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How do I know what size Tank to install with my Generator? What kind of Day Tank do I need?

You have questions - we have answers! In this article we discuss:

General education on Generators, Backup Generators, and specifically focusing on Tank Sizes, Importance, Purpose, & Differences between Main and Day Tanks

General Information about Generators:

Generators are used primarily for critical infrastructure where having backup power is not so much of a convenience as it is a necessity. This is true in most of the hospitals, nursing homes, the municipalities, and all of the other critical infrastructures you can think of. These infrastructures have access to the regular power from the utility company, but they also have a backup generator accessible to power in case of an emergency.

These generators are 99% driven by diesel fuel. The diesel fuel must be stored in order for the generator to immediately begin to operate when there is a power outage. The average generator should run for approximately 72 hours. This holds especially true for buildings that have elevators. If the power goes out, they need to be able to continue to operate without trapping anyone inside. However, this is only one example of why generators are imperative and absolutely necessary. What is even more necessary are the tanks that store the fuel so the generator can operate properly. If there is no fuel available, then the generator becomes useless and remains without power.

Another thing to consider is that these infrastructures are equipped with pumps and stations called “Lift Stations” because they help to lift waste and water to places where it doesn't flow naturally. When water doesn’t flow naturally, it needs to be pushed. These pumps regulate the flow and are very important so these stations cannot be without power. Generators ensure that these pumps and stations remain in operation during times of an emergency outage.

You can also consider many more infrastructures, such as bridges and other locations where electricity is available, but without it could result in injury to people if there is a power outage. Having a generator proves to be very useful in these moments. These are only a few reasons why generators are so imperative!

How do you know what size Tank to install with your Generator?

When considering what size generator to install you must consider how long you would need it to run in the case of an outage. After deciding the size of the generator you need, you then need to know what type of tanks will be required to store the fuel needed to power it during an outage. Generators that are able to run on diesel have approximately 72 hours of running time.

For example: If the intention is to have power for 72 hours without having to refill the tank on a generator that uses 100 gallons per hour, you would need a 7000 gallon tank or around an 8000 gallon tank because tanks can only be filled to eight 90 percent. In the event of a consumer that uses 100 gallons each hour and you need to operate for 72 hours, a tank with a capacity of 8000 gallons is required.

The calculation of Tank size for Generators that consume less power is also simple. The number of gallons per hour would be reduced by 10 if the tank size is smaller than 10000 gallons. This is how the computations are performed to determine the tank size that will be required.

Many generator tanks are often located far away from the generator so a lot of the generators are located in remote locations like the roof of a building. They're located on the inside of a building depending on how the original design was done.

Since the tanks for the generators are generally positioned far away from the generators, it necessitates the installation of generators in remote locations such as on top of buildings. Depending on how the original design was made, they can also be located inside of a building's interior when approved.

One huge thing to consider is that the generator may be located a considerable distance from the main tank where fuel is stored. In that situation, having reserve fuel nearby next to the generator is invaluable and convenient. These specific tanks are smaller in size, closer to the generator, and referred to as a “Day Tank.”

In a typical setup, there's a generator and an area where most of the fuel is stored called the Main Tank. Then there's a Day Tank, which is used to house a small amount of fuel right next to the generator. This makes it easy for the generator to siphon the fuel conveniently in an emergency and is an imperative asset to running generators safely and successfully.

What kind of Day Tank do I need?

The term Day Tank originally came from the idea that these tanks were created to successfully run a generator for a day without needing to siphon fuel from the Main Tank. However, in most cases, this doesn't prove to be true because you usually don't require an entire tank of fuel for one day. As long as you have pumps that are able to siphon the fuel from the main tank and automatically fill the small day tank next to the generator, then you don't always require a separate tank.

Day tanks are typically maintained at 50 gallons, and sometimes as low as 25 gallons. The Day Tank is created to offer quick and easy access to fuel if the generator would ever need to be used. And then, the fuel from it gets cycled by fuel from the Main Tank.

You'll also have pumps and sensors in the tank that will detect when the day tank's fuel level is low. When it reaches around 60% capacity, these sensors notify you. The pumps we keep running draw gasoline from the main reservoir and refill the tank. You'll also have pumps and sensors that will notify you when your fuel level is full, at which point the pumps will switch off until the generator runs out of gas again, and then the process will repeat itself as needed.

Overall, you can see why these tanks are so significant. I hope this post helps you understand the nature of Main Tanks and Day tanks as assets and the value they provide to generators. Thank you for taking the time to read! Contact us for more information or to request an estimate by calling 407-293-0891 or visiting

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