Choosing a generator for a commercial building sounds simple until you’re staring at a spec sheet full of kW, kVA, starting amps, fuel curves, and transfer switch options. Go too small and you’ll deal with nuisance shutdowns, voltage dips, and unhappy tenants. Go too big and you’ll pay more up front, burn fuel inefficiently, and potentially shorten the generator’s life through light loading and wet stacking (especially on diesel units).
This guide walks through a practical, step-by-step way to size a generator for a commercial facility—whether you run an office, retail space, restaurant, warehouse, clinic, multi-tenant building, or a venue with lighting and audio needs. It’s not meant to replace a stamped electrical design, but it will help you ask the right questions, gather the right numbers, and avoid the most common sizing mistakes.
Along the way, we’ll talk about load categories (life safety vs. legally required vs. optional), starting surges, power factor, future expansion, and the real-world decisions that affect generator performance. If you’re coordinating with an electrician, facilities manager, or engineer, you’ll be able to speak the same language and make smarter choices faster.
Start with the reason you need a generator
Before you pull out a calculator, get crystal clear on what the generator is supposed to do. “Keep the building running” can mean very different things depending on the business. A medical office might only need critical circuits (refrigeration for meds, a few exam rooms, and IT), while a grocery store might need nearly everything (refrigeration, lighting, POS, security, and sometimes HVAC).
Also think about outage patterns. If your area gets frequent short outages, you may care more about seamless transfer and ride-through than long-duration runtime. If outages are rare but can last days, fuel storage, refueling logistics, and maintenance access become just as important as size.
Finally, be honest about what “success” looks like during an outage. Do you need the building to remain open to customers? Do you only need to protect inventory and data? Do you need to meet code requirements for egress lighting and fire systems? This clarity will guide every step that follows.
Map your loads into three buckets (and don’t skip this)
Commercial generator sizing goes smoother when you categorize loads. Most buildings naturally fall into three buckets: critical/life safety loads, legally required standby loads, and optional/comfort loads. The exact definitions depend on your local code and occupancy type, but the concept is consistent: not everything needs to run on generator power.
Life safety loads typically include emergency lighting, exit signs, fire alarm systems, smoke control (where required), and sometimes elevator recall functions. Legally required loads might include certain ventilation, communications, or systems mandated for the building’s use. Optional loads are everything else—comfort cooling, decorative lighting, non-essential receptacles, and so on.
This bucketing matters because it affects both generator size and the type of transfer equipment you need. Many buildings can significantly reduce generator capacity by backing up only what’s truly essential—and by using load shedding or staged starting for larger motors.
Make a one-page “outage plan” that operations can live with
It’s tempting to size for “everything on,” but that often creates a generator that is oversized for 99% of real outages. Instead, write a simple outage plan: what stays on, what turns off, and what can be turned on later if the generator has headroom.
For example, a restaurant might keep refrigeration, a few kitchen circuits, POS, emergency lighting, and one HVAC unit for the dining area—but leave non-essential kitchen equipment off. A multi-tenant office might keep server rooms, core lighting, security, and one elevator, but not every suite’s plug loads.
Having this plan in writing also helps you avoid last-minute scope creep (“Can we add just one more panel?”) that balloons the generator size and budget.
Decide if you need whole-building backup or selective backup
Whole-building backup is straightforward operationally: everything stays powered (within the generator’s limits). But it often requires a much larger generator and a more complex installation, especially if there are large HVAC loads or multiple distribution points.
Selective backup uses an emergency distribution panel (or multiple panels) and only transfers the circuits you’ve chosen. This approach is usually more cost-effective and can be more reliable because the generator is supporting a defined, manageable load.
If you’re unsure, selective backup is a great default. You can still design it with expansion in mind—leaving space for additional breakers and conduit routes if future needs change.
Gather the right data (and avoid the “nameplate trap”)
Now you need numbers. The best sizing starts with real load data, not guesses. Ideally, you’ll have a list of equipment, their running watts (or amps), voltage, phase, and whether they’re continuous or intermittent. If you can get measured demand from your utility bills or building management system, even better.
