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Standby Generator Sizing: What Are They?


There is no doubt that standby generators are lifesavers. During power shortages and blackouts, our dependable “gennies” light up homes to keep us cozy and comfy and power our businesses to keep them running smoothly. But how well your standby generator does its job depends on whether or not it’s of the right size.

When purchasing a standby generator, many consumers make a critical error. They miscalculate their standby generator sizing and wind up with one that is either overpowered or underpowered. And it is for this reason that we have created this guide. Here you will discover how to size a standby generator so that you can get the suitable size.

Let’s start with understanding why sizing your standby generator is important.

Importance of standby generator sizing

When it comes to a standby generator, size does matter. A generator that is too large will result in unnecessary fuel consumption and an eventual waste of money. Remember that even if you are not utilizing its full power, the generator will still keep burning through fuel rapidly and keep generating its maximum output.

Oversized standby generators:

  • Lead to unnecessary fuel expenses
  • Produce surplus power, making it highly inefficient
  • Pose risks to electrical systems

A generator that is too small opens up a different can of worms. Its output won’t match your power demands, which can lead to the generator’s inverter repeatedly failing, and you will end up having to reset the breaker again and again to perform a toggle cycle. With your standby generator crumbling under the demands of power-hungry appliances, your ACs and heaters and microwaves may even fail to power on.

Undersized standby generators:

  • Don’t have enough wattage to meet minimum power demands, which may lead to voltage fluctuations that slow down power supply to appliances and systems.
  • Face the risk of overheating, which could even damage them.
  • May stall while running due to worn or dirty filters.

You can keep all these problems at bay through careful and accurate standby generator sizing. 

How to size a standby generator?

Standby generator sizing can be fairly simple, given that you are willing to indulge in some mental gymnastics and make sense of some important numbers. But we have simplified the process so that you can follow along with the tips on understanding how to size a standby generator.

Identify your essential appliances and find their maximum wattages

Everything starts with how much power your essential appliances need to operate uninterruptedly at the same time. Make a list of all the important circuits and appliances in your home or business establishment. Electrical appliances typically have a data plate on the back, which is where you can check their running wattages.

Do this for all essential appliances–HVACs, furnaces, refrigerators, well pumps, medical equipment (if there are any), microwaves, etc. And don’t let the size of an appliance throw you off. A hair dryer is much smaller in size compared to a TV but draws in much more power (1500–2000W) compared to the 600–800W of a TV.

Determine the highest starting wattage

Before you add up all the wattages, you will also need to consider the starting wattage. Some appliances, particularly those with electric motors, demand a high initial load when starting from a dead stop. This is often five to six times higher than running wattages. Pick the item in your list that requires the maximum starting wattage, multiply it by three, and then add the running wattage to it. For example, if your refrigerator has a starting wattage of 1000W and a running wattage of 300W, then its total wattage would be 3300W.

Calculate the total load requirement

Find the total wattage by adding all the individual wattages of each appliance, then top it off with a 20-30% margin. This margin will give your standby generator sufficient breathing space if required. For instance, if you’ve arrived at essential power requirements of 12kW (12,000W), your minimum generator capacity would be 15-7kW.

For commercial and retail applications, you can find the minimum requirement through the Square Footage Sizing Method. Assume 10 watts for every square foot used and add 50 kW as the base load to the total value. For instance, for 2,000 square feet of space, you get a minimum power requirement of 75kW (10W x 2,000 ft2 +50kW), which would be your minimum generator power requirement.

Check generator load management and performance charts

Now that you know how much power you need from your standby generator, you can shortlist the ones that can meet your power demands and examine their performance charts. Most manufacturers make them readily available with their generators.

Select a generator size that can meet your load requirements without exceeding its rated capacity or derating factors. This will help you avoid any potential issues with the generator’s performance and ensure a reliable power supply. If you are in Tennessee and need help making a choice, reach out to us, and we will help you find a generator that matches your load profile.

Other factors to consider

The process of sizing standby generators is not yet complete. If you want to properly grasp how to size a standby generator, you need to be aware of the following:

  • Space constraints: Make sure you have enough room to install a generator. The generator should have some open area on both sides for ventilation, cooling, and exhaust.
  • Local regulations and compliance requirements: Check that your generator and its installation adhere to local codes.
  • Noise levels: Generators can get noisy, which can be a problem in residential areas with strict noise ordinances. This is something you should watch out for.
  • Power phase: When choosing a generator for a business or store, you will also need to make sure that it works with the current electrical system’s power phase (single-phase or three-phase).
  • Integration with other equipment or generators: If you want to connect more than one generator or any of them to other equipment like a UPS, ATS, or PDU, make sure they are all compatible and can provide reliable power backup.

What sizes do generators come in?

Wondering “What sizes do generators come in?”, look no farther. Generators come in multiple sizes—from the smallest size to meet the bare essential power requirement to an absolute beast that powers an entire building. But there are three sizes that are the most popular:

7 kW Generators

For lighting up a few LEDs and maybe running a few small appliances, a 7-kilowatt generator will get the job done. These generators are fantastic at powering mid-sized homes but won’t handle larger appliances like washing machines or dryers.

14 kW Generators

A 14-kilowatt generator is a step up from its smaller brother in the sense that you can power one or two high-power appliances along with low-power devices. But you should take note that 14 kilowatts aren’t enough to drive two high-power appliances at the same time, so you may have to cycle their usage.

20+ kW Generators

For all intents and purposes, anything greater than 20 kW will deliver uninterrupted power to your home. It will drive most of your energy-hogging appliances and, at the same time. Generators with power outputs of 20 kW and above are the go-to sizes for several American families.

Looking for a standby generator in Tennessee?

Premier Generators is the #1 dealer of Generac generators in Tennessee. We have powered thousands of homes with our reliable products and dependable service.

We hope that this standby generator sizing guide has helped you understand how to size a standby generator better.

If you need more help making a choice, let’s talk. We would be happy to walk you through the different sizes of standby generators.