How to Size a Generator for a Commercial Building

A commercial building sits in an odd middle ground. It’s far more complex than sizing a single home, where one household makes every decision about what stays powered – a building has tenants, shared services, and building management pulling in different directions. But it’s also not bound by the strict, codified life-safety branch requirements that govern a hospital, where load categorization is dictated by clinical standards rather than business judgment. A commercial building’s sizing exercise is an engineered process, but the engineering serves business priorities that the building owner and facility manager largely get to define themselves – which is exactly why it needs its own methodology, distinct from both its neighbors in this series.

The clearest way to think about that methodology is the one every office worker already understands intuitively: an org chart. Every building has a hierarchy of who – or what – matters most when the power goes out, and sizing a generator correctly starts with drawing that hierarchy honestly, not with picking a kVA number off a rule of thumb.

The C-Suite: Life-Safety and Legally Mandated Loads

At the top of any building’s power hierarchy sits the equivalent of the C-suite – the loads that aren’t really optional, either because life safety depends on them or because local fire and building codes mandate emergency backup regardless of what the building owner would prefer to spend.

This tier typically includes:

• Fire alarm and detection systems
• Emergency lighting and illuminated exit signage
• Fire pumps, where the building’s fire-fighting system depends on powered pumps
• At least one elevator, in many jurisdictions, for emergency evacuation and accessibility compliance
• Basic security systems (CCTV, access control) at building entry points

These loads are usually a relatively small percentage of the building’s total connected load, but they’re non-negotiable in the org-chart sense – the “board” doesn’t get to vote this section down to save money, because it’s frequently a legal requirement rather than a business choice.

The Department Heads: Business-Critical Loads

One level down sit the loads that keep the actual business of the building running, even if a brief interruption wouldn’t literally endanger anyone. This is where building owners and tenants have real discretion, and where most of the sizing conversation actually happens.

• The IT floor / server room – often the single largest concentrated load in a modern office building, and frequently the one department head who insists on a dedicated, higher-priority backup path, sometimes with its own UPS bridging the gap before the generator picks up load
• Remaining elevators, beyond the one covered under life-safety, for basic building function rather than pure emergency use
• Building management systems (BMS) – HVAC controls, access control backbone, and monitoring systems that the building can technically run without briefly, but which create real operational problems if left dark for hours
• Security operations beyond basic entry monitoring – for buildings with a manned security desk, keeping that function powered is usually treated as business-critical rather than discretionary

Sizing at this tier requires an honest connected-load count per department, and this is also where our explainer on how to read a generator’s kVA and kW rating becomes useful – different vendors present capacity figures with different assumptions, and comparing quotes accurately requires understanding exactly what number you’re comparing.

Middle Management: Comfort and Productivity Loads

Below business-critical sits everything that makes a building pleasant and productive to work in but wouldn’t stop the business from technically functioning without it for a few hours: general HVAC across office floors, standard lighting circuits, workstation power, pantry equipment, and similar loads.

This is usually the largest connected load category by raw wattage in an office building, and it’s also where the demand-factor math (below) does the most work, because comfort loads are rarely running at their full rated capacity simultaneously across every floor at once – occupancy varies by time of day, HVAC cycles on and off by zone, and not every workstation draws its maximum load at the same moment.

The Interns: Discretionary Loads You Can Let Go Dark

At the bottom of the org chart sit the loads that simply don’t need to survive a power event: decorative facade lighting, non-essential signage, vending machines, and similar loads that add to connected load totals without adding to what actually needs backing up.

Deciding what belongs here – and being disciplined about actually excluding it from your backup calculation – is one of the highest-leverage decisions in the entire sizing exercise, because every kVA you don’t need to back up is a kVA you don’t need to buy, install, fuel, and maintain for the life of the building.

The Org Chart, Tabulated

Before moving to the math, it helps to see all four tiers side by side, with typical examples and how each tier is usually treated once an outage hits.

Org-Chart Tier	Typical Loads	Backup Priority	Who Decides
C-Suite (life-safety)	Fire alarm, emergency lighting, exit signage, fire pumps, one elevator	Always backed up	Code and fire authority, not optional
Department Heads (business-critical)	Server room/IT floor, BMS, security desk, remaining elevators	Backed up in almost all cases	Building owner, based on tenant needs
Middle Management (comfort)	General HVAC, standard lighting, workstation power, pantry loads	Backed up based on budget and tenant expectations	Building owner, balancing cost vs. comfort
Interns (discretionary)	Facade lighting, decorative signage, vending machines	Usually excluded from backup	Building owner, for cost efficiency

This table is also a useful tool when you’re explaining the sizing decision to tenants or ownership – it makes clear that “why isn’t the facade lit during an outage” is a deliberate cost decision, not an oversight.

Multi-Tenant Considerations: Shared Load, Separate Priorities

A single-tenant building has one decision-maker for where each load sits on the org chart. A multi-tenant commercial building rarely does, and this is where sizing gets genuinely more complicated than the underlying math suggests.

