How to Size a Commercial Generator: The Complete Buyer’s Guide

Buying the wrong size commercial generator is one of the most expensive mistakes a business can make in backup power.

Too small and the generator trips under real load, leaving your facility dark when you need it most. Too large and you have spent significantly more than necessary, and you risk wet stacking from running a diesel engine consistently below 30% of its rated capacity. Either way, the consequences land on your operations, your budget, and your credibility with the people who depend on the power staying on.

Correct sizing is not complicated, but it requires working through a specific set of questions in the right order. This guide gives you that process, step by step, so you can specify a commercial generator with confidence or walk into a supplier conversation knowing exactly what questions to ask.

Why Commercial Generator Sizing Is Different From Residential

Residential generator sizing is relatively simple. You identify which circuits you want to power, add up the loads, apply a safety margin, and you have your number.

Commercial generator sizing involves the same logic but several additional layers of complexity:

Motor Starting Loads

Commercial facilities almost always have motor-driven equipment: HVAC compressors, pumps, elevators, air compressors, manufacturing machinery. When an electric motor starts, it draws three to six times its running load for a fraction of a second. Your generator has to handle that surge without faulting or causing voltage to sag to the point where other equipment drops out.

Load Diversity and Demand Factor

Not every piece of equipment in a commercial building runs at the same time. A demand factor accounts for this. In a large office building, lighting and HVAC may run simultaneously, but not every outlet is fully loaded. Applying an appropriate demand factor prevents unnecessary oversizing.

Harmonic Loads

Modern commercial facilities have significant numbers of variable frequency drives (VFDs), LED drivers, UPS systems, and other non-linear loads that introduce harmonic distortion. Some generators handle this better than others, and oversizing may be required to maintain voltage quality with high harmonic content.

Future Load Growth

A generator sized exactly to today’s load has no headroom for expansion. Many commercial buyers size for current load plus 20 to 25% to accommodate growth without replacing the generator within a few years.

Step 1: Define What You Are Powering

Before you can size a generator, you need to know exactly what it is going to power. This sounds obvious, but many buyers skip this step and ask for a generator recommendation without being able to answer basic questions about their load.

Start by dividing your loads into two categories:

Essential loads are the circuits and equipment that must remain operational during an outage to protect people, property, or the ability to continue operations. These include life safety systems, refrigeration, servers, security systems, communications, and any production equipment that cannot safely shut down mid-process.

Non-essential loads are everything else. These are circuits that can be dropped during an outage without significant consequence: general lighting, non-critical outlets, decorative systems, supplementary HVAC zones.

Your generator should be sized to carry essential loads as a minimum. Whether it carries non-essential loads as well is a budget and priority decision.

Step 2: Calculate Your Running Load

With your essential load list in hand, the next step is calculating the total running load in kilowatts (kW).

For each piece of equipment, find the nameplate or specification sheet and note the following:

– Rated power in watts or kilowatts (convert horsepower to kW: 1 HP = 0.746 kW)

– Voltage and phase (single phase 120V, single phase 240V, or three phase 208/240/480V)

– Power factor for motor loads (typically 0.8 for commercial motors)

Add up all the running loads. This gives you your total connected load in kW. Apply your demand factor based on the realistic simultaneous usage. For most commercial facilities, a demand factor of 0.7 to 0.9 is appropriate, depending on the mix of loads.

The result is your estimated running load in kW.

Step 3: Account for Motor Starting Loads

This is the step most buyers miss, and it is the one most likely to cause problems.

For every electric motor in your essential load list, you need to calculate the starting kVA, not just the running load. Motors draw a large surge of current at startup, typically expressed as a multiplier of the running current:

– Small motors (under 5 HP): starting current is typically 3x running current

– Medium motors (5 to 50 HP): starting current is typically 4x to 5x running current

– Large motors (above 50 HP): starting current can be 6x or more, depending on the starting method

The generator needs to handle the largest single motor start without voltage dropping more than 15% (for most commercial applications) or the unit faulting out. If you have multiple large motors that could start simultaneously, or if the largest motor is a significant fraction of total load, motor starting becomes your sizing constraint.

Soft starters and variable frequency drives (VFDs) reduce starting current significantly and can allow a smaller generator to handle larger motors. If your equipment uses them, your sizing calculation changes accordingly.

Step 4: Apply a Safety Margin

Once you have your running load and you have accounted for motor starting, add a safety margin of 20 to 25%.