The “nameplate trap” is when you add up every piece of equipment at its maximum nameplate rating and assume it all runs at once. In reality, many loads cycle, some never run simultaneously, and many operate below nameplate under typical conditions. Nameplates are still useful, but you should pair them with realistic usage assumptions.
For newer buildings, you may have panel schedules and single-line diagrams. For older buildings, you might need a site walk and a bit of detective work. If you’re missing information, prioritize the big-ticket loads first: HVAC, pumps, compressors, kitchen equipment, elevators, and any IT/server room systems.
Use utility demand (kW) to sanity-check your estimate
Your utility bill often includes a peak demand number (kW). That peak is not the same as what you’ll back up, but it’s a great reality check. If your calculated “critical load” is somehow higher than your historic peak demand, something is off.
Demand data can also reveal seasonal patterns. If your peak is driven by summer cooling, you might choose a generator strategy that supports a reduced HVAC mode during outages rather than full cooling capacity.
When you share this demand info with your electrical team, it helps them validate assumptions and spot outliers quickly.
Don’t forget the “small” systems that matter during outages
Some of the most important loads are not huge in wattage, but they’re mission-critical: network switches, access control, security cameras, fire alarm panels, intercoms, and emergency communications. These are often fed from low-voltage and control systems that can be overlooked during generator planning.
If your building relies on card access, camera systems, or a managed Wi-Fi setup, plan for those circuits intentionally. It’s common to include a dedicated UPS for ride-through and then have the generator support the UPS for longer outages.
In many projects, coordination with a low voltage electrician helps ensure these systems are properly identified, powered, and tested under generator conditions—especially if you’re integrating fire alarm interfaces, door hardware, or network closets.
Calculate running load: kW, kVA, and power factor in plain language
Generators are often rated in kW (real power), but they also have a kVA (apparent power) rating. The difference matters because many commercial loads—especially motors and certain electronic equipment—have a power factor less than 1.0. Lower power factor means the generator must supply more current for the same real power.
A simple way to think about it: kW is the work you actually get, while kVA is what the generator has to deliver to make that work happen. Many generators are designed around a typical power factor (often 0.8), but your building’s mix of loads can push you closer to the limits if you ignore kVA.
If you’re doing a back-of-napkin calculation, you can estimate kVA by dividing kW by power factor. Example: 80 kW at 0.8 PF ≈ 100 kVA. For a real project, your electrician or engineer will model this more precisely, but understanding the concept helps you avoid under-sizing.
Single-phase vs. three-phase: why it changes the math
Most larger commercial buildings use three-phase power, and many big loads (HVAC, pumps, elevators) are three-phase. The current calculation differs between single-phase and three-phase systems, and that affects conductor sizing, transfer switch sizing, and generator selection.
Three-phase systems typically run more efficiently for large motors and distribute load more evenly. But they also require you to think about phase balancing—especially if you’re backing up a mix of single-phase loads across multiple panels.
If you’re not sure what you have, check your main service equipment or utility meter labeling. Your electrical contractor can confirm voltage and phase quickly during a site visit.
Continuous vs. non-continuous loads: the 80% mindset
Generators perform best when they run in a healthy load range—often around 30% to 80% of rated capacity, depending on the unit type and manufacturer guidelines. If you size a generator so that your normal outage load sits at 10% to 15%, you may run into efficiency and maintenance issues.
On the other hand, sizing so tightly that you’re constantly at 95% to 100% leaves no margin for starting surges, temperature derating, or future expansion. A practical approach is to size so that your expected running load lands in a comfortable middle zone, with headroom for starts and growth.
This is also where selective backup and load management shine: you can keep the generator in its sweet spot without sacrificing critical operations.
Account for starting surges (the step that breaks most DIY sizing)
Running watts are only half the story. Many commercial loads draw a much higher current when they start. Motors, compressors, and some HVAC equipment can have inrush currents several times their running current. If the generator can’t handle that momentary surge, you’ll see voltage dips, frequency droop, flickering lights, control faults, or outright shutdown.
Starting characteristics depend on the equipment type and starting method. Across-the-line motor starts are the most demanding. Soft starters and VFDs (variable frequency drives) can reduce inrush significantly, but they introduce other considerations like harmonics and sometimes a different relationship between kW and kVA.
