Buying Guide

Choosing the Right Size Generator

Types of Generators
Small generator powering a light at a campsite.

Generators come in several different wattage sizes. Although there are solar generators and battery generators, most use propane, natural gas and diesel fuels. There are basically three kinds of generators to choose from. 

  • Portable generators are powerful enough to provide electricity for outdoor activities such as camping, tailgating and other events. They’re easy to take just about anywhere and relatively affordable. They can also provide emergency power in the event of a blackout or storm.
  • Inverter generators provide electricity but with less noise and more fuel efficiency than other generators. Inverters also convert their AC power to DC power, making them ideal for RVs and battery-operated devices. 
  • Whole house generators provide a backup for your home’s power needs in case of a blackout or storm. They are hardwired to your home. They can quickly provide the power your home needs in case of an interruption to your regular power grid, whether through automatic generator transfer switches or manual generator transfer switches.

Tip: Never use any generator inside your garage, home or other structure, even with the windows or doors open. Carbon monoxide is extremely dangerous and virtually undetectable.

Understanding Starting, Running and Surge Wattage
A portable generator on wheels sitting outside a red barn.

The wattage or power your generator needs to produce will depend on the quantity and type of items or equipment you want to run. Before you can calculate your needs, you need to understand what starting wattage, running wattage and surge wattage are. 

Your running wattage is the amount of electricity you need to keep the device running. As the name implies, your starting wattage is the amount of electricity you need to start the device running. On average, the amount of power you need to start a device is usually three times the amount you need to keep it running. Surge wattage is the most power you can get from your generator. Now that you understand how wattage works, you can figure out how to choose the best generator size for your needs.

Safety Note: Protect your generator and your home with a circuit breaker. If a generator is not using a circuit breaker, it will eventually overload and overheat. If the generator is not shut off once overheated, it could catch fire. 

Calculating Generator Sizing
A home standby generator placed next to a brick house.

Most generator owner manuals include examples of what kinds of structures and devices they can provide power for. You can also calculate it yourself for smaller needs using the following formula. 

Identify the device you want to power. Check the product’s owner’s manual to determine how much wattage the device needs to run. Multiply the running wattage of the device by three. Add the running wattage to the starting wattage for the total wattage the device needs to operate.  

For example, a small refrigerator uses about 350 watts to run, so it’s starting wattage would be approximately 1050 watts, or three times its running wattage. Therefore, the total wattage you need to run a small refrigerator is about 1350 watts. In this case, you’d need a generator with a minimum of 1000 to 2000 watts. Written out formulaically it's: (Running wattage x 3) + Starting wattage = Total wattage needed. 

Tip: For best results, always use a generator that can comfortably handle all your power needs while using no more than 90 percent of its capacity. 

How Much Generator Power Do You Need?
A generator sits on a set of outdoor stairs.

If you’re wondering “how big of a generator do I need?” what you really want to know is how much generator power or total wattage you need. To determine the best generators for home use, refer to the list below and select which appliances and systems you want to power. Add all your wattage numbers together. This will give you an approximate wattage. For example, you might need something as small as a 2000-watt inverter generator or a couple of 5000-watt generators.  

Below is a list of approximate appliance and technology wattage needs:

  • Refrigerator/Freezer: 600-800 
  • Electric Range (one element): 2500 
  • Toaster: 1100 -1700 
  • Microwave: 1200 
  • Hot Plate: 1250 
  • Coffeemaker: 400-800 
  • Electric Oven: 5,000 
  • Television: 100-350 
  • Personal Computer: 500-2000 
  • Hair Dryer: 1200-1500 
  • Vacuum: 700-1400 
  • Space Heater: 1250 
  • Table Lamp: 150 

Below is a list of approximate HVAC and system wattage needs:

  • Electric Furnace: 5000-25000 
  • Heater (radiant): 1300 
  • Central Air Conditioning: 2000-4000 
  • Water Heater: 3000-4500 
  • Water Pump: 1000-2000 
  • Window Air Conditioner: 600-1500 
  • Outdoor Lighting: 500-1000 
  • Sump Pump: 1500 

Tip: Double the wattage for anything with a motor since it takes more power to start them. For an exact wattage number, consult your appliance and systems manuals.  

When it comes to choosing the right size generator, determine the usage and wattage your lifestyle or project will require. Ready to find the generator you need? We have options to deliver online orders when and where you need them. You can also rent a generator from your local Home Depot.