How To Size A Home Standby Generator

Generator Shop Cordova TN Generator Installation MemphisStandby generators are available in diverse sizes with costs ranging from a low of $1,900.00 all the way to $30,000.00 and higher for equipment alone. Installation costs often equal or exceeds the cost of the generator equipment.

Benchmark Electric recommends consulting an experienced generator installer to properly size a generator for your home or business.

In this article, we review a few basics on generator sizing with this single caveat:

“Do not rely on this post when sizing a standby generator for your home.
Consult an experienced generator professional for proper generator sizing.”

— Steve Baker | Benchmark Electric, LLC

This article is not written for technical or training purposes. This post is written to help homeowners understand the language of standby generators in a simple, easy to understand format. Included is a basic introduction to generator sizing.

Other readers of this post might think,

“Who care’s how to size a standby generator?
I just want a quote from a qualified professional.”

If that describes you, skip the reading below, score a home run, and call Benchmark Electric at (901)590-4968.

For those exploring and learning, continue reading and thanks!

How to Size a Home Standby Generator

Generators are sized in kilowatts (kW). A 20kW generator produces 20 kilowatts or 20,000 watts of power. A 40kW generator produces 40 kilowatts or 40,000 watts of power.

What size generator is needed for your home?

Some homeowners require “whole house generator coverage” while others might need to connect a few “essentials”. Here is a list of common “essential circuits” required by most homeowners:

  • Aquariums <- a popular request!
  • Medical Equipment
  • Well Pump <- very needed when on a well!
  • Sump Pump
  • Kitchen Refrigerator/Freezer
  • Standalone Freezer
  • Security System
  • Garage Door Opener
  • Gate Operator
  • Electric Fan Blower Motor for Gas Furnaces
  • Ceiling Fans
  • Some or all house lights and receptacles (for TV, PC, Internet, etc.)

Most of the circuits above do not require a large amount of power to keep them operational. However, it’s important to consider the sum of each of these loads when sizing a standby system. A budget-friendly air-cooled generator can handle the above loads just fine.

Air-cooled generators are considered “budget-friendly” because the cost of the Generator and Automatic Transfer Switch (ATS) ranges from around $3,000 to around $5,000.00, depending on options.

Liquid-cooled generator pricing generally starts around $10,000.00. Liquid-cooled generators are also more expensive to install than air-cooled generators because:

  • larger wire and conduit sizes are required to deliver the electricity,
  • special equipment handling costs due to the generator weight and handling,
  • a concrete mounting pad is required (Benchmark Electric uses a concrete pad on air-cooled generator installations),
  • trenching is often required on liquid-cooled installations, and
  • additional labor is required for all the above

Match the size of the ATS to the connected load center. Homes with a 200-Amp load center will need a 200-Amp ATS. If your home has a 150-Amp or 100-Amp load center, size the ATS appropriately.

Homes with multiple main breaker load centers will require multiple transfer switches for whole house coverage. Standby generators connected to all house circuits are referred to as a “whole house generator installation.” A “partial house generator” connects only part of the house.

Rough costs? Simply double the cost of the generator and transfer switches for a rough budget for your project. That’s the simple explanation. We’ll explore more on total project costs in a subsequent post. To reiterate, multiple installation scenarios can vary actual final costs up or down.

Let’s go over some examples:

Scenario #1: Partial House Generator Coverage

This home has one or two main breaker panels, natural gas heat, and NG hot water.

The following circuits are “must have essentials” while on generator power:

  • Kitchen Lights and Receptacles (including the refrigerator/freezer)
  • Security System
  • Garage Door Opener
  • Fan Blower Motor for Gas Furnace(s)
  • Downstairs Lights & Receptacles (incl. TVs, PCs, Internet)
  • Downstairs Central Air Conditioning

     Note: You might also have a second HVAC system for the upstairs, but the second floor A/C is not required while on generator power.

The above-listed circuits are located on both main breaker panels.

Step 1:
Mark each essential circuit with a sticker to identify those circuits you need connecting to a future generator.

Step 2:
Hire a licensed electrician to move each marked circuit to the main breaker panel connected to the generator. Non-essential loads can be moved to the other main breaker panel. Following this method of installation requires one Service-Entrance Rated ATS.

Is there A less costly alternative?

The following method may or might not be less costly due to labor factors. Here, the electrician moves “essential circuits” to a small subpanel to be connected to a future generator. Better still, check for an ATS with a built-in 10-16 circuit load center.

When connecting more than 16 essential circuits, it makes better sense to land your essential circuits on one of your two main breaker panels. The unconnected panel can house non-essential circuits like non-essential air conditioning for the upstairs, laundry equipment, and perhaps the oven(s).

In this scenario, a Service-Entrance Rated ATS sized to match the connected main breaker panel is required. In most situations, plan on the electrician to also install an outdoor disconnect switch (a Code requirement) for the unconnected panel(s).

