A commercial generator plays a critical role in your business continuity plan. By providing backup or emergency power to your building during a power outage, commercial standby generators enable critical facilities like elevators and security systems to continue operating. Standby generators also minimize business and data losses that arise from computer system failures.
However, determining how to properly size a generator depends on a number of factors. Before you proceed with your commercial generator purchase, you’ll need to consider the needs of your business and the technical constraints of your building first.
Contents:
- Why Proper Generator Sizing Matters
- Calculating Your Power Requirements
- Determining Your Starting and Running Wattage
- Examining Generator Performance Charts for Load Requirements
- Frequently Asked Questions About Generator Sizing
- Need Help Determining How to Size a Generator?
Why Proper Generator Sizing Matters
Commercial backup generators provide power to a number of critical safety systems that operate during an emergency, including fire alarms, fire pumps, security systems, and emergency lighting. Different buildings require different levels of backup power to keep these life-saving systems operational in the event of a blackout.
This is why most large-scale commercial generator installations require an engineering plan and an engineer’s supervision to ensure compliance with National Electrical Code (NEC) and National Fire Protection Association (NFPA) requirements in the United States. Beyond regulatory compliance, not knowing how to properly size a generator can also lead to a host of other problems.
Generators that are too large may cause:
- Damage to electrical systems
- Unnecessary operational expenses
- Inefficient power production
Generators that are too small may cause:
- Generator damage or overheating
- Insufficient or unreliable power
- Critical facilities and security system failures
Let’s look at what you need to know about estimating the right sized generator for your business.
Calculating Your Power Requirements
Start with making a list of everything that you plan to power with your backup generator. This varies widely depending on what type of business you operate, so don’t make the mistake of glossing over this step too quickly.
- For a retail establishment, you may want to power some or all of your payment terminals, lights, security systems and mission-critical data servers.
- For an office building, you may need to power lighting, telecommunications, security and other basic systems that will allow people to evacuate the building safely.
- For a restaurant or food establishment, you should consider refrigeration, HVAC systems or any other appliances that require power to prevent food from spoiling.
- For a healthcare facility or clinic, pay close attention to life-saving systems that require consistent power to operate, including breathing and dialysis machines.
Some factors for sizing a commercial generator include selecting single- or three-phase power, voltage selection and total power output. Be aware that most commercial applications require a reserve or surge capability, especially for large motors operated by several units independently.
Measurement Methods
Once you know which items you need to power with your generator, you will need to estimate your business’s power consumption at peak usage. Depending on the type of business and your power needs, there are a variety of methods you can use to determine your full load capacity.
Real-Time Measurement
- Use a clamp-on ammeter on each leg of the electrical service and add the measurements together to provide the total amps used by the facility.
- Divide the total amps by three for three-phase current, and by two for single-phase current. Multiply the result by the supply voltage, and again by 1,000 for kilowatts required.
- 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 (kW).
Full load kW = Total amps x supply voltage / 1,000
Reserve capacity = Full load kW x 0.25
For 100 percent power, generator size = Full load kW + reserve capacity
Utilize your utility company's billing system to find your maximum power usage.
Full Load Capacity by History
- Review your utility bill for the peak demand each month.
- Find the highest peak demand over the previous year and then add 25 percent for reserve capacity.
Full Load Capacity with Extensive Motor Use
- Multiply the starting current for the largest motor that turns on and off by the voltage for the number of watts required.
- For all other motor and non-motor loads, multiply the current by the voltage for watts.
- Calculate total watts used by the largest motor and all remaining motor and non-motor loads and multiply by 1000 for kilowatts.
- Add 25 percent for reserve/surge capacity and size the generator accordingly.
Square Footage Measurement
The square footage sizing method is commonly used for retail locations such as grocery stores, restaurants and convenience stores.
Retail application: 50 kW + 10 watts per square foot
Other commercial application: 50 kW + 5 watts per square foot
In addition to estimating your total power needs, it is critical to determine the starting load and running load for each item.
Determining Your Starting and Running Wattage
- Starting load: The initial high load to start items from a dead stop. For compressors and motors, the starting load can be up to six times the running load.
- Running load: The load needed to keep items operating after initial start.
For backup use, you can calculate the load on a staggered start for multiple units to spread out the load. Use the highest Locked Rotor (LR) rating from of all the items you want to run.
Steps for Estimating Starting and Running Wattage
- Select the items you wish to power at the same time, and add together for the total running watts.
- Selected the item with the highest number of starting watts.
- Add the two numbers together for total watts needed.
