Generator Sizing | Wattage Requirements

Have you ever thought about how reliant we are on electrical power? There are very few things we can do without it today; life would be almost impossible. But that is when generators come in handy. Whenever you need a source of power, you can rely on them to save the day. Whether you live in an area that often has storms and power outages or you just want to make sure that your day to day life can run as usual even with no power, a generator is a good choice. The first question you will probably ask yourself is, “What is the proper generator size that i need?”

Unfortunately, there is no one answer to this question. There are many variables when it comes to choosing the right generator size. However, in this comprehensive guide, we will cover them all to help you make a proper decision and stop you from wasting money. Although the following generator sizing guide will give you some idea about the average size range, however, no online manual is a substitute for a licensed electrician who can accurately calculate your electrical load requirements.

generator size capacity

Choosing a Generator Size 101

The size or power of a generator is determined by many factors, including the size of your space, the type of fuel incorporated, and of course, the load requirements of your applications. Taking into consideration some extra load growth. There are some steps that you should consider when it comes to choosing a proper generator size.

  • So, the first step is deciding what you are going to use the generator for. You can choose to use it for your home, business, RV/camping, or even outdoor events.
  • The next step is to make a list of what exactly you want to power and when.
  • The third step is to find out the running and surge wattage of everything you want to power with the generator.
  • Finally, the total addition of the watts will tell you the minimum output required from your generator. (You can also add to the total 25% more to anticipate future needs).

Of course, it would be lovely if the sizing process is more simple than that, but there is a bit more to it. So, instead of jumping into taking decisions, keep reading to find out absolutely everything necessary.

Explaining the Steps

Before we get into why the above-described steps should be taken with a grain of salt, let’s first clear some things up.

Why should you choose what you will power and when?

calculate the total wattages

Every appliance or device has a specific wattage (eg. a dishwasher has a running wattage of 1200). This number is what you need to know to calculate the minimum size of the generator.

When you will power these applications is also essential. You can add up the wattage of every appliance/device you have. But the fact is that your TV, microwave, and dishwasher are not going to work at the same time. So, if you add the wattage up, you may end up with a much larger generator than you actually need.

Instead, when the dishwasher is off, something else can be powered. So, turn the dishwasher off when you use the microwave, for example. This way, you will be able to avoid paying extra money for a large generator that you may not actually use its full potential.

What are the running watts and surge watts of appliances?

These are also important for calculating the generator size. Running wattage is what an appliance/device needs to work. However, some appliances also have a surge wattage, which is the amount of power they need to start up their internal motor. Surge watts doesn’t apply on all applications; some devices require the same wattage to start and continue working. While other applications will require extra wattage when starting, mainly applications that has an electric motor.

For example, a computer has a running wattage of 600 watts, but it does not need any additional power to start. However, a house fan needs 600 watts to start and only 300 watts to keep running. Think of the surge watts as extra energy for the appliance to begin operating. This additional energy is only required momentarily then it goes down to lower stable running watts.

So, this is important because you need to take into account that the fan will initially pull 600 watts to start working. But then it will require only 300 watts to continue. And the other 300 watts can go into powering something else. As a result, it is vital that the initial starting wattage is taken into consideration in your generator sizing process. Primarily, that starting watts can sometimes be two or three times more the running watts.

What are the maximum watts and rated watts of generators?

wattage of generators

Generators also have surge and running watts. Usually, you will find manufacturers advertising for their new models with the maximum/surge watts. To learn more, let’s explain what these outputs are.

Surge watts for generators represent the maximum power output they can produce. However, this power is limited, and it can last for a maximum of 30 minutes. This additional power limit is useful for the extra wattage required to start up a machine or a device. And as we explained in the above section that appliances require this extra starting energy only momentarily.

On the other hand, rated watts for generators represent the continuous power output they can produce. Usually, the rated watts are 80-90% of the maximum watts. You should always build your sizing calculations on the rated watts of a generator to get adequate continuous power for your needs.

How to find information about the surge and running wattage?

The wattage information is usually written somewhere on the device or in the user manual. Keep in mind that sometimes there is only one number which takes both wattages into account or there is simply no surge wattage for the device. If there is no information about wattage, it is enough if you find information about the amps, then you can calculate the running wattage with the following formula:

Watts = Amps x Volts

Amps = Watts / Volts

Volts = Watts / Amps

The voltage differs depending on the place you live, but it is typically 120 volts in the US. With these formulas, if you have any of the two numbers, then you can find out the third.

It’s also important to note that most electrical motors will need additional surge/starting wattage which can be up to three times it’s running wattage. Also, old machines tend to use more than the indicated power as they become less efficient over time. Remember, you can also use an appliance load tester to determine the exact rated watts needed for each application.

How to calculate the approximate size needed?

