage and operating conditions can change the result

Voltage requirements must match the building’s distribution system and connected equipment. A voltage mismatch can damage equipment or prevent critical systems from operating. The assessment should confirm phase, voltage, frequency, and connection requirements before a generator is selected, rather than relying only on the facility’s total kW.

Environmental conditions can reduce usable output. Altitude and ambient temperature may de-rate a generator, meaning its actual power capability is lower than the published rating under standard conditions. The design therefore needs to apply the manufacturer’s de-rating factors for the planned installation location. The generator should also have enough reserve to respond to changing loads and maintain voltage and frequency regulation.

A practical design goal is to avoid running at absolute maximum capacity for extended periods. Often called the 80% rule, this approach leaves operating headroom and helps protect engine life. It does not replace a load calculation or a manufacturer’s rating guidance. The final selection should keep total kVA within the nameplate limit, account for environmental de-rating, and leave reasonable capacity for dependable operation.

Fuel Options and Storage for Commercial Backup Power

Fuel selection is part of generator sizing, not a decision to postpone until installation. Commercial standby systems may use natural gas, propane, or diesel, and each option creates different requirements for supply, storage, site planning, and outage endurance. The right choice depends on the building, available utilities, environmental conditions, local requirements, and how long critical operations must continue.

Natural gas, propane, or diesel

Natural gas can be a practical option where the utility service is reliable and the building has adequate capacity. However, the supply should be tested and documented during design. Fuel pressure must be verified so a natural gas generator can reach its rated output when demand rises. A connection that appears adequate during normal building operation may not provide the pressure or volume required during an outage.

Propane can provide a stored-fuel solution for sites without dependable natural gas service. Diesel is another common commercial option, particularly when a facility needs substantial on-site fuel capacity. It requires space for the tank, appropriate delivery and containment planning, and coordination with applicable site and fire requirements. Cummins and Kohler commercial generator lines offer configurations that can be evaluated against the facility’s load and fuel strategy.

Storage and outage duration belong in the sizing plan

Commercial generators often require special fuel storage arrangements, so storage should be considered during the initial sizing and planning phase. That means reviewing tank location, access for refueling, clearances, ventilation, containment, and the physical limitations of the property before selecting equipment.

Start with the outage scenario the business must be prepared to withstand. Planning for short interruptions is different from planning for extended outages caused by storms, regional utility failures, or infrastructure damage. The duration you need to cover affects both generator and fuel sizing. A system intended to support a few hours may rely on a smaller fuel reserve. While a critical facility that must operate for days may need larger storage, staged refueling, or a fuel plan that keeps essential loads running without interruption.

A qualified electrical contractor can match the fuel system to the generator’s rated output, essential load sequence, and resilience goals. This integrated approach helps prevent a generator that is electrically capable but unable to run for the duration your commercial building requires. EPA resilience guidance also emphasizes planning for both short- and long-duration power outages.

Common Sizing Mistakes: Undersizing and Oversizing

A commercial generator should be sized around the electrical conditions your building will face during an outage, not a convenient average from the utility bill. An undersized unit may appear adequate during normal operation but fail when several essential motors start together. That peak demand can create voltage dips, trip breakers, or prevent the automatic transfer switch from completing the transfer. The result is a generator that exists on paper but cannot reliably carry the loads the business depends on.

Peak demand matters because starting load is temporarily higher than running load. HVAC compressors, pumps, refrigeration equipment, and other motor-driven systems can draw substantial inrush current at startup. A load study identifies which essential systems may start simultaneously and evaluates their sequence. It also ranks loads by priority, so critical operations receive power when every connected system cannot run at once. The assessment should account for the building’s current requirements and outage plan.

Oversizing creates a different problem. When a generator operates for extended periods at a very light load, the engine may not burn fuel completely. This condition, known as wet stacking, can allow unburned fuel and combustion byproducts to accumulate in the exhaust system. The problem is not solved simply by choosing the largest available Cummins or Kohler unit. The generator needs an application-specific design that keeps loads within an appropriate operating range while preserving capacity for starting demand.

Plan for the building you expect to have

Commercial electrical needs rarely remain fixed. New HVAC equipment, refrigeration, production machinery, kitchen equipment, electric vehicle infrastructure, tenant improvements, or an expanded workspace can make an initially adequate generator obsolete sooner than expected. Future electrical load increases should be considered during the initial sizing process to avoid replacing the system prematurely. That does not mean buying excessive capacity without a plan. It means documenting realistic expansion assumptions and confirming that the generator, distribution equipment, conductors, and transfer equipment can support the intended growth.

Placement can limit the practical options

Physical site conditions also influence the design. Noise ordinances, property lines, air movement, access for installation, clearances, fuel arrangements. And local site constraints may affect which generator configuration can be used and where it can be placed. A unit that fits the calculated electrical load may not fit the approved location or satisfy the site’s noise requirements. These constraints should be reviewed before the final equipment selection, not after the purchase.