If you can't determine the running watts of an item, use the formula of watts = volts x amperes. Only motor-driven items have an additional starting voltage requirement. Remember: For accurate sizing, convert all amps to kilowatts.
Examining Generator Performance Charts for Load Requirements
Once you have calculated the amount of power you will need from a commercial backup generator, the next step is to identify a generator unit that will meet your needs. To help guide your selection, manufacturers offer performance charts for each product they sell.
First, look for a unit with the necessary power rating to run each of your selected items. Generators are usually rated in kilowatts and come in a wide variety of capacities. If your needs fall between common ratings, choose the next highest capacity.
Other Generator Factors to Consider
- Operation: Generators can operate automatically or manually. Nearly all commercial generators utilize an automatic switch that automatically switches a building’s power to your backup generator when your primary power fails.
- Power phase: Make sure to determine if you need single-phase vs. three-phase power. Most commercial backup systems will require three-phase service to provide the voltage levels required.
- Fuel source: Typically you can choose from diesel, propane, natural gas or gasoline. Diesel and gasoline are both more efficient than propane, but propane is ideal for sporadic use of a generator since it does not degrade in storage.
- Noise rating: Depending on the installation site, consider the noise rating of the unit when it's running.
Frequently Asked Questions About Generator Sizing
Myth 1: Bigger is always better for generators, right?
Not necessarily. While it might seem like a good idea to have more power than you need, an oversized generator can lead to several problems. For diesel generators, this can cause "wet stacking," where unburned fuel and soot accumulate in the exhaust system due to consistently low load operation. This reduces efficiency, can damage the engine over time, and increases maintenance costs. Oversized generators also have higher upfront purchase costs and may consume more fuel than a correctly sized unit, leading to unnecessary operational expenses. Accurate sizing to match your actual load requirements is crucial for optimal performance and longevity.
Concern 1: How do I account for future business growth when sizing?
This is an important consideration for long-term planning. To account for future growth, you should first accurately calculate your current maximum power demand. Then, project your anticipated load increases over a reasonable timeframe (e.g., the next 3-5 years). Add a percentage for this projected growth (commonly 10-25%) to your current calculated load, on top of the standard reserve capacity (typically 25% of the full load). It's also wise to discuss your expansion plans and specific future equipment needs with a power generation expert. They can help you select a generator that either meets these future needs or has provisions for modular expansion if your needs are expected to grow significantly.
Myth 2: Can I just use the square footage calculation for any commercial building?
While the square footage method (e.g., 50 kW + X watts per square foot) can provide a very rough, initial estimate for some basic retail or office applications, it is often not precise enough for many commercial and industrial facilities. Buildings with specialized equipment, significant motor loads (like manufacturing plants or facilities with large HVAC systems), or critical power requirements (such as healthcare clinics or data centers) require a much more detailed load calculation. Relying solely on square footage can lead to significantly undersized or oversized generators, resulting in the problems mentioned earlier. A thorough assessment of all electrical loads, including their starting and running wattage, is essential.
Concern 2: What are the most common mistakes people make when sizing a generator?
Several common mistakes can lead to improper generator sizing. These include:
- Underestimating starting wattage (kVA): Motors and equipment with compressors often require significantly more power to start than to run. Failing to account for the highest starting kVA can lead to an undersized generator.
- Missing critical loads: Overlooking essential systems that must run during an outage (e.g., emergency lighting, security systems, fire pumps, specific IT infrastructure).
- Ignoring reserve capacity: Not adding a buffer (typically 20-25%) above the calculated peak load for unexpected demands or to prevent the generator from running at 100% capacity constantly.
- Not considering load diversity: Assuming all equipment will run simultaneously at peak load, which might lead to oversizing. Conversely, underestimating simultaneous operation is also a risk.
- Disregarding voltage dips: Not accounting for allowable voltage dips during motor starting, which can affect sensitive equipment.
- Failing to comply with codes: Overlooking specific National Electrical Code (NEC) and National Fire Protection Association (NFPA) requirements for their building type, occupancy, and location.
- Relying on a single calculation method: Especially for complex loads, using multiple methods for cross-verification is advisable.
Consulting with a qualified power systems engineer or generator specialist is the best way to avoid these pitfalls.
Need Help Determining How to Size a Generator?
To avoid the pitfalls that come with not knowing how to size a generator, consider seeking out the assistance of a backup power expert. General Power is here to help you find the right generator for your specific needs.
Speak with an Expert: 1-888-819-5646
Our power experts will partner with you to understand your needs and help you select the right unit for your business. Call 1-888-819-5646 to speak to one of our friendly staff members.