How to Size a Commercial Generator

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Sizing A Generator

A generator is a significant investment, not only because of the installation process and price but also because of its intended use. Whether this is for commercial or home use, your generator will play a substantial role in either your business plans or during emergencies. Sizing a generator is therefore a key step.

It is natural for there to be a lot of confusion when first purchasing a generator as it is not a regular occurrence for most. The process can include a lot of questions such as:

  • Which fuel type to use?
  • Which brand is the best? And most importantly,
  • What size will you need?

To help you throughout this process, our engineers here at Edge Technology will be happy to support and inform you through the course of selecting your generator. Our team can offer you advice and essential services such as commercial generator installation. Please subscribe to our Youtube channel for future tips and guidance on sizing a generator.

Commercial Vs Residential Generators

Naturally, a residential generator differs from a commercial one. You can usually expect a home to only require a light-to-moderate power back up, which can be supported by a smaller, portable generator.

A commercial building will require a power back-up system that will ensure the smallest possible gap for important equipment to continue running when there is a power shortage.

A commercial generator must be equipped with an automatic operation and automatic voltage regulator. By providing these, you are not only ensuring a seamless power supply when there is a shortage, but you are also protecting the equipment by guaranteeing a continuously-controlled voltage.

Why Generator Size Matters

Generators supply power for hospitals, retail, manufacturing and various other commercial applications when there is a power cut, which is why it is crucial when sizing a generator to ensure these can continue seamlessly.

It is paramount that you do not select a generator too small for the intended task. It will damage the appliances and devices that are pulling the power from the generator and can potentially damage the generator itself.

Here are some of the benefits of a generator that is powerful enough, if not more so, for the demands of the intended task:

  • Increased generator life span.
  • Consistent performance.
  • Limited occurrence of capacity overloads.
  • Limited occurrence of unexpected system failures.
  • Reduced prospect of asset damage.
  • Decreased risk of overheating and electrical fires.
  • Increased ease and speed of maintenance.


What to Consider When Sizing a Generator

There are a variety of factors involved when purchasing a generator. Below, we’ll be discussing an array of considerations that you should weigh up when buying a commercial generator.

Before purchasing a generator, ensure that you speak to an engineer, like one of our expert team members at Edge Technology, to review your options and ensure that your generator meets your commercial needs!

Power Requirement

As all business are different, not all facilities will require the same power supply. The more electricity that is required for the function of your enterprise, the higher the demand will be on the backup power.

If your business is in an area where there is no electricity, then you will most likely need a continuous supply of power, which will require a much more powerful and larger generator.

Voltage Sag

Also known as a voltage dip, voltage sag is a temporary drop that will occur when there is a short circuit or overload of electric motors.

A voltage sag is a reduction of voltage of 10% or more below normal. These can have a ripple effect across the equipment connected to the power source such as monitors being turned off, dimming lights or temporary short circuits.

Frequency Sag

The generators frequency is determined by the engine rpm (revolutions per minute) which will affect the quality of electrical power. As the voltage increases so does the power, which then also raises the frequency.

The probability of a frequency sag is heightened by the amount of equipment used throughout an industrial building. To avoid any issues when this happens, you should ensure that the generator can cope with the maximum drop in frequency and speed.

The Surrounding Area

The surrounding area of the generator will affect its performance because the combustion engine will need a sufficient amount of air to properly function.

When considering the surrounding area of your generator, you should bear in mind:

  • The humidity
  • The temperature
  • The altitude of the building

These factors can collectively create a slight power shortage. The manufacturer of the generator will supply you with more information about this.


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How to Size a Commercial Generator

After taking everything into account, you will now need to calculate the necessary size of your generator.

Below are some methods you can use to calculate the size of your generator. Our experts here at Edge Technology will be happy to assist you and ensure you find the correct generator for your commercial needs.

Full Load Capacity Method

This mathematical equation will assist you in determining the correct generator size. This method will need the full-load current measurements during peak usage to estimate the capacity of the generator.

The full load measurements account for the power that is being consumed from the generator. The reserve capacity refers to the excess of equipment that will be required to carry the peak load, this will maintain the reliability of the electricity.

Full Load Kilowatts = Total Amps x Supply Voltage x 1000

Reserve Capacity = Full Load Kilowatts x 0.25

100% Power / Generator Size = Full Load Kilowatts + Reserve Capacity

Power Usage History Method

Monthly electricity bills are a good indicator when determining the size of the generator. Collect the bills from the past year and sieve through to find any patterns and fluctuations.

Once you have determined the peak power demand, ensure you add 25% so you have sufficient reserve capacity.

Extensive Motor Use Method

For this method, you will need the largest currents for the largest motor. The largest motor is an active component in turning the generator on and off, also referred to as a starting current.

Starting Current x Voltage = Watts Required

For the remaining motor and non-motor load:

Current x Voltage = Watts

Total Power Kilowatts = Total Watts (largest motor + all remaining motor + non-motor loads) x 1000

Add 25% for reserve/surge capacity and size the generator accordingly.

Square Footage Measurement Method

One of the more common methods used to size generators for retail applications.

Retail Application: 50 kilowatts + 10 watts per square foot.

Other commercial application: 50 kilowatts + 5 watts per square foot.

We hope this article has helped you to size your commercial generator. If you require any further assistance, you can contact our team of experts. Here at Edge Technology, we are happy to help with our array of services for commercial and residential generators!


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