What Size Propane Tank You Need for a Whole House Generator

Typically, most homeowners have a propane tank ranging from 100 to 500 gallons. However, how much propane your whole house generator will use will depend on many things. To select the best propane tank for your generator, you must consider a couple of factors before making a final decision on which one to buy.

Generally, it’s good to have a tank that holds between 100 and 500 gallons of propane for a whole house generator. A 100-gallon propane tank will last 1-2 days, while a 500-gallon tank can last 7-10 days. To use more portable propane cylinders, hook multiple together with a switchover valve.

Consider the following factors before you settle on the size propane tank you need for a whole-home generator:

  • Manufacturer’s designation
  • The durability of propane gas
  • The length and frequency of power outages
  • Residential energy requirements
  • Your budget
  • Property size
  • Generator’s power capacity and load

It would be best to select and install the correct size propane tank. This article discusses the factors that influence your choice of a storage tank, installation requirements, and propane’s unique characteristics that facilitate proper sizing.

What Size Propane Tank Do You Need for a Whole House Generator?

Most whole house generators will use 2 to 3 gallons of propane per hour. 5,000-watt generators will use closer to two gallons of propane per hour, while more powerful 9,000-watt generators will use around three gallons per hour. With these simplified numbers in mind, here are some calculations:

Propane Tank SizeHow Long It Will Power a Generator
100 lbs1-2 days
500 lbs7-10 days
1,000 lbs14-21 days

The size of the propane tank you opt for to serve your whole house generator depends on the factors below:

1. Manufacturer’s Designation

Manufacturers may state the average amount of propane gas that your whole house generator will consume per hour within the documentation. They may also provide information concerning how much electricity the stated fuel consumption rate will produce. But you could also determine the fuel usage rates by researching online and checking through different charts.

Once you estimate how much fuel your generator will use under various conditions, you can calculate how much you need per month or year. And that would help you determine the size of the propane tank you need.

For example, if you determine that your whole house generator will use an average of three gallons of propane per hour under load, you would need to have at least five days of continuous backup power. Multiply 3x5x24, and you get 360 gallons.

But since we know that you can only fill propane tanks up to 80 percent, you would require a tank of at least 450 gallons. Since that size is unlikely to exist, you would opt for the next biggest size, which would be a 500-gallon tank. And that is what you would need for your whole-home generator.

2. The Durability of Propane Gas

Unlike other types of generator fuel, propane does not degrade over time. So that means it can stay within its storage container for years with no problem. All you would need to do is have it inspected every decade.

Due to the durability of propane gas, you can afford to err on the side of caution and opt for a bigger storage tank. That way, if you end up dealing with unexpected lengthy power outages, you would still have a standby power source to carry you through significant weather events. And if you have fewer power outages, your propane gas can remain within the tank, ready for use the next time around.

For example, if you estimate you will need 500 gallons of propane each year but anticipate having more issues this year due to an extreme weather event, you could buy a 1000-gallon tank. Should Mother Nature have mercy on you, you can use what remains in the tank the following year. And you won’t have to buy additional gas to fuel your whole house generator.

3. The Length and Frequency of Power Outages

It would be prudent to determine the average number of times your power often goes off and how long the outages last. That would give you an idea of how much fuel you would need to carry you through each year with minimal hassles.

Usually, the average American will experience around 106 to 118 minutes of power outages each year. But that excludes extreme weather events, such as severe snowstorms, heat waves, or hurricanes.

Texas’ 2021 winter blackout crisis is an example of how bad things can get when people lack power for prolonged periods. As a result of the blackout that affected millions of households and businesses, people went without water, food, and heat. And in the end, over 700 people are said to have died as a result.

Unfortunately, experts believe that the electric grid, as it currently exists, is not equipped to handle extreme weather events. So, if they occur, you should prepare for lengthier and more severe blackouts.

To ensure you do not end up in a similarly vulnerable position, learn to calculate how much energy you tend to consume per day and estimate how much fuel you would need for a prolonged blackout event.

For example, if you need two gallons of propane per hour, you could plan for a week of blackouts by having about 336 gallons of gas. In this case, you could opt for a 420lb propane gas tank to accommodate the gas expansion when temperatures rise.

4. Residential Energy Requirements

How much energy your home requires daily will also influence your choice of propane gas tank size. The higher your power consumption, the bigger your tank should be.

The average American household usually needs 10,715kWh per year. Of course, your generator does not need to produce all that since it’s only backup power. So, you would need to use other guidelines to help you estimate how much fuel you would need for your whole-home generator.

How Climate Influences Your Energy Requirements

Heating and energy conditioning tends to account for a significant portion of your energy bill. And while it is easier to cool a home, it’s much more challenging to heat it.

If you live in a warm climate, you will need less energy to make your home livable. But if you live in a colder area, expect your energy requirements to be significantly higher.

