If you’ve ever wondered to yourself, “What size generator do I need,” the first step is figuring out how much total power — or wattage — you need.
This includes how much power you need to start up your equipment (starting wattage) and to keep it running (running wattage). In addition to generator power, you may also want to consider other factors like physical size, noise level and fuel type.
In this guide, we explain how to determine the correct generator size, how to calculate total wattage and what factors to consider when choosing a generator.
How to Determine Your Generator Size
Generators work by providing electric power in case of an outage. To calculate your power needs, take inventory of the devices that you want to run on the generator. Maybe you need a generator to run heavy machinery at a construction site. Or perhaps you want a generator set to power your whole house or an entire building during a power outage.
Since total wattage determines the correct generator size, make sure to have a solid understanding of what devices you need to power and the energy requirements of each.
1. Take Inventory of What Your Generator Will Need to Power
Appliances typically have wattages stamped on the bottom, back, or nameplate of the unit. Devices can have a range of power settings, so the actual amount of electricity consumed may vary depending on the setting being used. For example, a fan running at high speed may use more power than a fan running at low speed.
If you cannot find the wattage on the appliance, check to see if the unit lists amperes instead. If so, you can find the wattage by multiplying amperes by voltage using the formula:
WATTS = VOLTS x AMPS.
If you have a device listed at 10 amps that uses 120 volts, you will have a wattage of 1,200 watts (10 x 120 = 1,200).
If the device does not list amps or volts, you can check the owner’s manual or manufacturer’s website to find the wattage of your specific product. Regardless of how you discover your unit’s electrical rating, it’s an essential step for determining your energy usage (and costs), ensuring the safe and efficient operation of your device, and calculating the correct generator size.
2. Calculate Your Total Wattage
To select the right generator size, you’ll need to know both your required running watts and starting watts.
- Running (Rated) Watts: Running watts refer to the continuous energy needed to keep your devices running
- Additional Starting (Surge) Watts: Starting watts refer to the extra watts needed for the first few seconds that it takes to power up motor-driven devices.
Disclaimer: These are estimates only. Please refer to your tool or appliance’s nameplate for the exact wattage or check your owner’s manual or the tool’s manufacturer.
|Tool or Appliance||Running (Rated) Watts||Additional Starting (Surge) Watts|
|Air Compressor 1½ HP||2,500||2,500|
|Airless Sprayer ⅓ HP||600||1,200|
|Table/Radial Arm Saw – 10 inches||2,000||2,000|
|Central Air Unit||1,500||4,500|
|Color Television – 27 inches||500||–|
|Dishwasher – Hot Dry||1,500||1,500|
|Electric Drill ½ HP||1,000||1,000|
|Electric Stove – Single||1,500||–|
|Electric Water Heater – 40 Gallon||4,000||4,000|
|Miter Saw – 10 inches||1,800||1,800|
|Planer/Jointer – 6 inches||1,800||1,800|
|Personal Computer With 17” Monitor||800||–|
|Quartz Halogen Work Light – 300||300||–|
|Table Fan – 14 Inches||200||400|
|Water Well Pump ⅓ HP||1,000||2,000|
|Window AC – 10,000 BTU||1,200||1,800|
3. Choose the Right Generator Size
To know how much generator power you require, add the total running watts of all devices you want to run at the same time to your highest single starting watts. You need a generator that can handle that amount without using 100% of its wattage. According to the U.S. General Services Administration, generators should be sized to approximately 110% of design load, but should ideally run at a range of 50% to 80% of their rated capacity.
Generators in the U.S. are typically rated in kilowatts (kW), which refers to the amount of actual power or working power that a machine produces, and kilovolt-ampere (kVA), which reflects the apparent power, or the amount of power a device uses.
The two get confused because in a world of perfectly efficient systems, kW would be the same as kVA, but because no electrical system is 100 percent efficient, kW tells you the useful power the system converts into actual power output. So to avoid confusion, using the operating power (kW) plus voltage, you can estimate the output amperage of a generator.
