What Size Portable Generator Do I Need?

The world runs on power—whether you’re running your HVAC system at home, adorning your RV with outdoor lights, or operating power tools on the job site, you need reliable access to electricity.

But, electrical power availability isn’t always a given. When your power goes out during a summer storm or when you need to charge electronics at a remote campsite, a portable generator can be a saving grace, keeping you connected wherever and whenever you need it.

But, shopping for a standby generator isn’t without its challenges. As you browse your options, you may ask, “What size portable generator do I need?” 

In this guide, we’re breaking down everything you need to know about portable generator sizing. In a market with nearly countless options, we’ll help you choose the perfect size by exploring the importance of portable generator size and wattage considerations, calculating your needs, and finding the sweet spot. 

Demystifying portable generator sizes can be challenging—our experts are here to help.

Why Does Generator Size Matter?

Before we jump into the importance of size, let’s break down how a generator works:

1. A mixture of fuel (gasoline or liquid propane, for instance) and air combine in a combustion chamber.

2. The fuel is ignited by a spark plug, creating a combustion reaction that forces the pistons to move down and the crankshaft to rotate.

3. As the crankshaft turns, it spins the rotor inside an alternator, which generates an electrical current—power you can use.
   
   

Regardless of size, all fuel-powered generators perform two crucial conversions:

1. Converting chemical energy (the energy stored in fuel’s chemical bonds) to mechanical energy (the energy produced when the pistons, crankshaft, and rotor move)

2. Converting mechanical energy to electrical energy (via the alternator, which spins a rotor in an electromagnetic field to produce a charge)

So, all generators do the same thing, but they produce different amounts of power (wattage) based on size. Why? The larger the generator, the more fuel it can hold, and the larger the pistons are. With more fuel and larger pistons (which produce additional energy), a larger generator produces more power.  

Understanding Wattage

Wattage is the amount of electricity required to operate a device, tool, or electrical appliance. A device’s wattage is equal to the voltage (the “strength” of the charge, measured in volts) multiplied by the current (the charge’s flow rate, measured in amps). 

Generator size is generally measured in watts—the amount of electricity they can produce. But, there are two different elements to consider when calculating the wattage your portable generator will need to provide. 

The Wattage Calculation

To determine how many watts your ideal backup generator should produce, perform a simple calculation:

Total of all running wattages for all devices you wish to power + Highest starting wattage of the motor-powered devices you wish to power = Minimum required wattage.

Let’s explore both elements of this equation in more detail. 

Running Wattage

Running wattage is the continuous power required to operate a device that’s already running steadily. For instance, once your HVAC system starts, it will require running wattage to maintain cooling or heating functions after the initial start up.

A critical part of the wattage equation above is the total running wattage of all the devices you want to power at a given time. Let’s explore an example.

If the power goes out during a summer storm, you may want to run the following devices simultaneously:

• Air conditioners
• Electric water heaters
• Refrigerators
• Televisions
• Lights
• Cell phone chargers
• Septic system sump pumps
• Well water pumps
• Computers
• Electric cooktops
• Microwave ovens
  
  

While it’s unlikely that all of these devices will be running simultaneously (if you live alone, you’re unlikely to be running your electric stove, taking a hot shower, and using the microwave all at the same time), you should choose a generator size that can hypothetically manage the simultaneous power demand of all of your chosen devices. 

Thus, you should calculate the total running wattage of all of the devices you imagine you’ll run on your generator at one time. 

Each device should include wattage information in the user manual. If you can’t find wattage information for a device, err on the side of caution and use a high wattage estimate. 

Starting or Surge Wattage

Electronic devices with motors or heating elements require extra energy for a few seconds when they start up—these are called starting watts, or surge watts.

When you calculate how many watts your generator must produce to meet your power needs, you must account for starting wattage for devices that rely on motors or heating elements, like:

• Refrigerators
• Dishwashers
• HVAC equipment
• Washing machines
• Vacuum cleaners
• Microwaves
• Electric and ceiling fans
• Blenders
• Power tools, including:
• Circular saws
• Some sanders
• Electric jackhammers
• Electric weed eaters
  
  

It’s important to make a list of starting wattages for all motored devices you plan to power with your smaller generator. But, your equation only needs to account for the device with the highest starting wattage requirement. Why?

• While you may be running numerous devices simultaneously, you’re unlikely to be starting them all simultaneously. 

• Choosing the highest starting wattage covers your bases for all of your other devices’ starting power needs. 

• Adding the highest starting wattage to the sum of all running wattages will ensure that your generator can start your devices one by one and sustain their running wattage requirements.