Exterior Energy & Power Generators

How to Calculate How Big a Generator You Need to Prepare for an Emergency

When the electricity goes out, an emergency generator can power a home so that essential tasks aren’t put on hold. Luckily, homeowners who are wondering “How big of a generator do I need?” can easily find the answer.
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How Big of a Generator Do I Need? Solved! Standby Generator and Portable Generator Sizes Explained

Photo: Kohler

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Q: Last year, we endured a power outage that lasted for days. After that experience, we decided to buy a generator, but we’re just not sure what to buy. How big of a generator do I need to power my entire home?

A: Homes can experience inconvenient power outages that last for a few hours. When a power outage lasts for days, though, it can be dangerous. An emergency generator will keep the power on and make frozen pipes, spoiled food, flooded basements, mold and mildew, or extreme temperatures a thing of the past. For those who have a medical condition that requires equipment powered by electricity, a power outage can be life-threatening. Having a backup plan in the event of harsh weather, wildfires, or a power-grid failure means that life can go on even if the power is out.

Generators come in different wattages, and there are various types of generators for every need. Small generators can power an appliance or two, while a whole-home generator (sometimes called a standby generator) can power an entire house. The size of the generator a homeowner will need to purchase will depend on how they plan to use it. Wondering “How big of a generator do I need?” If so, this guide will offer information and guidance on choosing one of the best standby generators for home use and what will be the best fit.

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First, understand starting, running, and surge wattage.

The wattage, or power that the generator needs to make, is based on the number and type of devices or appliances that it must operate.

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Photo: amazon.com
  • Starting wattage. Starting wattage is the wattage needed for an appliance to start. This wattage is typically two to three times more than the running wattage or the amount required to keep the appliance running. Many electronic devices and appliances need more wattage to start up and require less wattage to keep running. When the power goes out, a generator will have to deal with a surge from all the appliances that run all the time: water heater, refrigerator, freezer, HVAC unit, etc. If a dishwasher, microwave, or washing machine is running at the time of the outage, the generator will need to power those on as well.
  • Running wattage. Running wattage, sometimes called rated wattage, is the amount of electricity that’s needed to keep an electronic device or appliance running. Many appliances will list the running wattage to help shoppers calculate how much wattage is needed to power everything. It’s also wise to look up the surge wattage to help determine the correct generator size.
  • Surge wattage. Surge wattage is the highest amount of wattage that a generator can produce. If all of the home’s appliances and devices will need to run off the generator, it’s advised to add 1,000 to 2,000 additional watts to cover the initial surge.

Know the difference between watts, amps, and volts.

Electric current is the total amount of energy passing through a wire over a given period of time—but there are a few different units that measure how the energy passes. Understanding the difference between watts, amps, and volts can make it simpler for a homeowner to recognize the differences in electrical systems. Homeowners who are having a whole-house generator installed won’t need to do any of this math or use a generator size calculator—the generator installer will take care of it all—but knowing these measures can help customers make a more informed decision.

It’s helpful for homeowners to think about the flow of electricity like water flowing in a river. Voltage measures the electric potential between two points, and it’s measured in volts. A charge moving from one point to another is measured in voltage. Like a river flows from a high point to a low point, voltage always travels from a high voltage (lots of potential) to a low voltage (little to no potential).

Amps, or amperes, refer to how strongly a charge flows. In the river analogy, amps would be how much water passes a certain point in a given period of time. Amps and volts are often used together in a measurement known as volt-amperes, or VA.

The goal of a generator, though, is not to just create electricity for no purpose—it needs to be put to work as a useful energy source, like heat or light. As current is created and flows from the generator into a home, the current will power whatever the generator is connected to. Wattage measures how much power a device actually uses. Wattage is measured with this formula: wattage = amps x voltage.

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Determine the number of appliances and devices you’ll want to run, and calculate the total wattage.

To determine whole-house generator sizing, the first step is for the homeowner to determine the number of appliances and electrical devices that will be run and calculate the total wattage. First, they’ll want to list all the appliances that need to be running during a power outage. Next, they’ll write down the number of watts that are required in order to power up and run each device. This information can be found in the owner’s manual or on the machine itself. It’s recommended that a homeowner use a generator calculator to determine an accurate wattage estimate or hire an experienced local electrician to calculate the exact wattage that is necessary. It can also be helpful for a homeowner to refer to a generator size chart to get a visual of what each size can handle.

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Photo: amazon.com

Homeowners who are concerned about overloading the generator may consider staggering the use of certain appliances or unplugging some devices that are not being used. It’s also important that homeowners remember to factor in items such as a sump pump that may be needed during a strong rainstorm or a threat of flooding. The approximate running watts of some common household devices and appliances are shown in the table below.

Appliance or DeviceRunning Wattage
Coffee maker400 to 800
Refrigerator/freezer600 to 800
Space heater1,250
Toaster1,100 to 1,700
Electric oven5,000
Personal computer500 to 2,000
Television100 to 350
Table lamp150
Hair dryer1,200 to 1,500
Cell phone battery charger10
Video game console200
Washing machine750
Clothes dryer1,500 to 5,000

The approximate wattage requirements for a home’s heating and cooling systems are as follows.

