Most generators on the market have a number in their model name, which we assume is the wattage that they can provide. But what if I told you this isn’t the case?
Upon further inspection, we find that each generator has two wattage values in their specs: running watts and starting watts.
What do these values mean? How do they differ? And how do they impact your choice of a generator?
In this article, we’ll go through some physics basics before explaining the difference between running and starting watts, and giving you some advice on how to choose the correct wattage for your portable generator.
You may want to skip this section depending on your knowledge of physics, or simply because you just want to know the basics and aren’t bothered with the basics behind it. However, for the purpose of completeness, I am adding this short refresher of what work and power are and how we quantify them to arrive at a good measurement of what we would commonly call effort.
While we all understand the concept of work as something that takes effort, quantifying effort is somewhat challenging. After all, it takes effort to do many different things – Move an object, heat water, perform a chemical reaction, etc. Yet, we need to use the same scale for all!
Luckily, at least for our brief intro, all we need to be concerned with is that if we move an object from a place A to a place B, it requires work (opens in a new tab) . This we can all agree on. Thus, we conveniently quantify work as the force necessary to move an object multiplied by the distance by which we have moved it.
Work Formula:Where W=Work done (Joules), F=Force (Newtons), d=Distance (meters)
This formulation gives us the following results: It takes more effort to move a heavier object and likewise, to move the same object further. It doesn’t matter if it is done by a human, water, steam or electricity, nor does it matter if we are moving a rock, our arm, or a rotor.
However, this formulation fails to address that there is a difference between moving an object instantly, in a minute, and across an hour, with three short breaks in between. Therefore, to more accurately describe effort, we must account for the time across which we perform the amount of work.
This is done by simply dividing our work by time – The faster we perform work, the more effort it takes. This measurement of effort is called power (opens in a new tab) , measured in watts – and therefore commonly dubbed wattage.
Power Formula:Where P=Power (Watts), W=Work done (Joules), t=Time taken to do the work (seconds)
Since humans and electricity perform the same work by different means, we must allow for some reformulations where needed. Without going through the mathematical explanation, we shall simply state that equivalently to human power, electrical power can be calculated by multiplying current (amps) and voltage (volts). Even though these two formulas seem very different at a first glance, both human watts and electrical watts can move the same rock or engine rotor, across the same distance, in the same time.
Electrical Power Formula:Where P=Power (Watts), I=Currrent (Amps), V=Voltage (Volts)
*Note that this formulation is the most basic one
This gives us the interconversion and some basic understanding of how much power can be provided by electricity. By adding energy to the mix, we can also to explain phenomena such as heating or providing light using the same toolset, though we’ll leave this to the interested reader’s own research and move on to the topic of our article.
With that out of the way, let’s move to the meat of the topic, which is the difference between running and starting watts. As briefly as possible, for any given electrical appliance with an engine:
Starting watts are the wattage needed to get a device running, or in other words, the work over time necessary to create momentum required by a device to run. A generator can provide this wattage temporarily, in a brief surge (which is why the starting watts are also called “surge watts” or “peak watts”).
Running watts are the wattage needed to keep a device running, or in other words, the work over time necessary to keep its momentum from falling. A generator can provide this wattage continuously.
After taking time to explain what wattage is, I hope that this wording will leave everything clear, rather than leaving you with unanswered questions. In short, it takes less effort to keep something moving than to actually get it moving. Remember the last time you had to push start your car for further reference.
Conveniently, generators enable you to exceed their maximum running wattage for a brief moment (opens in a new tab) without triggering their circuit breaker, so that you can actually start a device, even though you are already running your generator at a significant capacity.
Since generator manufacturers are in the business to sell us their products, they usually choose starting watts as the value we first see when choosing a generator, simply because it is the higher of the two. Only after reading the actual specs will you find the wattage that a generator can continuously provide.
While this trick is somewhat cheeky, since one would expect that a 4000W generator will run 4000W worth of appliances, starting watts are not a useless number and still must be considered. You need to have enough power to start all your devices after all!
