Solved! How Big of a Generator Do I Need?

When the electricity goes out, an emergency generator can power your home without having to put your life on hold. Are you googling, “How big of a generator do I need?” Here’s how to find the answer.

By Katie Flannery | Updated May 18, 2022 11:47 AM

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how big of a generator do I need


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: Many 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 so you don’t have to experience frozen pipes, spoiled food, flooded basements, mold and mildew, or extreme temperatures. And if you have a medical condition that requires equipment powered by electricity, a power outage can be life-threatening. A backup plan in the event of harsh weather, wildfires, or a power grid failure means you don’t have to put your life on hold.

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 home standby generator) can power an entire house. The size of the generator that you purchase will depend on how you plan to use it. Wondering, “How big of a generator do I need?” This guide will offer information and guidance on choosing the best generator for home use and what will work the best for you and your family.

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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 you want to operate.

  • 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 calculate how much wattage is needed to power everything. You can also look up the surge wattage to help determine generator size.
  • Surge wattage. Surge wattage is the highest amount of wattage that a generator can produce. If you want to run all of your appliances and devices 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.

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Electric current is the total amount of energy passing through a wire over a given period of time—but there are a few different measurements that measure how the energy passes. When you understand the difference between watts, amps, and volts, you can recognize the differences in electrical systems. If you’re having a whole-house generator installed, you won’t need to do any of this math yourself—your generator installer will take care of it all for you—but knowing these measures can help you make a more informed decision.

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. When a charge is moving from one point to another, this difference 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, is how strongly a charge flows, measured in coulombs per second. In the river analogy, amps would be how much water passes a certain point in a given period of time. You may see amps and volts 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 your home, the current will power whatever the generator is connected to. Wattage measures how much power a device actually uses. You can measure wattage 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.

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To determine whole-house generator sizing, you need to determine the number of appliances and electrical devices you want to run and calculate the total wattage. First, list all the appliances you want or need to be running during a power outage. Next, write down the number of watts that are required in order to power up and run the devices. You can find this information in the owner’s manual or on the machine itself. It’s recommended to use a generator calculator to determine an accurate wattage estimate or have an experienced electrician calculate the exact wattage you need for your home. The total wattage you determine you need shouldn’t be more than the wattage output of the generator you buy.

If you’re concerned about overloading the generator, consider staggering the use of certain appliances, or unplug some devices if you’re not using them at that moment. Don’t forget to factor in items like 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:

  • Coffee maker: 400–800 watts
  • Microwave: 1,200 watts
  • Refrigerator/freezer: 600–800 watts
  • Space heater: 1,250 watts
  • Toaster: 1,100–1,700 watts
  • Electric oven: 5,000 watts
  • Personal computer: 500–2,000 watts
  • Television: 100–350 watts
  • Table lamp: 150 watts
  • Hair dryer: 1,200–1,500 watts
  • Cell phone battery charger: 10 watts
  • Video game console: 200 watts
  • Washing machine: 750 watts
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The approximate wattage requirements for a home’s heating and cooling systems are:

  • Central air conditioning: 2,000–4,000 watts
  • Window air conditioning unit: 600–1,500 watts
  • Electric furnace: 5,000–25,000 watts
  • Water heater: 3,000–4,500 watts
  • Sump pump: 1,500 watts
  • Radiant heater: 1,300 watts
  • Outdoor lighting: 500–1,000 watts

Keep in mind that these running wattage requirements are approximate; you should refer to the actual appliances or owner’s manuals for the manufacturer-specified wattage requirements. Adding the total number of starting watts together will typically be higher than what you need your generator to produce. Once you have an estimate of the highest wattage usage that you’ll need to power your home during an outage, add another 100 or 200 watts just to be safe.

Keep in mind that a bigger generator is not always the best option. Choosing a generator that can meet your basic needs will save on the cost of fuel. Getting the correct-size generator for your home 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, your 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 avoid costly repairs and ensure you get your money’s worth out of your investment.

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

how big of a generator do i need


Once you have the total number of starting watts that you need from a generator, you can determine the best size for your needs. Keep in mind not to overload the generator to prevent it from overheating and automatically shutting off. To do this, 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 to power your home.

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

If you experience frequent and prolonged power outages, consider a large inverter or a home standby generator, also known as a whole-house generator. These can provide enough energy to power your entire house. They can be connected directly to your home’s circuit breaker panel to run devices that are hardwired in your 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 your 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 generator specialist will also typically perform regular maintenance checks to ensure the generator is running properly.

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Whole-house generators run on natural gas or propane, can produce up to 20,000 watts, and typically cost between $2,000 and $6,000, with the installation costing between $500 and $3,00. 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 you’ll need stabilized gasoline to prepare for a power outage. Keep in mind that 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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If you encounter occasional power outages that may or may not last an extended period, a large inverter or a large portable generator may work for you. These generators can save you money if you 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 also be connected to a breaker panel, and they’re louder than a large inverter generator. Some portable generator options can produce up to 7,500 watts and can cost between $700 and $2,800.

If you rarely lose power but want 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. These 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 they cost between $400 and $1,000. They can also be paired together to produce more wattage. Some have smartphone apps and handy features like 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.

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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 costs between $1,000 and $1,700. This efficient generator can power 110-volt devices with a standard plug, but it can’t power items like sump pumps or an HVAC system.

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

To find the best generator for home use, consult our list of favorites and the reviews behind each—or reach out to a local pro if you’re considering a whole-house generator.

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

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