This margin serves three purposes:

– It provides headroom for load growth without requiring a generator replacement

– It keeps the generator running at 75 to 80% of rated capacity under full load, which is the optimal operating range for diesel engines

– It reduces the risk of overload during unexpected load additions

A generator running consistently at 75 to 80% of its rated load runs cleaner, runs cooler, and lasts longer than one running at 95% or above.

Step 5: Choose Your Configuration

With your kW requirement established, the remaining specification decisions are:

Fuel Type

Diesel is the most common choice for commercial standby generators above 20 kW. It is energy-dense, widely available, and stores well in on-site tanks. Natural gas and LP (propane) are alternatives where a reliable gas supply is available or where diesel fuel storage is constrained by space or permit requirements. Natural gas eliminates the need to manage fuel inventory but introduces dependency on the utility gas supply, which may also be disrupted during a major outage.

Standby vs Prime Rating

A standby-rated generator is designed for outage duty: it starts automatically, runs for the duration of the outage, and returns to standby. Most commercial backup generators are standby rated. A prime-rated generator is built for extended or continuous operation as the primary power source. If your facility has unreliable grid access or needs continuous generation, specify a prime-rated unit.

Single Phase or Three Phase

If your facility is wired for three phase or you have three phase equipment, your generator must be three phase. For single phase commercial applications, a single phase unit is appropriate. See our guide to single phase vs three phase commercial generators for a full breakdown.

Enclosure

Outdoor installations require a weatherproof enclosure. If noise is a concern, a super-silent or acoustic enclosure reduces noise output significantly. Americas Generators can supply generators with appropriate enclosures for your installation environment.

Common Commercial Generator Sizing Examples

These are approximate ranges to illustrate how sizing works in practice. Every facility is different and these should not replace a proper load calculation.

Small restaurant (2,000 sq ft): Essential loads typically include refrigeration, cooking equipment, POS systems, and basic lighting. Running load commonly falls in the 30 to 60 kW range for single phase applications.

Small office building (10,000 sq ft): HVAC, server room, lighting, and general outlets. Running load typically 60 to 150 kW depending on HVAC type and server density.

Large retail store or supermarket: Refrigeration is the dominant load in food retail. A medium-sized supermarket may require 150 to 400 kW to maintain refrigeration and basic operations.

Hospital or medical centre: Life safety loads plus HVAC and clinical equipment drive significant demand. Community hospitals commonly specify generators from 500 kW to over 1,000 kW, often with redundancy.

Light manufacturing facility (30,000 sq ft): Depends heavily on equipment. Motor-driven production equipment can add up quickly. Facilities in this range commonly need 200 to 500 kW, with motor starting a critical sizing factor.

Using the Americas Generators Power Requirement Calculator

For a structured starting point, the Americas Generators power requirement calculator allows you to enter your key loads and get an estimated kW requirement. It is not a substitute for a full load assessment by an engineer, but it gives you a working number to bring into a supplier conversation.

Once you have an estimated kW range, browse our commercial generators by kW range to see what is available in stock and ready to ship.

Frequently Asked Questions: Commercial Generator Sizing

What happens if my commercial generator is too small?

An undersized generator will overload when the connected load exceeds its rated capacity. This typically causes the generator to fault and shut down, or to run at excessive temperatures leading to accelerated wear and potential failure. In a standby application, an undersized generator that faults during an outage provides no protection at all.

What happens if my commercial generator is too large?

An oversized diesel generator running at low load, typically below 30% of its rated capacity, is at risk of wet stacking: the accumulation of unburned fuel and carbon in the exhaust system. This causes fouled injectors, reduced power output, and increased maintenance costs. Correct sizing, or load bank testing to manage wet stacking in unavoidably lightly loaded installations, prevents this problem.

Do I need an automatic transfer switch with my commercial generator?

For standby applications where the generator needs to start automatically when grid power fails, yes. An automatic transfer switch detects the utility outage and transfers the connected load to the generator within seconds. Americas Generators stocks transfer switches to match our generator range.

How long does it take to receive a commercial generator after ordering?

Americas Generators maintains over 200 commercial generators in stock. Most orders can be processed and shipped significantly faster than industry-standard lead times. Contact our team for availability on specific kW ranges and configurations.

Can I finance a commercial generator purchase?

Yes. Financing is available for qualified buyers. Contact our sales team to discuss options.

Ready to Specify Your Commercial Generator?

Americas Generators has been supplying correctly sized commercial backup power solutions to businesses across the US and internationally since 1994. Our technical sales engineers can review your load requirements and help you specify the right commercial generator for your facility.

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