Factors to Consider When Choosing a Generator
While wattage determines the size generator you need in terms of power, other considerations may also affect your final decision.
The main consideration when determining the correct generator size is your total wattage required. You’ll want a generator big enough to safely and efficiently meet your energy needs, but you also don’t want to waste money and fuel running an unnecessarily powerful generator.
Generators come in different power capacities, so you’ll want to consider if you need a small, medium-sized, or powerful industrial unit. For comparison, small generators can power 20,000 watts (20 kW), large generators can power about 100,000 watts (100 kW), and extra-large generators can power 240,000 watts (240 kW) and higher.
To help you choose the right generator, we’ve compiled a handy list of different-size generators and their common uses.
- 1 kW to 10 kW: If you’re looking for a backup generator, one in this size range can help you run electricity through a few rooms of your home. A portable 5 kW generator allows you to operate a refrigerator, four lights, a fan, and an electric motor. For the average-sized home, however, you’ll probably want at least 10 kW of power. Most of these smaller generators run on natural gas, propane, or diesel fuel.
- 10 kW to 50 kW: Generators in this class permit homeowners to run more appliances simultaneously. Perhaps you have multiple energy-intensive devices such as HVAC units, water pumps, and washing machines that you want to use all at once. If so, you may opt for a propane, diesel, or natural gas generator in this size range.
- 50 kW to 100 kW: Typically diesel-powered, these units make a good choice to provide backup power generation for offices and restaurants.
- 100 kW to 200 kW: This class of generators can provide power solutions to industrial sites, larger offices, or any establishment with a significant number of appliances like computers, lights, fans, and air-conditioning units. They take both diesel and gas, and can also power fishing boats.
- 200 kW to 300+ kW: If you need to keep the power on 24/7, industrial generators of this size make a good choice. These large generators run on diesel or gas and can provide energy sufficient for large industries and high-rise buildings equipped with elevators and/or escalators.
Different types of generators run on different types of fuel, including gas, natural gas, diesel, or propane. Each kind has both pros and cons. For example, diesel generators may cost more upfront than gasoline models but have better fuel efficiency and can save money in the long run. While propane generators can burn through fuel three times faster than diesel, propane excels in cold weather and is available even during outages when diesel and gasoline pumps may not work.
Physical Size of Generator
Most types of generators fall into two main categories: portable and standby. Portable generators are easy to move. There are small versions that you can take on a camping trip or store in your garage for home use. Another type of portable unit, towable diesel generators come with wheels and are designed to hitch directly to the back of a vehicle.
Standby generators stay in one spot but have the capacity to provide emergency backup power for houses, construction sites, and large buildings like hospitals.
Type of Generator
Generators come in all different types, sizes, and energy sources. A small diesel generator might supply electricity to a single power saw, while a large propane unit might serve as a home standby generator in case of blackouts.
Generators fall into the following categories:
- Inverter generator: Inverter generators use special technology to create clean, stable energy. These models tend to be quieter, more fuel-efficient, and safer for charging sensitive electronics like laptops or phones.
- Portable generator: These smaller units can supply power for less energy-intensive tasks, such as charging your phone on a camping trip or powering an electric tool while working at home. While the more compact size makes them transportable and affordable, portable generators cannot power an entire home or construction site.
- Standby generator: These larger, stationery units can serve as whole-house generators and even power entire buildings in case of an emergency. They can also turn on automatically to assure no downtime. Their large size (typically bigger than an air-conditioning unit) often also comes with a large price tag.
- Towable diesel generators: Designed to hitch right to the back of your vehicle, towable diesel generators can go with you wherever you need power. Whether your construction site is completely off the grid or you need to supply electricity for an outdoor event, these mobile units offer a portable, fully integrated power system.