Different tenants often have different priorities for what counts as “business-critical” for their own operations – a law firm’s server room and a retail tenant’s point-of-sale system are both business-critical to their respective occupants, but neither is a legal life-safety requirement for the building as a whole. Building management typically handles this by offering tiers of backup service: a base level included in common-area maintenance charges (covering the C-suite and shared department-head tier loads), with additional backed-up capacity available to individual tenants at extra cost if they want their own space’s comfort or business-critical loads covered beyond the shared baseline.

This also affects how the generator’s capacity gets billed and sub-metered, and it’s worth deciding this commercial structure before finalizing the kVA figure, not after – a building that later wants to offer more tenants an opt-in backup tier needs headroom built into the original sizing decision, since retrofitting additional generator capacity into an occupied building is considerably more disruptive than sizing generously (within reason) from the start.

Calculating Headcount: From Connected Load to Real Demand

Here’s where the org-chart metaphor earns its keep. A company’s total headcount on paper is never the number of people in the building at any given moment – people are on leave, working from other locations, or simply not all in their seats simultaneously. Electrical demand works the same way, and this is the single most misunderstood part of commercial generator sizing.

Connected load is the sum of the nameplate rating of every piece of equipment across all four tiers above – the equivalent of total headcount on the payroll. Almost nobody should size a generator to this number; doing so produces a generator that’s dramatically, expensively oversized for what the building will ever actually draw.

Demand factor scales connected load down to a realistic maximum demand, because not everything runs at its full rated capacity at the same time. Diversity factor goes a step further, accounting for the fact that peak demand in different parts of the building (different floors, different tenant spaces, different times of day) doesn’t happen simultaneously either. Electrical Engineering Portal’s explanation of demand factor, diversity factor, utilization factor, and load factor is a solid technical reference for the underlying formulas, and ECMag’s explainer on diversity vs. demand factor is useful for clarifying a distinction that even experienced facility managers sometimes blur.

Applying both factors in sequence – connected load, then demand factor, then diversity factor – is what turns an intimidating raw wattage total into a realistic generator sizing target. Our explainer on understanding generator load factor and duty cycle covers the related concept of how consistently the generator will run once sized, which matters for both fuel planning and engine longevity.

Redundancy: Does the Building Need a Deputy?

Some buildings need more than a correctly sized single generator – they need a deputy who can step in if the primary unit is down for maintenance or fails outright. This is a business decision as much as a technical one: a building with a data center tenant, a hospital-adjacent clinic, or any tenant whose business genuinely cannot tolerate an unplanned gap in backup power may justify an N+1 configuration (one additional unit beyond calculated demand). A standard office building without such tenants often doesn’t need this redundancy and can accept a well-maintained single unit with a solid AMC in its place.

A Worked Example: A Six-Floor Office Building

Consider a six-floor commercial office building: ground floor lobby and retail, floors two through five as leased office space, and floor six as a server room and building management suite.

A connected-load audit across all floors totals roughly 950 kVA, once every HVAC unit, lighting circuit, workstation load, elevator, and the server floor’s IT equipment are added up individually. Applying a demand factor (accounting for the fact that not every floor runs its HVAC and lighting at full rated capacity simultaneously, and that the building rarely reaches 100% occupancy across every leased floor at once) brings this down to roughly 620 kVA of realistic maximum demand.

From there, the building’s facility team separates life-safety and business-critical loads (fire systems, one elevator, the server floor, BMS) – roughly 280 kVA of that 620 kVA total – as the absolute floor that must be backed up regardless of budget, with the remaining comfort-tier load (general HVAC and lighting across leased floors) evaluated separately for how much of it the building wants to back up during an extended outage versus accepting some discomfort for tenants.

The final decision in this example: back up the full 620 kVA of realistic demand (rather than only the 280 kVA life-safety-and-critical floor) because tenant satisfaction and lease retention depend on maintaining reasonably normal working conditions during outages, not just technical survival. Add a standard sizing margin of roughly 15-20% for future tenant fit-outs and equipment additions, and the building lands on a requirement in the 720-750 kVA range – likely specified with standby duty rating rather than prime, since even a building with frequent short outages rarely needs continuous-duty engineering the way a factory running multiple daily shifts does.

Avoiding an Overstaffed Power Budget

Oversizing is the most common – and most expensive – mistake in commercial building sizing, and it usually comes from skipping the demand and diversity factor steps entirely and simply summing connected load. Beyond the upfront capital cost, a generator that’s chronically oversized for the load it actually serves runs at low relative load most of the time, which increases the risk of incomplete combustion and carbon deposit buildup in the engine over time (commonly called wet-stacking) – a real mechanical problem, not just an inefficiency. Reducing fuel burn and running the unit closer to an efficient load band also has a direct cost benefit; our tips to reduce diesel generator fuel consumption guide covers practical steps once your sizing is locked in.

