How to Size a Generator for Commercial Building

Commercial generators enable your business to continue operating even during power outages. If your primary power source fails, the generator will activate, providing backup power for your systems and equipment.

When it comes to installing a generator for a commercial building, size matters. To determine the appropriate size for your commercial generator, you’ll have to consider single- or three-phase power, voltage selection, and total power output. Discover how to choose the best size generator for your business to ensure you’re prepared to meet power demands and load requirements during an outage.

Commercial Generator Sizing Methods

Whether due to a storm or local power blackout, the right size generator will make a difference in your business’s ability to continue functioning.

There are several methods for sizing industrial generators:

Full load capacity by measurement
Full load capacity by history
Full load capacity with extensive motor use
Square footage measurement

What to Do Before Sizing a Generator

Before you calculate appropriate commercial generator sizes, consider the following:

Voltage drop: When motors start, they create a current surge that step loads the generator and creates a voltage drop. Most commercial applications should be limited to about 15% voltage drop.
Amps: Never add amps when sizing a generator. Convert amps to kilowatts (kW) and add kW to determine the required generator size. When sizing a generator for a commercial building, you want to calculate the total number of power output rating in kW you would need in order to ensure that your generator can provide power to all required equipment and machinery. Simply adding the needed amps to the total kW value will provide inaccurate power calculation needs and result in the purchase of a generator that does not meet or exceeds your total power requirements.
Compliance: Ensure that your commercial generator and installation are compliant with the National Electrical Code (NEC).

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How to Calculate the Size of a Generator

While the right size generator will depend on factors like the type of business you are running and your specific power needs, there are several ways and resources to calculate what size you need.

You’ll want to consider and take into account several factors before getting started:

The types of items your commercial generator will power
The starting (startup/surge) and running wattage of those items
The total power requirements for those items
Utilities such as lights, emergency lights and HVAC
Peak usage in real time
Monthly power bills
The facility’s square footage

Method 1: Full Load Capacity

Merely follow these mathematical steps to figure out the right size for your commercial generator!

Step 1: Estimate the mandatory generator capacity by taking full-load current measurements during peak usage at the service panel. Use a clamp-on ammeter on each leg of the electrical service and add the measurements together. This provides the total amps used.
Step 2: Divide the total amps by three for three-phase current, and by two for single-phase current, then multiply that by the supply voltage, and again by 1000 for Kilowatts (kW) Required.
Step 3: Add the power in kilowatts used by each emergency safety system according to articles 700, 701, 702 and 708 of the NEC to the kilowatts required to obtain Full Load Kilowatts.
Step 4: Remember, full load kilowatts = total amps x supply voltage / 1000. Reserve capacity = full load kilowatts x 0.25.
Step 5: For 100 percent power, generator size = full load kilowatts + reserve capacity.

Method 2: Square Footage

If you run a retail store or restaurant, this may be the best and simplest method for you.

For a retail application: 50 kilowatts + 10 watts per square foot.
For a commercial application: 50 kilowatts + 5 watts per square foot.

Importance of Achieving Correct Commercial Generator Sizing

No matter the size of your business or what industry you’re in, achieving the correct generator size is crucial because it can impact several aspects:

Performance: Undersizing will cause your generator to work harder than it needs to, potentially leaving your equipment unprotected if the generator struggles or fails to provide backup power. Similarly, oversizing can be a costly mistake because your business will be paying for increased installation, equipment and maintenance costs for a generator that’s larger and more powerful than you need. A general exception to this is if you are planning on scaling up and expanding, in which you would be future-proofing your facility with a greater initial investment cost.
Efficiency: Your commercial generator is responsible for keeping your business running even during power outages. If it isn’t capable of handling the task, your business won’t be productive and profitable.
Downtime costs: Especially for businesses with continuous operations, if the generator is down, your business can potentially lose revenue and have to deal with costs related to downtime.

Discover the Woodstock Power Company Experience

At Woodstock Power, we specialize in buying, selling and servicing industrial and commercial power generators from a variety of brands. While we operate globally, we are headquartered in Philadelphia, Pennsylvania, serving as a reliable and trustworthy source for generators and accessories from a variety of brands.

As the most powerful brand in the industry, we have relationships with resellers and dealers from all around the world, which enables us to find the exact generator solution our clients need. We handle delivery, installation and maintenance as well as provide financing options. No matter the size project, you can trust that our generator specialists will support you throughout every step of the process.

If you’re interested in learning more about commercial generator sizes and finding the right

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