Generators can make substantial noise, so you’ll need to factor in where you intend to use them. Large generators tend to be louder, while smaller (or inverter) generators tend to be quieter. If you’re working on a construction site, noisy generators may not pose a problem. However, if you plan to use them at an outdoor festival or in a suburban area with noise restrictions, you may need a quieter model. Some manufacturers include advanced noise-reduction features, or you can purchase a separate generator silencer.
Where Are Generators Used?
Generators perform all sorts of functions, from giving us access to modern conveniences while camping to providing life-saving medical care during emergencies. This list reveals the wide variety of ways generators can be used:
- Hospitals and medical centers: Standby generators provide a critical backup power supply to hospitals and medical centers in emergency situations. They help the lights stay on in operating rooms, keep vital machines like respirators and heart monitors working, and prevent refrigerated medications and food from going bad.
- Homes: Home generators can keep your house from going dark during a storm, and even enable you to do laundry, run your AC, and cook dinner on an electric range at the same time.
- Construction sites: Portable generators allow construction crews to have electricity even where no outlets are available and power different machines at different locations on the job site.
- Events: Generators can provide an energy source at outdoor events, powering lights, heating devices, audio and visual units, cooking equipment, and more.
- Campsites: While you might like to spend time in the great outdoors, you may enjoy a few modern conveniences at the same time. Portable generators can power up your RV’s appliances, light up your campsite, and allow you to charge your devices.
FAQ About Generator Sizing
Have other questions about choosing a generator size? Here are answers to some of the most common questions. You can also check out our free e-book to learn more about choosing a generator.
Download our FREE generator rental guide e-book today!
What Size Generator Do I Need for My House? (Chart Included)
If you’re looking for a generator to power a house, you first need to know what appliances you’ll need to power. Here are a few rules of thumb to follow when deciding on a generator:
- A 5,000-watt generator is enough to power the essentials — lights, refrigerator and freezer.
- A 15,00-watt generator is enough to provide a more comfortable experience — lights, refrigerator, freezer, central AC, water heater, oven/stove, microwave and TV.
- A 25,000-watt generator is enough to power most household needs — lights, kitchen appliances, central AC, water heater, washer/dryer, garage door, and household electronics.
What Size Generator Do I Need to Run a Refrigerator?
On average, refrigerators require about 2,200 starting watts, so any generator with at least this much power will work. To be safe, you may want to purchase a generator capable of producing at least 3,000 watts.
What Size Generator Do I Need for My RV?
An 8,000-watt generator will be enough for most RV needs. It will power the lights, AC, fridge, microwave, a phone or tablet charger and a laptop charger.
What’s the Difference Between Watts and Amps?
Watts and amps (amperes) are different measurement systems for energy — think yards versus meters. If you find an item with its energy needs listed in amps when trying to estimate your energy needs for a generator, you can convert it to watts with this equation:
Watts = Amps x Volts
In this equation, volts will almost always be 120V, although some power tools do run at higher voltages. You can also use our ampere conversion chart.
What’s the Difference Between Resistive and Reactive Loads?
Resistive loads draw a consistent amount of energy from the generator. This includes appliances like light bulbs, a coffee machine or a water heater.
Reactive loads draw a variable amount of energy from the generator. This is often due to motors that require an extra energy draw to start up, as with refrigerators, freezers or washing machines. Refrigerators also use an internal fan that occasionally turns on to keep a consistent temperature.
If you do calculations manually, remember to account for additional energy needs in your starting watts numbers.
Rent a Towable Generator
Hopefully, this guide has put the question, “What size generator do I need,” out of your mind. Remember to properly maintain your generator after getting it.
If you need a portable power source to light up your site or run machinery, consider renting a towable diesel generator. From 20 kW to 240 kW, we have a variety of generators available in different sizes to meet your power requirements.
So whether you need to power a single electric saw or an entire building, you can hitch the right generator to your vehicle and transport a convenient, efficient power solution to your location.