Understanding portable generator power limits vs. advertised wattage

All portable generators have an advertised wattage limit (or power capacity) included in their description or Spec Sheet. This number represents the maximum total electricity, measured in watts, that a portable generator can provide with any combination of two or more of its available receptacles (or outlets).

Most portable generators also have starting watts (peak or surge wattage) listed. Starting watts refers to the extra watts a generator can provide momentarily to turn on an appliance. This is a temporary burst of power that a generator can produce to help start energy intensive appliances, like A/C units.

To use an example, the GP8000E produces 8,000 watts of continuous power; and up to 10,000 watts of starting (or surge) power:

Calculating watts

Watts are calculated with a simple formula: Volts x Amps = Watts. For example, a 120-volt outlet with a 20-amp capacity will produce a maximum of 2,400 watts. 120 x 20 = 2,400.

Understanding how watts are calculated is crucial in understanding the power limits of individual outlets (or receptacles) on portable generators. The difference between 'outlet' and receptacle' is shown in the image below:

Power limits of outlets and receptacles

Let's continue using the GP8000E portable generator as an example. Looking at the image below, we see that the GP8000E includes the following outlets:

• (2) 120V, 20A Duplex Outlets
• (1) 120/240V, 30A Twist Lock Outlet
  

To understand the total power that can be supplied by each outlet, in watts, we will use our formula from above. Using the 120V, 20A Duplex Outlet as an example, we can determine the following:

Volts x Amps = Watts

120V x 20A = 2,400 Watts

This means that the entire 120V, 20A duplex outlet can produce a maximum of 2,400 watts of power. 

• If a user exceeds 2,400 watts, this outlet will be overloaded, and the breaker will trip.

We can also conclude that the 120/240V, 30A receptacle below can produce a maximum of 7,200 watts of power.

• If a user exceeds 7,200 watts, this outlet will be overloaded, and the breaker will trip.

In reality, most appliances connected to these outlets will not demand this much electricity.

For example, the average refrigerator requires 300-800 watts to run. While each individual outlet can produce a large amount of electricity, the important thing to keep in mind is that this generator can produce a maximum of 8,000 watts total. If a user is powering multiple appliances, the total electricity available is still capped at 8,000 watts. If an individual outlet is overloaded at any time, the breaker will trip and need to be reset. It's crucial to understand that breakers protect their corresponding outlets, and not the entire generator. Users are be cautioned not to regularly overload a portable generator, as this can damage the unit.

Planning

Understanding the power limits of a portable generator's individual outlets in relation to the electricity required by the appliances is key to choosing the right size generator.

Let's imagine a real-life scenario where we are intending to power an RV with our GP8000E.

• The RV has a multiple circuit 30-amp electrical system. The total energy demands of all electrical systems combined, when running simultaneously, is 7,500 watts.
• We are considering using the GP8000E from above to power the RV. The generator can provide 8,000 watts of continuous power, and 10,000 watts of starting power.
• The generator has a 120/240V 30-amp receptacle, which is compatible with the 30-amp plug from our RV.

Will this generator be able to power all of the electrical systems in the RV, using only the 120/240V receptacle?
The answer is no.

This is because the RV's electrical system demands more power than the 120/240V, 30-amp receptacle can provide. Remember that 240V x 30A = 7,200 watts.

• While the generator itself can provide up to 8,000 watts of power with all of its outlets combined, 7,200 watts is the maximum power output of this receptacle.
• Because the RV requires 7,500 watts of power when utilizing all of its electrical systems, this receptacle alone cannot meet those demands.
• However, if using multiple extension cords and incorporating additional outlets, this generator WILL be able to provide the necessary power, as it can produce up to 8,000 watts.

Why does the amperage of the outlet not match the amperage of the circuit breaker protecting it?

On certain portable generator models, the amperage of an individual outlet may not match the amperage of the circuit breaker that is protecting it. 

Examples include:

• The GP9500TF (50A outlet with a 40A circuit breaker)
• The GP3300i (30A outlet with a 25A breaker)

Image of the GP9500TF control panel, showing the 50A outlet with the 40A circuit breaker

While relatively uncommon, this amperage mismatch exists because the plug pattern is standardized for convenience (50 amp plugs and receptacles are the standard, while 40-amp plugs and receptacles are rare), but the circuit breaker is sized for the generator’s real continuous output capacity and to protect its internal components from an overload.

Using the example of the GP9500TF above:

• The generator is capable of producing up to 9,500 running watts, and 12,000 starting watts.
• The 120/240V, 50A outlet is protected by a 40A circuit breaker. If the electrical load demand exceeds 40 amps, the circuit breaker will trip to prevent an overload.
• While the outlet's plug pattern accepts a compatible 50A plug, the power output of this individual outlet is limited to 9,600 watts. (240V x 40A = 9,600 watts).
• However, 9,600 watts technically exceeds the generator's 9,500 watts of continuous power capacity. In the event that a full 9,600 watts was being pulled from this outlet, the circuit breaker may trip to prevent an overload.

For additional help in sizing a portable generator, we recommend contacting a Generac Authorized Service Dealer.