But fear not, my friends! Even though the two wattage values may seem to unnecessarily complicate our lives, they are here for our own good and help us overcome unwanted circuit breaks. Counting them in is pretty simple.
The most straightforward way to go about this is the following:
You can use a wattage calculation sheet, such as the one above from LCEC (opens in a new tab) , or simply use our generator sizing tool, which allows you to add custom appliances.
To not neglect any points, I will also mention a few useful side notes:
To figure out the running and starting watts of any appliance, you must first understand the type of electrical load that they represent to determine if they’ll need additional starting watts. We can distinguish three main types of load:
If your device falls under the 1st (a light bulb, radiant heater, kettle, etc.) or 2nd (a laptop/cell phone charger, etc.) case, you’re pretty much all set. Your device won’t need additional starting watts and you can simply calculate your running watts by multiplying amps by volts.
If your device falls under the 3rd case, it most likely features some sort of an electrical motor or a compressor. You’ll also have to consider that in addition to the running watts, you’ll have to calculate the starting watts.
There are three ways to go about this: a lazy one, an easy one, and a more complicated one.
The lazy way will not be the quickest nor necessarily the easiest one. It simply consists of getting in touch with your appliance’s manufacturer to ask them for its running and starting watts.
The easy way, which is the one that we recommend, is to get a good quality clamp meter that has the ability to measure the “inrush current”. This will allow you to easily see the amps that are drawn upon startup. Simply attach the clamp to the appropriate wire (use an AC line splitter if needed), select the “inrush” function of your clamp meter and start your appliance to get a reading. Write down the amps and multiply them by the voltage to get the wattage.
The more complicated way consists of looking at the data plate of your appliance.
If you decide to follow this route, you’ll need to know the following terminology:
Roughly speaking, the RLA would be your running amps, and the LRA your starting amps. In fact, it’s a bit more complicated than that. LRA are defined at full voltage. However, when a generator is used to start a motor, the surge of current causes a drop in the voltage, which lowers the LRA. As we won’t get into lengthy details here, we recommend you this read (opens in a new tab) to learn more about the topic.
In summary, we have explained that power, or wattage, is a good measurement of the effort it takes to start and run your appliances. Naturally, starting something from the scratch requires more effort than simply keeping it running and this must be accounted for when buying a generator, which can provide an additional, but limited surge of power – hence the difference between starting and running watts.
It is important to never rely on the number in a generator’s model name, since this is more often not the power that can be provided continuously. For marketing purposes, this number usually refers to the starting watts. Instead, you must check the generator’s specs for running watts, which are the actual value of continuously provided wattage.
However, neither can you neglect the starting watts, since you may need the additional surge of power to start your appliances. Refer to our three-point list of how to account for both correctly.
Did you calculate your power needs before buying your generator? How did you do it? Lets us know in the comments below!
Manager & Editor of generatorbible.com. Early retired from the OPE industry, living in South Carolina. He now mostly spends his time traveling and taking care of his wife and grand-children.
8 CommentsWhen figuring size of wire for an interlock switch, should I consider starting watts or running watts?
Great Valuable information. It is immensely appreciated.Thank you for the great explanation and theory behind generator wattage terminology. It helped me considerably.
Good basic info. Question: Can a product like Watt Rescue be used with a portable generator to “clean” or improve the THD. This type product uses capacitors to smooth out cycles. Thanks in advance for any info.
Edward Gerry October 27, 2020 at 8:07 amHi so my tranfer switch is 30 amp but the generator states 9000 start up 7250 running. Should the 30 amp set up be changed to 50 watts?
Boone County Generators January 31, 2022 at 12:34 pm7250 watts at 240 volts is just a hair over 30 amps. (7250/240=30.2)
Your 30 amp transfer switch can handle slightly higher current for brief periods (motor startup), so I think it’s an appropriate size for your generator.