The goal is not to size the generator for the sum of all possible inrush events at once. Instead, you plan the sequence: what starts first, what can wait, and what can be shed if needed.
Identify the “top three” hardest-starting loads
In many buildings, just a few pieces of equipment drive generator sizing: a large rooftop unit, a fire pump, an elevator motor, a big air compressor, or a walk-in cooler compressor bank. Find these early.
Ask for motor horsepower (HP), locked-rotor amps (LRA), and starting method. If you can’t get LRA, a qualified electrician can often estimate based on motor type and size, but manufacturer data is best.
Once you know the worst offenders, you can decide whether to change the starting method (soft start/VFD), stagger starts, or accept a larger generator.
Staggered starting and load shedding: cheaper than oversizing
Staggered starting means you deliberately sequence loads so they don’t all kick on at once during transfer. Many automatic transfer switches and generator controllers can be configured to start certain loads after a delay.
Load shedding takes it further: if the generator is near capacity, it temporarily drops lower-priority loads to keep critical loads stable. For example, you might shed one HVAC unit if a second one starts, then bring it back later.
These strategies often allow a smaller generator to perform reliably, and they can reduce fuel consumption during long outages.
Choose your generator type and rating based on how it will be used
Not all generator ratings are the same. Manufacturers may list standby, prime, and continuous ratings. Standby is common for emergency backup—shorter duration, variable load. Prime is for longer, more regular use. Continuous is for constant, steady operation.
For most commercial buildings seeking backup power, the standby rating is the relevant one, but you still need to consider expected runtime and whether your outages could require extended operation. If you anticipate multi-day outages, you’ll care about fuel consumption curves, maintenance intervals, and the practicality of refueling.
Fuel type also matters. Diesel units are common for higher capacities and can be robust, but they can dislike very light loads. Natural gas units avoid on-site fuel storage but depend on gas supply reliability and may derate more in hot conditions or at elevation.
Diesel vs. natural gas: what changes for sizing
Diesel generators often handle step loads well and are widely available in commercial sizes. However, they need proper load to avoid wet stacking, and you’ll need a plan for fuel storage, fuel polishing, and compliance with local regulations.
Natural gas generators can be convenient because the fuel supply is “always there,” but you should confirm gas line capacity and pressure. In some areas, gas utilities may curtail service during extreme events, so it’s worth discussing risk tolerance and any code requirements for on-site fuel.
Either way, your generator should be sized with derating in mind—ambient temperature, altitude, and enclosure airflow can all reduce available capacity.
Portable, towable, or permanently installed: match the operational reality
Permanent standby generators paired with an automatic transfer switch offer the smoothest experience: the generator starts automatically and transfers within seconds. That’s usually the right answer for buildings with life safety requirements or where downtime is expensive.
Towable or portable generators can work for certain facilities, especially if outages are rare and you have staff who can deploy equipment safely. But they require manual connection, safe cable routing, and clear procedures—plus you may still need a transfer switch or interlock system.
Be realistic about who will handle the generator at 2 a.m. during a storm. If the honest answer is “no one,” automatic standby is worth the investment.
Step-by-step sizing workflow you can actually follow
Let’s turn the concepts into a repeatable process. This is the workflow many electrical teams follow, simplified into steps you can manage as a building owner or facilities lead.
As you go, document everything in a spreadsheet: load name, quantity, voltage/phase, running amps or watts, starting method, priority level, and whether it must run during an outage.
Step 1: List the circuits and equipment you want backed up
Start with your outage plan and turn it into a list of panels/circuits. If you have panel schedules, great—use them. If not, do a walkthrough and identify what each panel feeds.
Keep the list practical. “All receptacles” is not a load—it’s a category. Specify “server room receptacles,” “POS receptacles,” “security system power supply,” and so on.
This is also where you decide what will be on UPS versus generator. UPS can cover short interruptions and keep sensitive electronics stable while the generator starts.
Step 2: Calculate total running kW (and estimate kVA)
Convert the running loads into kW. If you have amps, voltage, and phase, you can compute kW with standard formulas (your electrician will do this precisely). If you have watts already, sum them and convert to kW.