When two air conditioners share the same circuit, most manufacturers of air-cooled generators include load-shed capabilities for non-essential 2nd air conditioners or non-essential electric hot water heaters. While upstairs lights and receptacles are still “hot”, these circuits will not consume power from the generator until a light is turned “on” or an appliance is operated from an upstairs receptacle.

It’s important to properly shed larger non-essential loads. Failure to do so can overload the generator and result in a complete loss of generator power. Failures of this sort often occur while doing the laundry or roasting a feast in the oven with the air-conditioner running.

Scenario #2: Partial House Generator Coverage for Larger Homes

Let’s envision a home with four (4) 200-Amp Main Breaker Panels. The customer’s total power requirements while on generator power can be handled while connecting just two of the four panels.

Partial house coverage for larger homes relates to the number of connected panels to achieve the homeowner’s requirements. Other key factors include connected air conditioning compressors, gate operators, sump pumps, pool equipment, elevators (if any), ovens, and a host of other factors.

Scenario #3: Whole House Generator Coverage

As mentioned previously, a whole house generator will require a service-entrance rated ATS for each main breaker panel.

Let’s be clear. ATS gear connected to all main breaker panels makes any connected generator a “whole house generator”. It becomes critically important to size the generator capable of handling potential house loads.

Scenario #4: Managed Partial House and Whole House Generator Coverage

This scenario pertains to air-cooled generator installations when connected air conditioners are shed to make room for other heavy electrical loads like an electric oven or clothes dryer.

When partial house coverage is desired, identify essential electrical circuits.

The sample scenario below looks at typical electrical loads during a sizzling summer afternoon with a 5-ton A/C running. Laundry equipment is not being operated, but some light cooking on the electric range is occurring during dinner prep. The TV is on and so are a couple of ceiling fans.

Let’s look at these electrical loads:

Generator Shop Cordova TN Load Calculation Worksheet

The above illustrates how well a 20kW air-cooled generator can handle the job with power to spare.

Keep in mind, too, a 20kW air-cooled generator produces a full 20,000 watts when the fuel source is LP. The same generator produces 18,000 watts on natural gas.

Benchmark Electric typically recommends an air-cooled 20-22kW generator when air conditioning is connected. A smaller generator might handle the job to, but you might also be stretching the performance of the smaller generator. A 20kW generator may run a few hundred dollars more than a 17kW, but you will likely save that much and more on future repair costs when the generator is not under warranty.

Even in the above illustration, turning on one more ceiling fan, TV, or computer could begin to stretch the performance of a 20-22kW air-cooled generator. Keep your power requirements conservative while on generator power.

Take Note of This …
Think of the larger 20-22kW air-cooled generators as a suitable “managed whole house generator” for homes 2500 square feet or less. You will need the load management feature properly installed to avoid accidental overloads and shut-downs.

Should a homeowner expect to be able to turn everything in the house “on” and not overload an air-cooled generator? If load management is properly installed, you can expect the generator to continue to perform, but do not expect the air conditioning to work while operating the electric oven or clothes dryer. When the oven or dryer is switched on, the generator senses the need for additional power immediately and switches off the air conditioning to free up power to run these high-load appliances.

Here’s another explanation. Your generator is connected to your home’s 200-Amp service. However, because the generator is rated at 20kW, the maximum electrical load produced is 75-Amps on natural gas or 83.3-Amps on LP. Most homes with 200-Amp service might only pull, say, 120-Amps max on sizzling summer afternoons.

If you want a true Whole House Generator, you will be able to turn on anything at any time without overloading the generator. So, look for a generator capable of pulling at least 110-Amps.

Here’s the math for analytical readers (assumption: 120/240V Single-Phase Service):
kW or Total Watts / 240 Volts = Rated Amps
18kW or 18,000 Watts / 240 Volts = 75-Amps
20kW or 20,000 Watts / 240 Volts = 83.3-Amps
25kW or 25,000 Watts / 240 Volts = 104.2-Amps
30kW or 30,000 Watts / 240 Volts = 125-Amps
36kW or 36,000 Watts / 240 Volts = 150-Amps

If the peak electrical load is 110-Amps and you want a “whole house generator”, consider a 30kW liquid-cooled generator.

Are you considering a future new addition, shop, or guest house? If so, consider these future additional loads and consider going a step higher to accommodate the additional future loads.

Helpful Final Tips from Steve:

  • Purchase a generator designed to do what you want it to do
  • See a great deal on a generator at a Big Box Store? Great! But does it really fit your needs?
  • Consult a generator professional for accurate generator sizing.

Hopefully, the above helps illustrate the importance of generator sizing. Carefully consider the loads you need to be connected to the generator. Better yet, call an experienced generator professional for assistance with sizing.

Better still, call me, Steve Baker, at (901)590-4968 to chat about standby power or to schedule an in-home consultation and free estimate. I’m happy to work with any homeowner or business within a 100-mile radius of Memphis, Tennessee.

When you are ready to explore Project Costs, click here.

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