For a closer look at the planning and installation process, review our commercial generator installation services. A qualified electrical contractor can coordinate the load calculation, future-use assumptions, site review, and equipment selection so the finished backup system is neither dangerously undersized nor needlessly oversized.

Code Requirements and Working With a Commercial Electrical Contractor

A commercial generator is not sized in isolation from the building, site, or authority having jurisdiction. Local building and electrical codes should be reviewed before the final equipment size is selected. Those requirements can affect clearances, fuel systems, installation location, emergency circuits, emissions controls, and the permitting package. The U.S. Department of Energy guidance also emphasizes reviewing local building codes before finalizing generator sizing.

Permitting is part of the design, not an administrative step added after the calculations. A qualified commercial electrical contractor can coordinate the load calculation, one-line diagram, equipment specifications. Site constraints, and permit documents so the proposed system is practical to install and inspect. Emergency lighting and communication systems must be included in the total load calculation, alongside the equipment needed for essential business operations. Leaving these circuits out can produce a system that appears adequate on paper but does not support safe egress or critical communication during an outage. The EPA power resilience guide identifies both systems as part of the essential load review.

Match the ATS to the complete electrical system

The automatic transfer switch (ATS) is the component that transfers selected circuits between utility power and generator power. It must be correctly sized to match the capacity of both sources, rather than being chosen only from the generator nameplate. An undersized ATS can constrain the installation, while an incorrectly specified switch can create compatibility and inspection problems. The selected ATS should also align with the distribution equipment, voltage, phase, and the loads designated for backup.

Verify output before permanent service

Before the system enters permanent service, a load bank test is recommended to verify actual generator output under load. This test gives the installation team an opportunity to confirm that the Cummins or Kohler equipment performs as designed and that controls. Transfer equipment, and connected emergency circuits respond as expected. It is a verification step, not a substitute for accurate design calculations. See the commercial electrical services offered by Pro-Tech-Power for a broader view of commercial electrical project support.

Design the space for safe operation

Ventilation must be engineered into the generator installation plan. The enclosure or generator room needs a practical approach to combustion air, heat removal, and exhaust routing. The design must also preserve adequate service access space around the equipment. Crowding a generator into an inaccessible area may complicate inspection, future work, and safe operation. Pro-Tech-Power designs, permits, and installs commercial backup power systems using Cummins and Kohler equipment. Its role is installation, not generator repair or servicing, so the project should establish any ongoing service responsibility separately before work begins.

Contact Pro Tech Power to schedule a commercial generator sizing consultation for your facility.

Frequently Asked Questions

How do you calculate the power requirements for a commercial building generator?

Start by listing the equipment that must operate during an outage, then identify which loads must run at the same time. Calculate both the continuous running load and the temporary starting load for motors such as HVAC and refrigeration equipment. Historical utility usage can help establish a baseline, but the final calculation should prioritize essential equipment and peak demand rather than average consumption. A qualified electrical contractor should also check voltage, power factor, connected kVA, and planned future expansion. EPA guidance recommends conducting a power assessment and identifying essential loads before selecting a generator.

What is the 80% rule in commercial generator sizing?

The 80% rule is a common planning practice that avoids operating a generator continuously at its absolute maximum capacity. Leaving practical capacity headroom can support steadier operation and help protect engine life during extended use. It is not a substitute for a complete load calculation, and it should not be applied mechanically to every project. The design still needs to account for starting current, power factor, voltage, environmental de-rating, and the building’s actual emergency loads. EPA preparedness guidance advises avoiding extended operation at absolute maximum capacity.

Why do motor starting requirements matter when sizing a generator?

Electric motors can require a temporary surge of power when they start, even though their ongoing running load is lower. If the generator cannot handle that inrush current, voltage may dip and sensitive electronics can reset or equipment may fail to start. The sizing review should identify the largest motors, determine their starting method, and consider whether loads will start together or in sequence. This is especially important for commercial HVAC, refrigeration, pumps, and compressors. The generator must support the highest realistic startup event, not only the building’s steady-state wattage. DOE guidance links starting-current capacity with avoiding damaging voltage dips.

How does power factor affect commercial generator sizing?

Power factor compares useful working power, measured in kilowatts, with apparent power, measured in kilovolt-amperes. Because generators are rated in kVA as well as kW, a building with a lower power factor may require more apparent-power capacity to serve the same useful load. The calculation should include the power factor of connected equipment, including non-linear loads such as variable frequency drives. The combined kVA must remain within the generator’s nameplate rating so voltage and frequency regulation are maintained. EPA technical guidance identifies power factor as a generator-capacity consideration.

Schedule a Commercial Generator Sizing Consultation

The right generator size depends on your building’s essential loads, starting demands, electrical characteristics, fuel plan, and future needs. A careful review helps your team plan a backup power system that supports critical operations and fits the site’s requirements. To request a commercial generator sizing consultation, contact Pro Tech Power with details about your building and backup power goals. Our team can help identify the information needed for the next planning step.