How Many Watts Do I Actually Get to Use with My Portable Generator?

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Once you have determined your wattage need you will want to consider what your generator will actually have available for use in outlet/receptacle combiniations.

If you haven’t yet looked at other factors that influence wattage output you do need to do so and which I cover in this post: Watts Needed for Your Generator?

Limitations of Circuits and Receptacles

There are limitations to what you can actually get out of your portable generator. It is based on several considerations such as the receptacle output and rating, the circuit breaker amperage, and the combination of circuits and output.

You will need to make sure you understand what output you are getting. Some generators will allow you to use a combination of outlets to be able to use the full output, others will only let you get the full output on a larger twist lock such as an L14 receptacle.

So let’s build a ground up way to look at the generator usable output under consideration.

There are two wattage levels stated for generators which are the rated continuous and the surge. You will focus on rated continuous first and then surge as a second consideration for some appliances.

Typical NEMA 5-20 Receptacle Output

As stated in the post on Watts the formula is this calculation:

Voltage x Amps = Watts

WH7500E-outlets

5-20R and L14-30 outlets

The duplex or 5-20R receptacle is the normal standard for most 120V systems which you will find in portable generators. It’s what your home appliances and outlets use.

Given that this is a 120V (125V rated) output the maximum wattage available in a plug set (2 outlets per set, 1 circuit) will be restricted by the circuit breaker tied to the 2 plug receptacle.

Since it is a 20 amp receptacle, the circuit breaker will be 20 amps or less. Your output for this circuit (which can supply more than one receptacle (duplex and part of an L14 twist lock) is what restricts the maximum watts available up to the generator output maximum.

Note that A circuit can surge above the rated wattage for short periods and this is tied to appliance that need that short burst of extra power to get started.

Here are some typical continuous wattage based on a few circuit breaker limitations.

Typical Circuit Limitations based on amps
Volts Amps Watts
120 30 3600
120 25 3000
120 20 2400
120 15 1800
120 12 1400
120 10 1200

Although an outlet may have a rating for higher amps, the circuit breaker will limit the available wattage output which is usually tied to the generator output capacity.

L14 Receptacle Output

NEMA L14 Receptacles

NEMA L14 Receptacles

Example: An L14-30R has a 30 amp rating, but if the generator only has a 25 amp circuit at 120 volts you will only be able to use 3000 watts rather than 3600 watts. And respectively at 240 volts it’s 6000W rather than 7200W.

Additionally if your generator has an underpowered engine and you approach the max rated output your available wattage may be lower than the circuit breaker allowed. Most generator manufactures try to make it as close as possible, but sometimes you can come up short. At higher elevations, with a 3% to 3 1/2% loss of power per 1000 foot elevation, it can be more pronounced. In the mile high city you could lose 15% plus to of the rated output (3000W now is only 2550W or less)

So far you’ve face a few factors that can lower your available power due to circuit breakers, engine performance, and elevation. However, there is another consideration and that is dealing with the dual circuit and load balancing.

Portable Generator Circuits Using Multiple Outlets

We touched on the issue of circuit breakers before and on circuit for individual plugs and the possibility of a second circuit breaker for multiple plugs. Let’s take a bit of a closer look.

Multiple circuits

Portable Generator Panel Showing Multiple Circuits

Each manufacture may provide a different wiring of the outlets. As an example you may have to switch to be able to use 120V or 240V circuits. Or it may be wired that the individual circuits are wired to provide 120 to duplex, but still allow 240 to an L14 outlet.

Another might wire 3 sets of plugs into a single circuit with a duplex (5-20R), a TT RV socket, and a L5-30R. If your full circuit allowed only 25 amps, that is your first restriction. But if you used a full 15 amps on the duplex you would have 10 amps left on the other 30 amp rated plugs. Still enough for 1200W but not likely enough to power major equipment.

Turn that around and you could power a major appliance and then still have enough in small appliance on the duplex plug.

So your restriction is really tied to a combination of plugs on any one given circuit and circuit breaker.

This consideration of how you’ll use plugs/ transfer boxes, and appliance load becomes a bigger concern for heavy power users.

Briggs and Stratton even shows a wiring diagram where it would appear that each top and bottom of a single standard duplex dual plug set is wired to a different circuit. Where many manufactures such as WestPro put a grouping on the control panel to indicate plug set circuit grouping the B & S has circuit breakers in line with the individual top and bottom outlets.

Unless you dig into the physical wiring or test the plugs you don’t know for sure. In many cases the wiring diagrams are not available so you have to make assumptions. This may cause you to add margin in your needed wattage calculation.

The higher the wattage and number of plugs the more complicated it can be. However, in general if there’s no voltage switch all plugs may be used up to a total of the rated wattage. If a switch is included, you can expect some restrictions on which plugs are usable. The owners manual should help make it clear, but that is not always the case.

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Summary

In many cases you won’t have to worry about the plug sets and what is available for use unless you become a full power user and are constantly pushing your unit to the limits on output.

When you need full power of a portable generator unit then the combination of circuits, circuit breaker size and outlets can become a limiting factor.

For the most part the generators are sized to have circuit breakers and output closely related, however, the combination and use of outlets can be a limiting factor based on how the power is being used by each outlet.

Consider this if you have two circuits to your duplex plugs limited at 20 amps each the maximum available output is 4800W or the generator maximum output or circuit breaker whichever is less. As an example if your rated output is 3600W, it is likely you will be limited by this maximum and the circuit breaker may be only be 15 amps.

Do not rely on the outlet rating, but on the circuit breaker and generator output adjusted for other influential factors.

In our reviews we find that most manufactures provide a set that allows for maximum use, but it is not always clear on how you can use it. In the larger wattage models switches will often times restrict the use as to not to overpower the circuits. In the small units all plugs will often be available, but you’d better understand the additive nature of total power use and where you are drawing that power from.

Hope this helps shed some light on the issue of output restrictions.

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