Then estimate kVA using a reasonable power factor assumption if you don’t have exact PF data. Many mixed commercial loads land around 0.85 to 0.95, but motor-heavy loads can pull lower.
The output of this step is your “steady-state” generator demand—what the generator must supply once everything is running.
Step 3: Add starting kVA requirements for motors and compressors
For each motor load, identify the starting method and estimate starting kVA. The key is to model the worst-case start scenario based on your planned sequence.
If you can stagger starts, you don’t need to add every inrush at once. Instead, identify the largest single step load and any overlapping starts that could happen during normal operation (for example, two compressors cycling on close together).
This step often changes the generator size more than the running load does, especially in HVAC-heavy buildings.
Step 4: Apply derating and add a realistic growth margin
Now account for environmental derating (hot weather, altitude, enclosure restrictions) and installation-specific factors. Manufacturers provide derating charts; your contractor can apply them based on site conditions.
Then add growth margin—but keep it realistic. A common approach is 10% to 25% depending on how likely expansion is. If you plan to add a tenant, new equipment, or EV chargers, plan for that explicitly rather than adding a vague oversized buffer.
At the end of this step, you should have a target generator rating range rather than a single exact number.
Step 5: Confirm transfer switch sizing and distribution strategy
The generator is only part of the system. The transfer switch must be sized for the loads it will carry, and the distribution must be designed so that critical loads are fed reliably.
Sometimes the best solution is multiple transfer switches: one for life safety loads, one for legally required loads, and one for optional loads. This can make testing and maintenance easier and can reduce the chance of non-essential loads interfering with critical ones.
This is also where load shedding controls, breaker coordination, and phase balancing get engineered properly.
Code, permits, and why life safety changes everything
Commercial generator projects live at the intersection of electrical code, building code, fire code, and sometimes healthcare or occupancy-specific standards. If your generator supports life safety systems, the rules get stricter: separation of wiring, dedicated transfer equipment, testing requirements, and sometimes specific runtime requirements.
Even when you’re not strictly “life safety,” insurers and AHJs (Authorities Having Jurisdiction) often have expectations about installation quality, signage, clearances, and maintenance documentation. Planning for this early avoids painful redesigns later.
Permitting also affects timeline. Utility coordination (for natural gas or electrical service changes), pad and enclosure requirements, sound attenuation, and emissions rules can all shape what generator you can install and where it can go.
Emergency systems testing is part of sizing decisions
Generators aren’t “set and forget.” Most commercial standby systems require periodic testing under load. If the generator is oversized and never sees meaningful load, testing becomes less effective and can even create maintenance problems.
Some facilities use load banks to test generators properly. If you anticipate needing load bank testing, consider space and connection points in the design. This is another reason not to oversize unnecessarily.
A well-sized generator is easier to test, easier to maintain, and more likely to perform the way you expect during a real outage.
Noise, location, and neighbors: practical constraints that affect capacity
Generator placement isn’t just about convenience. Sound ordinances, exhaust routing, clearances for service access, and distance to fuel supply or electrical gear all matter. Longer conduit runs and voltage drop can influence design choices, particularly for larger units.
If you’re in a dense commercial area, you may need sound attenuation or a specific enclosure type. Those choices can affect cooling airflow and, in turn, derating.
It’s worth doing a site walk early with your electrical contractor and (if needed) a generator vendor to confirm feasibility before you commit to a size.
Special load types that deserve extra attention
Some loads behave differently on generator power than they do on utility power. They may be sensitive to frequency variations, harmonic distortion, or transfer timing. Identifying these early can prevent frustrating commissioning issues.
If your building includes any of the following, treat them as “special” and confirm compatibility: elevators, fire pumps, large VFD-driven HVAC, medical imaging equipment, data centers, stage lighting dimmers, and high-power audio systems.
Elevators: talk to your elevator contractor early
Elevator backup can be as simple as providing power for one elevator car for emergency use, or as complex as supporting normal elevator operations. The elevator controller may have specific requirements for generator power quality and may need a soft-start approach.
