Last Updated on October 11, 2025
Having consistent and reliable access to power is crucial for any industry. From first responders to grocery stores, healthcare facilities and data centers, sudden or lengthy power outages can wreak havoc on day-to-day operations. It also makes difficult on the lives of those who depend on these people and organizations.
Unfortunately, power outages are becoming increasingly common. What’s to blame? A combination of increased demands for electricity, an aging grid, and more frequent natural disasters like heatwaves, hurricanes and wildfires add to the penchant for power outages.
When bad weather or other electrical grid problems strike, renting a high-quality commercial generator may be the only thing keeping things going. But only if you select the right size of power generator for your power consumption.
In this article, you’ll learn all about commercial generator sizing so you can keep your facility up and running.
How Do I Know What Size Commercial Generator I Need?
In general, you will need a generator that provides at least 25% more kilowatts (kW) than you expect your facility or equipment will use.
Why Does Commercial Generator Sizing Matter?
There are a number of ways to determine which size generator you need, and it’s important to get it right.
If you select a generator that is too small, it will be unreliable because it won’t be able to create the amount of electricity that you need. It will be running at full capacity for too long, which could cause the generator to overheat or cause critical equipment and systems to fail.
On the other hand, selecting a generator that is too large and powerful could overpower and damage delicate electrical systems or simply produce electricity inefficiently. Both of these scenarios could create unnecessary costs or unsafe situations for employees.
Knowing what size generator to rent is also a critical aspect of ensuring overall site safety and compliance with national electrical codes and regulations. For example, OSHA has several requirements for working safely with standby power generators:
- OSHA Standards for Electric Power Generation and Portable Power Equipment
- Working Safely with Electricity
- Using Portable Generators Safely
4 Tips for Sizing a Backup Generator
If you’ve ever wondered, “What size of portable generator do I need? How do I know how big of a generator to rent?” then read on to learn four important tips for correct commercial generator sizing.
1. Estimating Load Size: Know How Many kW You Might Need
The amount of electricity an appliance or a facility uses — also known as a “load” — is calculated in watts (W) or kilowatts (kW; 1 kilowatt = 1,000 watts). It’s impossible to select the correct commercial generator size if you don’t know your facility’s load size.
Load size the maximum kilowatts (kW) that the facility will use plus a margin of reserve. You always need to factor in a margin of reserve power. Doing so makes sure equipment will keep running if the power is out for an extended period.
Here are the simplest ways to estimate the load size, or total kilowatts, you’ll need:
- Utility Bill’s Peak Demand + 25%: To get a realistic estimate of maximum load size for all electric components, look at the facility’s electric bills from the past year. Find the highest peak demand, which is the point in time that the most kilowatts were used. Then, add 25% as a reserve margin.
- Square Footage: If you don’t have direct access to utility bills or don’t know exactly how many kilowatts all your equipment will use, you can get a simple rough estimate based on square footage of the building.
- For Retail Locations: 50 kW + 10 W per square foot
- For Other Commercial Locations: 50 kW + 5 W per square foot
- Consult a Power Expert: Kilowatts, amp, supply, voltage selection — electrical terminology is complex and confusing. When creating a backup power plan, enlist the help of a power expert like those at Herc Rentals to help you calculate your power consumption and power generator load size.
2. Converting kW to kVA: Account for Inefficiency in the Generator
No generator is 100% efficient 100% of the time. To account for the inherent inefficiencies of generating electricity, standby power equipment is often listed by its apparent power, which is the amount of kilovolt-amperes (kVA) it produces, instead of kW.
If the generator you’re considering is listed by kVA instead of kW, you will need to convert the total kW you need into kVA to determine if the generator can produce sufficient power.
Dvide the total kW you need by the power factor of the generator (the power factor is usually 0.8).
- How to Convert kW to kVA: Total kW ÷ power factor of 0.8 = kVA
- Example: 120 kW ÷ 0.8 = 150 kVA
3. Don’t Forget Startup and Runtime Requirements
Some equipment uses more power during startup than it does after it has been running for some time. In fact, the amount of kilowatts needed during startup can be six times what’s needed while the equipment is running. Double-check that your load size estimates account for these power surges and that the maximum capacity of the generator can accommodate them.
Similarly, generators should not be operated at maximum capacity for more than 30 minutes. If a generator will be used as a primary power supply for any extended period of time, then increase your load size’s reserve margin so that you won’t use more than 70-80% of the generator’s capacity.
4. Analyze Project Site Conditions and Access
Lastly, don’t forget to account for the logistics of the project site. Commercial generators, whether portable generators or standby generators, tend to require more room to operate safely than you may think. Plus, local safety and noise regulations may dictate where you’re allowed to place the generator.
When deciding, make sure to consider where the generator will be stored, and where and how it will be installed. You may also keep in mind if it will need to be moved at some later point.
For example, if you need a large towable generator, but difficult terrain or narrow spaces prevent access to the best spot for the unit, you may need to find an alternate location or rent multiple smaller generators. You may also need to rent additional power equipment, such as load banks.
Find the Right Size Commercial Generator at Herc Rentals
From construction sites to medical facilities to 24/7, a standby generator ensures teams, facilities and equipment will keep running even when power goes out.
At Herc Rentals, our power experts are ready to help you determine exactly what size generator your project needs. From portable generators to large towable or standby generators, we’ll help you execute your project’s power or emergency backup plan with reliable, high-quality power generation equipment.
Explore Herc Rentals backup power generation solutions, or give us a call at 1-800-432-6215 to talk with a power expert today.