Since there are a lot of variables, we understand that determining a generator size can be difficult. But there is one recommendation that can help you. After you determine the running wattage for most of the appliances/devices you are planning to use, add them up, and then add the highest surge wattage for the highest appliance/device that draws the most power. We do this to be sure that the generator has enough power to start up even the biggest appliance/device of them all. This is how you determine the average generator size you need, and you should definitely not go below it.

If you schedule the use of your devices well, you can get by with the minimum need. But, if your budget allows you, it would not be a bad idea to go a bit beyond the minimum. This way, you can be more relaxed about seasonal loads or other extra device usages.

Now that we cleared all the questions regarding these basic steps. Let’s look into what your options are in some specific situations.

Available Sizes of Generators

First, we have inverter generators. They range from 1,000 watts and up to 6,000 watts. This range is usually enough to power most essential appliances in many situations. They have the advantage of being quieter, smaller in size, and also some models can be paralleled with other portable inverter generators to achieve more power output.

Next, we have portable generators. They range from 1,000 watts and up to 20,000 watts. These are the most popular, and they are widely used in many situations. Portable generators have the advantage of being more budget-friendly than other types and offer wider wattage flexibility.

Lastly, we have standby generators. They range from 5,000 watts and up to 50,000 watts. They are widely used in households, industrial/commercial businesses, and in places prone to frequent power outages. Standby home generators have the advantage of being able to supply uninterrupted power for extended periods and they have the capacity to power many applications.

  • Households & Cabins

What generator you will choose for your home depends on two factors. The first is what you want to power (so everything we mentioned above) and second is how often you will actually use it.

If your home experience power outages once or twice a year, you may not want to spend a lot of money on a standby generator, but still would like to be sure that the essentials will work. In this case, you can estimate an average of 3000 and 5000 watts. Or, if your home relies on large electrical heating/cooling systems, a well pump, etc. then an average of 5000 and 7000 watts should do.

If on the other hand you experience power outages more frequently, or you simply want everything to work all the time, then definitely choosing a standby generator will satisfy all your needs. An average wattage used in large households can go up towards 7000 and 20000 watts. All are broad estimates, but if you follow our step by step guide, you will be able to find the exact size you need.

  • RVs & Boats

When it comes to powering appliances/devices in your RV, the process of calculating the size of the generator is pretty similar to what we previously discussed, so you can use the same instructions we have given. The size will again vary depending on how many applications you have in your RV and when you want to power them.

In general, you will need between 3500 and 7000 watts, or if you want to power more applications at the same time, then go for a larger one. Keeping in mind that the main factor in choosing a generator for your RV is the air conditioning unit. For example, a 3500-watt generator will have enough power to run a 13500 BTU (British thermal unit) AC and a couple of simple devices. But, if your needs and wants are different, and you want to power a larger AC, fridge, TV, and lights at the same time, then opt for a 7000+ watts.

The same applies a boat’s generator. Check the power requirements of the main appliances in your boat, such as adequate emergency power supply for the boat’s engine, heater, or coffee pot, and choose a size that can accommodate your needs.

  • Camping & Tailgating

When you go camping, you usually bring smaller appliances/devices with you, so you do not need so much power. In general, you will need while camping between 1000 and 2000 watts. If you are planning to use small lights, fans, or a TV, then you can go with a small generator. However, for coffee makers, grills, hair dryers, and heaters, you should go for a more powerful one. Also, don’t forget to look for a quiet running generator so you would be allowed to use it in noise enforced campsites.

The same applies for tailgating or outdoor events. If you use a mini-fridge, sound system, lights, and a heater, then choose a generator size that can supply all these devices at the same time.

  • Business & Industrial

Industrial power needs can vary greatly, based on the machines and power tools you rely on in your business. Usually, the average power needed is far higher than that of household or recreational use. This is due to the motors incorporated in industrial machines; they require far higher watts to start and to keep them functioning properly. As a result, to successfully determine the power requirements for your business, carefully look for enough information about wattage estimation, power efficiency factors, and some power management guides. As it can be a waste if your generator does not meet the necessary power requirements for your industrial equipment.

However, to get an average estimate, you will follow the same steps, which is to add up all the watts of all the power tools that you will need to run at the same time and then add the surge watts of the biggest load drawing tool. The result will give you an average estimate.

Wrap Up

As you can see, deciding on a generator size is a bit difficult since it involves a lot of variables. In any case, if you follow the steps we explained, you will surely get the right one for you. However, make sure you consult with an expert in a store or an electrician before you make any significant investments. Ultimately, the most important thing is not to overload your generator and break it. So, it is always a good idea to go with a slightly more powerful generator than your calculations. Still, you do not need to waste money on the power you do not need, stick to our recommendations, and you will not go wrong. Good luck!

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