For example, if you live in a 1000-square foot cottage in a warm climate, you would need 30,000BTU per hour to heat it properly during the winter season. However, if you live in a much cooler environment, your home will need 60,000BTU of heat to achieve a similar result.

But this is an hourly requirement. So, if your winter season lasts about 90days, you would require 60,000X24x90 for a cold area and half that for a warmer climate. That translates to 129,600,000BTU for the entire winter season for the former and 64,800,000BTU for the latter.

Since we know that each gallon of propane gas produces 91,500BTU, if we assume that we will lose power for at least 10% of the winter season (in this case, nine days), we can estimate that we will need 141.64 gallons of propane in a cold climate and 70.81 gallons of gas for warmer weather.

Therefore, you would need a 100-gallon tank of propane to survive in a warm place, but two such tanks or one 250-gallon tank, to deal with frigid wintry seasons.

How Your Home Size Influences Your Energy Requirements

The size of your home also affects how much energy you use for heating, lighting, and cooling. The bigger the house, the more power you would require.

For example, a standard climate would need 45,000BTU to heat a 1000-square foot home, 90,000BTU to heat a 2000-square foot home, and 225,000BTU to heat a 5000-square foot home.

Suppose you live in a 2000 square foot home and experience nine days of power outages each winter. In that case, you would need to generate 90,000X24X9 BTU, which equals 19,440,000BTU, to cover the days when there is no power.

We have already established that one gallon of propane will generate 91,500BTU of heat. So, you would need 212.5 gallons of propane to heat a 2000 square foot home in a standard climate. But remember, a tank can only be 80 percent full. So, you would require a tank with a capacity of at least 265 gallons.

That would necessitate some creativity on your part. Therefore, you could buy a 100-gallon and a 150-gallon tank to cater to your needs. Alternatively, you could err on the side of caution and buy two 150-gallon tanks.

How Electrical Appliances Influence Your Energy Requirements

Another way to determine your electrical requirements is by checking out the wattage or BTU requirements of each appliance you intend to run using a whole house generator. And these values will vary depending on what devices you use, how much space they must cover, their brand, and energy efficiency.

For example, an air conditioner will require an average of 20 BTU to cool each square foot of space. So, if you need to condition 2000 square feet, you would need 40,000BTU per hour. On the other hand, a dishwasher would require 1200W to 1500W, while a fridge would need 150W to 400W.

If you have appliances that use wattage and BTU, you will do well to convert everything into BTU and then calculate how many gallons of propane fuel you would need per day during a power outage.

But first, you could invest in an energy monitor or use the one you have already to determine how much electricity you consume daily, weekly, and monthly. And then, you could convert the total wattage into BTU.

Therefore, suppose you consume an average of 10,715kWh per year, as most people tend to do. In that case, you would need 29.356kWh per day for a year. So, if you experience ten days of power outages each year, your whole house generator must make 293.561kWh or 293,561watt hours.

We know that two horsepower will produce 1000 watts per hour where propane is concerned. So, 293,561 watts will require 587.122HP. But we have also established that each horsepower will consume 10,000BTU of fuel per hour. So, you would need 5,871,220BTU to create the necessary power for your home for ten days.

Knowing that each gallon of propane produces 91,500BTU, you will need 64.17 gallons for ten days of power outages if you are a homeowner with standard energy needs. In this case, a 100-gallon tank would do.

It is worth noting that you can convert the watts into BTUs directly. However, you are likely to get different results.

That formula is:

1W=3.412BTU/hour, which translates to 1Wh=3.412BTU after multiplying both sides by one hour.

In this case, 293,561watt hours will equal 1,001,630.132BTU. Since one gallon will produce 91,500BTU, you would need 10.946 gallons that would fit in a 20-gallon tank. But that does not make sense for a whole house generator.

So, because this formula gives a lower amount of gas, it would be best to err on the side of caution and opt for the higher value from the previous calculation.

5. Your Budget

It would be crucial for you to budget for a propane tank accordingly. That means having enough money not just for the tank but to cover any delivery, installation, and permit costs that arise. Generally, the larger the tank, the more expensive it would be.

Depending on the size, brand, or new or used, propane tanks can cost anywhere from $30 to $3,500. For example, you could pay $400 to $800 for a 100-gallon tank while having to dig deeper to cater for the 250-gallon tank that would cost $500 to $1,000. On the other hand, if you desire a 1000-gallon tank, it would be best to be prepared to pay anywhere from $1,500 to $3,500.

Do remember that the above cost estimates only cover the tank itself. Installation of residential propane tanks can cost an additional $275 to $1,725 for above-ground tanks and $1,125 to $5,150 for underground tanks.

In addition, you also have to consider the cost of gas permits. Luckily these tend to cost only $25 to $50 and are valid for the life of the gas tank.

6. Property Size

There is only so much you can do with a small space. So, if your property is pretty small, you will be limited in what size of tank you can install above ground or underground. Not only will you have less space to maneuver within your yard, but you’re also restricted by what you can do below ground due to buried utility lines.