Central air conditioning2,000 to 4,000<
Window air conditioning unit600 to 1,500
Electric furnace5,000 to 25,000
Water heater3,000 to 4,500
Sump pump1,500
Radiant heater1,300
Outdoor lighting500 to 1,000

Homeowners will want to keep in mind that these running wattage requirements are approximate; it’s important to refer to the actual appliances or owner’s manuals for the manufacturer-specified wattage requirements. The total number of starting watts added together will typically be higher than what the generator will need to produce. After determining an estimate of the highest wattage usage that will be needed in the event of a power outage, homeowners will want to add another 100 or 200 watts just to be safe. Some of the most common generator sizes and their capabilities are as follows:

  • 10,000 watts: Can simultaneously power a fridge, freezer, dishwasher, electric stove, window air conditioning unit, washing machine, furnace, sump pump, small appliances, television, and lights.
  • 7,500 watts: Can simultaneously power a fridge, freezer, water heater, electric stove, air conditioning, washing machine, dryer, space heater, small appliances, and lights.
  • 2,200 watts: Can power a fridge, electric stove, microwave, washing machine, space heater, sump pump, and a television one at a time.

While there are a wide variety of generator uses, not every homeowner needs the largest or most powerful option. Choosing a generator that can meet basic needs will save on the cost of fuel. Getting the correct-size generator is important to avoid overloading and overheating a generator that’s too small and is trying to produce more energy than it can, which can lead to an automatic shutoff. If this happens, any connected devices and appliances could be damaged. If a generator isn’t stressed from trying to meet a high demand for energy, it will last longer. This will help homeowners avoid costly repairs, or even replacements.

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Choose a generator that can handle your power needs using no more than 90 percent of its capacity.

After calculating the total number of starting watts needed from a generator, it will be possible to determine the best size for a homeowner’s needs. Homeowners will want to keep in mind not to overload the generator to prevent it from overheating and automatically shutting off. To do this, they’ll want to choose a generator that needs to use no more than 90 percent of its capacity. That way, the generator will not have to overwork to provide energy. The best generator brands will have a wide selection of size and wattage options to choose from. Homeowners can compare portable vs. standby generators to find which would work best for their home.

  • Portable generators: These units are the smallest and most cost-effective option and can power one or two essential appliances or devices in the event of an outage. Some smaller portable generators are also handy for camping and other outdoor recreation.
  • Inverter generators: A happy medium between small portable generators and whole-house generators, these units are quiet but powerful enough to keep lights on as well as an appliance or two. They do not need to be installed alongside the house and can be moved relatively easily.
  • Whole-house generators: A whole-house generator is a permanent installation that is wired to the home’s power supply and can take over the power needs of an entire home. Some models, called standby generators, will switch on automatically as soon as they sense that the power has failed.
Generator Sizes for Standby Generators, Like This Kohler 26kw
Photo: Kohler

Consider how often you would use a generator and your power needs, and decide which type of generator is most suitable.

Homeowners who are still wondering “What size generator do I need?” will want to consider how frequently they anticipate needing backup power. Those who experience frequent and prolonged power outages may choose a large inverter or a home standby generator, also known as a whole-house generator. The size of a whole-house generator can vary, but most will be able to provide enough energy to power an entire home. They can be connected directly to the home’s circuit breaker panel to run devices that are hardwired in the home and are essential during a power outage, such as a water heater, HVAC unit, and sump pump. A generator specialist will determine the right size generator for the home, and they’ll complete the wiring job. These generators can be permanently installed and will automatically kick in when there’s a power outage to provide seamless energy. A common generator mistake that homeowners make is neglecting routine maintenance service, so it will be necessary for them to schedule regular appointments with a generator specialist to ensure the unit is running properly. It may also be worthwhile for a homeowner to invest in one of the best generator covers to extend the lifespan of the unit.

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A top-rated local pro can do it for you. Get free, no-commitment estimates from experts near you.

Whole-house generators run on natural gas or propane, can produce up to 20,000 watts, and typically cost between $1,458 and $8,239, with the installation costing between $1,500 and $5,000. Large inverter generators cost between $1,400 and $4,000 and can produce as much as 7,500 watts. These generators do not run on natural gas or propane, so it’s important to have stabilized gasoline on hand to prepare for a power outage. It may be necessary for a homeowner to prepare the house for a home backup generator by pouring a concrete pad or having an electrical subpanel installed. Home standby generators cannot be installed in low-lying areas, and they can’t be moved if there is a threat of flooding.

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Photo: amazon.com

For homeowners who encounter occasional power outages that may or may not last for an extended period, a large inverter or one of the best portable generators may be the best option. These generators can save homeowners money if they don’t mind having a generator stored in a shed or garage and manually hooking it up during a power outage. Large inverter generators are efficient and quiet but can be expensive. Some large portable generators can be connected to a breaker panel, and they’re louder than a large inverter generator. Also available are portable generator options that can produce up to 7,500 watts.

For someone who rarely loses power but wants peace of mind just in case, a recreational inverter or a midsize inverter generator has enough power to run a few essential items. These portable models can also be used for tailgating to power a cooktop or a TV. Such generators are lightweight and quiet but can only power a few small items at a time. A recreational inverter generator can produce up to 2,000 watts and can be paired together to produce more wattage. Some have smartphone apps and handy features such as fuel-level indicators. These models can’t be connected to a breaker panel, and they can only power appliances or devices that have a standard plug.

Need a generator installed?
A top-rated local pro can do it for you. Get free, no-commitment estimates from experts near you.

Homeowners will want to keep in mind that the higher-priced recreational inverter generators cost as much as a portable generator, which can power many more devices. A midsize inverter generator can produce up to 3,500 watts and can power 110-volt devices that have a standard plug, but it can’t power items such as sump pumps or an HVAC system.

For safety purposes, it’s recommended for homeowners to keep a running generator at least 20 feet away from the house and direct the exhaust away from an HVAC unit, doors, and windows.

To find the best generators for home use, homeowners can read generator reviews to see which would be the best fit—or reach out to a local pro to install a whole-house generator.

Sources: Consumer Reports, The Home Depot, Static1, Rural Living Today, Energy Today