Some buildings choose to power only the elevator needed for accessibility or emergency response. Others use an elevator management strategy that limits operation to one car at a time during generator mode.
Because elevator loads can be spiky, they can influence generator sizing disproportionately. Early coordination helps you avoid oversizing “just in case.”
Fire pumps and smoke control: when the load is non-negotiable
If you have a fire pump, it’s often the single most important and demanding load in the building. Fire pump starting can create a large step load, and codes can dictate how the generator and transfer equipment must be arranged.
Smoke control systems (where required) can also be significant. They may involve large fans, dampers, and control systems that must operate reliably during emergencies.
In these cases, generator sizing is not just an operational decision—it’s a compliance and life safety decision. Your design team will likely need to coordinate closely with the AHJ.
IT rooms and sensitive electronics: pair generator with UPS thoughtfully
Generators typically take several seconds to start and stabilize. That’s fine for many loads, but not for servers, network gear, and certain security systems. A UPS bridges that gap and also cleans up power quality.
When sizing the generator, remember you’re often powering the UPS input, not the IT load directly. UPS systems can have inrush and battery recharge demands that affect generator loading after an outage.
Plan for the “recharge wave” when power returns—if every UPS starts charging at once, it can create a significant load step. Staged recharge settings can help.
Right-sizing in the real world: common scenarios and how to think about them
Let’s make this concrete. The same step-by-step method applies, but the priorities change depending on the building type. These examples aren’t exact prescriptions—think of them as patterns you can adapt.
In each case, the best results come from combining smart load selection, realistic usage assumptions, and a plan for motor starting and sequencing.
Office buildings: focus on egress, security, and one “business continuity” zone
Many offices don’t need every floor fully powered during an outage. A common strategy is to back up emergency lighting, life safety systems, security/access control, and a designated continuity zone (like one suite or one floor) with outlets and network access.
HVAC can be limited to one unit serving the continuity zone, or even skipped entirely if the goal is safe shutdown rather than staying open. This can dramatically reduce generator size.
Because offices often have a lot of plug loads, be careful about assuming all receptacles are used at full capacity. Real demand is usually far lower than the theoretical maximum.
Retail and restaurants: refrigeration and cooking equipment change the math
For retail, the must-haves are typically lighting (at least partial), POS systems, security, and sometimes HVAC depending on whether you plan to remain open. For restaurants, refrigeration is often the top priority to protect inventory.
Kitchen equipment can be tricky. Electric ovens, fryers, and large hood systems can push loads up quickly. A practical approach is to choose a limited menu during outages and power only the equipment needed for that menu.
Refrigeration compressors can have high starting surges, so sequencing and soft-start options can be valuable in keeping generator size reasonable.
Warehouses and light industrial: motors, compressors, and process loads dominate
Warehouses may only need lighting, dock equipment, security, and IT—unless there are process loads like compressors, conveyors, or refrigeration. Light industrial spaces can swing from modest to very demanding depending on the equipment mix.
If you have large motors, spend extra time on starting method and whether you can stagger starts. In some facilities, the generator is sized primarily around the biggest motor start plus essential running loads.
For these projects, it’s common to work with specialists such as industrial electrical contractors St. Louis who are used to motor control, VFD integration, and the kind of commissioning that ensures everything behaves correctly on generator power.
Work with the right electrical partner (and what to ask them)
Even if you’re comfortable doing preliminary calculations, a commercial generator project should be validated by a qualified electrical professional. They’ll confirm code requirements, perform detailed load studies, coordinate with the utility, and ensure the system is safe and maintainable.
When you’re interviewing contractors, ask how they approach load calculations, whether they model motor starting, and how they handle commissioning and testing. The quality difference between “install a generator” and “deliver reliable standby power” is huge.
If your building is in Missouri and you’re coordinating local expertise, working with a commercial electrician St. Louis who regularly designs and installs standby systems can help you navigate everything from transfer switching to load management and final testing.
Questions that reveal whether a contractor is thinking beyond the box
Ask what happens during the first 30 seconds after an outage. A strong contractor will talk about transfer timing, motor starting sequence, voltage/frequency stabilization, and which loads are delayed.