Typically, the bigger your propane tank is, the larger the space you will need to accommodate it. For example, a 120-gallon propane tank is about five feet and eight inches long, with a diameter of two feet. But as you move up the sizing chart, you will realize you need more space. A 250-gallon tank could be seven feet and inches long with a diameter of about two and a half feet.

On the other hand, a 1000-gallon tank could be 15 feet and ten inches with a diameter of three and a half feet. Remember, these are just the basic tank sizes. You have to factor in additional space for proper tank installation.

7. Generator’s Power Capacity and Load

A generator’s capacity refers to the maximum amount of electrical power it can create when running at full blast. And it is usually measured in kilowatts or megawatts (kW or MW). So, a 22kW propane generator can produce up to 22,000 watts of power at a given time.

The higher a generator’s capacity, the more appliances it can power. However, that means it will also consume more fuel, which would result in the need for a larger propane tank.

That said, generators should never run for more than 30 minutes at full blast. And that is where load comes in.

The load refers to the electricity being consumed from the whole house generator by whatever is running at any given time. It can be less than or equal to the maximum generator capacity, but not more.

The basic rule of thumb is to keep the load at 70 to 80 percent of the total capacity. That not only prevents the generator from overloading but also accommodates future increases in energy requirements.

So, if you operate your generator at a load less than 100 percent, you would consume less power and, thus, less fuel. And that means that even if someone recommends a bigger tank by considering the generator’s total capacity, you could work with a smaller one.

For example, if you have a 10HP 500-gallon tank (assuming it was full), you could run a 5kW generator at 50% capacity for 920 hours. However, if you were to run that same engine at full blast, you would require a 1000-gallon tank to cover the same period.

Propane Characteristics That Influence Tank Sizing

Before deciding which propane tank size you need for your whole house generator, it’s best to learn about propane gas and consider how it features in various energy calculations.

1. Expansion and Contraction of Propane Gas

Propane gas tends to expand within its housing container when the temperature rises. So high is its rate of expansion that it expands 17 times as much as water’s volume over the same temperature increase.

Due to its ability to expand, suppliers can only fill propane gas tanks up to 80 percent regardless of the size you choose. For example, if you buy a 500-gallon tank, you can only have 400 gallons of propane gas. That ensures that the gas volume will not overwhelm its storage container even when temperatures are high.

However, when it’s cold, propane gas tends to contract. And when it does, the volume of the propane gas reduces, and so does the internal tank pressure. For that reason, you may end up with inaccurate gauge readings. But what you need to remember is that the amount of the gas will remain unaffected, provided there is no leakage.

2. The Weight of Propane Gas

Propane gas weighs 4.2 pounds for every gallon. Therefore, 96 gallons of propane within a 120-gallon tank would weigh 403.2 pounds. That weight is exclusive of the empty tank weight.

Generally, propane tanks tend to have the tare weight (TW) stamped on their sides. But when a value is designated for a propane tank, it usually indicates the amount of gas you can put in. For example, a 20-pound propane tank can accommodate up to 20lbs. of propane gas. And that is equal to about 4.76 gallons of propane.

3. The Horsepower of Propane Gas

A horsepower is a unit of measurement that quantifies the energy produced by engines like generators. Typically, one horsepower is equal to 746 Watts. So, a 500HP propane generator can produce 373,000 Watts or 373kW.

However, things work a little bit differently when using propane gas. Under load, a propane gas generator will use two horsepower to produce 1000 watts of energy (1kW). So, a 500HP, in this case, would make up to 250,000 watts or 250kW of power using propane gas.

4. The British Thermal Unit (BTU) of Propane Gas

British Thermal Unit (BTU) measures heat quantity that you can use to determine how fuels produce energy.

Each gallon of propane contains approximately 91,500 BTU of heat (but you can use 92,000BTU for ease of calculations). So, a 500-gallon tank containing 400 gallons of gas can eventually produce as much as 36,600,000BTU of heat energy.

Also, 10,000BTU of fuel per hour will make one horsepower of mechanical energy under load.  Therefore, 400 gallons of gas can create 3,660HP of energy. And since we know that two horsepower can make1000 watts of power, we can conclude that 3,660HP from 400 gallons of propane gas will eventually produce 1,830,000 watts or 1,830kW of power.

Final Thoughts

There is no easy answer concerning how much fuel you need for a whole house generator. So, it would be best to consider all the relevant factors before determining what size propane tank you would need for your needs.

And remember, it is better to have a larger tank than a smaller one since propane does not degrade. You could always use the leftover gas next year.


Hubert Miles

I've been conducting home inspections since 2002. I'm a licensed Home Inspector, Certified Master Inspector (CMI), and an FHA 203k Consultant. I started PickGenerators.com to help people better understand whole-house and portable generators.

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