Choosing the Right Management Structure: Phase, Enclosure, and Location

Once the kVA figure is set, a few structural decisions remain. Most commercial buildings of any real size run a three-phase electrical supply, so the generator needs to match – see our single-phase vs. three-phase generators guide if you’re unsure which your building has, though at this scale it’s almost always three-phase.

Enclosure and placement matter enormously for a commercial building, both for acoustic comfort (tenants and neighbors won’t tolerate an open-frame unit at close range) and for space planning – our comparison of open-frame vs. enclosed canopy generators is worth reviewing before finalizing rooftop or basement plant-room placement. If the building sits in Delhi NCR or another jurisdiction with specific DG-set operating restrictions, review our guide to GRAP rules for DG sets in Delhi NCR as part of your compliance planning, since operating restrictions during high-pollution periods can affect how much you can rely on backup generation as a primary fallback in those windows.

Statutory approvals also apply at this scale – our guide to factory license requirements for DG set installation covers the documentation trail relevant to larger commercial installations, not just industrial ones, since many municipal approval processes don’t distinguish sharply between the two once you’re above a certain capacity threshold.

Fuel storage and day-tank sizing also need to be planned against the same demand figure – a building sized for 720-750 kVA needs a fuel storage and refueling plan that supports its expected runtime during an extended outage, not just the tank size the vendor happens to offer as standard. This is worth confirming explicitly during the vendor evaluation stage, since undersized fuel storage is a less visible sizing mistake than an undersized generator but has the same practical effect during a genuinely long outage.

Reviewing the Org Chart Annually

An org chart goes stale the moment a company reorganizes, and a building’s load profile goes stale just as fast. A tenant fit-out that adds a dense server rack, a floor converted from general office to a call center running dozens of workstations around the clock, or a new ground-floor tenant with heavy kitchen equipment can each shift the building’s real demand meaningfully away from the figure the original generator was sized against.

Facility teams that treat the sizing calculation as a one-time exercise at construction, rather than a living document revisited at each major lease change, are the ones most likely to discover – usually during an actual outage – that their backup capacity no longer matches their real building. Building a simple annual review into your facility management calendar, cross-checking the current tenant and equipment mix against the original demand-factor assumptions, catches this drift well before it becomes an emergency.

Frequently Asked Questions

Should a commercial building size for 100% of connected load to be safe? No – this is the most expensive and least necessary approach. Properly applying demand and diversity factors, as described above, produces a realistic sizing target that’s both cheaper and more appropriate to how the building actually draws power, without meaningfully compromising reliability.

Does every commercial building need N+1 redundancy? No. Redundancy needs depend on the tenant mix and the cost of an unplanned outage to those tenants. A standard office building can often operate safely on a single well-sized, well-maintained unit; buildings with data-center, healthcare, or similarly outage-sensitive tenants often justify the additional cost.

How often should the load calculation be revisited after the generator is installed? Whenever the building’s tenant mix or floor usage changes meaningfully – a new large tenant with heavy IT load, a floor converted to a different use, or a significant renovation – rather than only when the original generator reaches end of life.

Is a commercial generator sized differently from an industrial one of similar capacity? The underlying math (connected load, demand factor, diversity factor) is similar, but the load categories and duty expectations differ meaningfully – an industrial facility often has heavier motor-starting loads and may need prime-rated, continuous-duty engineering, while a commercial building’s load profile is generally steadier and lighter relative to its size.

Who typically pays for backup power in a multi-tenant building – is it part of maintenance charges? This varies by building, but a common structure bills a base level of backup (life-safety and shared department-head loads) through common-area maintenance charges to all tenants, with any additional tenant-specific backup capacity offered as a separate, opt-in charge. Deciding this commercial model early makes the sizing conversation with ownership considerably more straightforward.

What happens if the building’s actual demand grows beyond what the generator was sized for? Running a generator meaningfully beyond its rated capacity risks nuisance trips, accelerated wear, and in the worst case an inability to start reliably under load. If a periodic review (see above) shows demand approaching the original sizing ceiling, the right response is planning a capacity upgrade or a second parallel unit before demand actually exceeds it, not after.

Where to Go From Here

Once you know your building’s size requirement, our diesel generator buying guide for large industries covers the procurement process for this capacity range in more depth, since a 700+ kVA purchase typically involves the same multi-stakeholder evaluation as an industrial buy. For a smaller standalone commercial space – a single shop or clinic rather than a multi-tenant building – our diesel generator buying guide for small businesses is the more directly relevant next read. And once the generator is installed, our monthly maintenance checklist for diesel generators is worth building into your facility management schedule from day one.

If you’d like help auditing your building’s actual connected load or reviewing a vendor’s sizing proposal before you commit, reach out to our team – a second set of eyes on the demand-factor math is cheap insurance against a very expensive sizing mistake, and it’s considerably cheaper to catch an oversizing or undersizing error on paper than to discover it during a live outage with tenants on-site.