Ask how they plan to test the system. Will they perform a building load test? Will they recommend a load bank? How will they verify that life safety circuits behave correctly?
Ask how they design for maintenance. Can the generator be serviced without shutting down critical operations? Is there bypass/isolation for the transfer switch if needed?
Commissioning and documentation: the stuff you’ll be grateful for later
Commissioning is where the design meets reality. It should include verifying load transfer, confirming sequences, testing alarms and monitoring, and ensuring the generator remains stable under expected load changes.
Documentation matters too: updated one-lines, panel schedules, labeled circuits, and a written operating procedure for outages. When staff changes, good documentation keeps the system usable and safe.
If you have multiple tenants, clear documentation prevents accidental overloads (“we plugged a space heater into the emergency circuit”) and makes it easier to enforce what belongs on generator power.
Common sizing mistakes (and how to avoid them)
Most generator sizing problems come from a few predictable mistakes. The good news is they’re easy to avoid once you know what to watch for.
Use this section as a checklist before you finalize a purchase or approve a design.
Mistake: sizing for everything without a load plan
If you don’t define what must run, the default becomes “back up the whole building,” and the generator size (and cost) balloons. Worse, you may end up with a generator that runs lightly loaded most of the time, which can create maintenance headaches.
Fix: create an outage plan and prioritize circuits. Use selective backup, staged starting, and load shedding where appropriate.
Bonus: your building operations team will thank you because they’ll know exactly what to expect during an outage.
Mistake: ignoring motor starting and step loads
A generator that can handle the running load may still fail during the first minute of operation if a large motor starts abruptly. This is one of the most common reasons people say, “The generator is big enough, but it still trips.”
Fix: identify hard-start loads early, model starting kVA, and sequence starts. Consider soft starters or VFD strategies where they make sense.
Also confirm voltage dip tolerance for sensitive equipment—some devices will fault even if the generator technically stays online.
Mistake: forgetting derating, fuel logistics, and future growth
A generator that looks perfect on paper can be underpowered on a hot day if derating isn’t considered. Or it can be operationally impractical if fuel deliveries can’t reach the site during a prolonged outage.
Fix: apply manufacturer derating, design for realistic runtime, and add a sensible growth margin based on actual plans (not vague “maybe someday” expansion).
If growth is likely, consider designing the distribution and transfer equipment for expansion even if you don’t buy the larger generator today.
A quick example workflow (so you can picture the process)
Imagine a mid-sized commercial building that wants to keep essential operations running: emergency lighting, fire alarm, security, network closet, one elevator for emergency use, refrigeration for a small café, and one HVAC unit for a designated continuity area.
Step one is listing those loads and confirming where they’re fed from. Step two is calculating running kW and kVA for each. Step three is identifying the biggest starting surge—maybe the HVAC compressor or elevator motor—and deciding whether to delay one until the other is stable.
Then you apply derating for summer temperatures and add a growth margin for a future tenant improvement. The final generator size might land in a range (say, 80–100 kW) rather than a single number, and the team chooses the model that best handles step loads and expected runtime.
The real win is that the building gets reliable performance without paying for unnecessary capacity—and the system is designed so it can be tested and maintained without drama.
Final checklist before you commit to a generator size
Use this checklist as a final pass. If you can confidently answer each item, you’re in a strong position to approve a generator size and move into design and permitting.
Load planning: Have you defined exactly which panels/circuits are backed up and which are not? Do you have an outage plan that operations agrees with?
Numbers: Do you have realistic running kW estimates (ideally supported by measured demand)? Have you accounted for power factor and kVA?
Starting: Have you identified the largest motor starts and planned sequencing or soft-start strategies? Have you considered UPS recharge loads?
System design: Is the transfer switch strategy appropriate (single vs. multiple switches, bypass/isolation if needed)? Is phase balancing considered?
Real-world constraints: Have you considered derating, noise, placement, exhaust routing, fuel supply, and maintenance access?
Testing: Do you have a plan for acceptance testing and ongoing periodic testing under load?
If you treat generator sizing as a system design problem—not just a single number—you’ll end up with standby power that behaves predictably when it matters most.
