36000 BTU Vs. 40000 BTU Water Heater: What's The Difference?

Ryan Womeldorf
by Ryan Womeldorf
Water heaters are a modern convenience and necessity, but only when you have one with the right heating capabilities. Choosing between a 36,000 BTU water heater and a 40,000 BTU water heater comes down to capacity, and the latter can handle 50 gallons. Whether it be heating capability or water capacity, let’s explore the difference between a 36,000 BTU and a 40,000 BTU water heater.

Choosing the right water heater for your home comes down to more than a few factors. One of the most important factors, however, is how many BTUs it has. Short for British Thermal Unit, a BTU is a unit of energy.

But is that level of energy the only difference between a 36,000 BTU water heater and a 40,000 BTU water heater? Aside from slightly better heating capability, a 40,000 BTU water heater also tends to have a slightly larger tank. The 36,000 BTU water heater is about 40 gallons while the 40,000 BTU water heater is around 50 gallons.

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What is a BTU?

A BTU is short for British Thermal Unit. This is a measurement of the quantity of heat that is required to raise a single pound of water by a single degree Fahrenheit. The higher the level of BTU, the more heating power that the unit possesses.

BTU for appliances is generally used to quantify the heating and cooling ability of that unit. You will see BTU referenced not only for water heaters, but for air conditioners, and any other type of similar equipment used in the home.

Significance and Recovery Rate

Water heaters are typically ranked in some way by their BTU input. What that means is that they are ranked by their level of BTU per hour. So, if you were to have a 36,000 BTU water heater, it is capable of heating up to 36,000 pounds of water by a single degree each hour. That equates to 360 pounds by 100 degrees.

The recovery rate, meanwhile, is how long it takes the water in the tank to heat back up again. When a water heater is used, the hot water dissipates from the unit, filling with cold water. The recovery rate entails how long that it takes to re-heat that water.

Recovery rate is generally calculated by the number of gallons of water that can be heated up to 100 degrees Fahrenheit within an hour. The typical recovery rate for a standard 40-gallon water heater tank is about 40 minutes.

Size and Usage

The most common way of calculating the size of a water heater tank, however, is simply the size of it. Generally speaking, this is referred to in gallons. For a two-person home, the standard is a 30- to 40-gallon tank. For five people or more, you would need something ranging in the 50- to 80-gallon range.

Typically speaking, BTUs are referred to more commonly in ventilation, heating, and air conditioning communities to discuss the rating of the equipment. For the most part, homeowners refer to the size or the recovery rate when comparing water heaters.

The Main Differences Between 36000 BTU and 40000 BTU Water Heaters

For the most part, a water heater this close in BTU rating is going to be largely the same. After all, BTU is a relatively flat determining factor. It has no role in the features of a given water heater, so those would not come into play here.

The two differences between a 36,000 BTU water heater and a 40,000 BTU water heater is heating capacity and tank size. Let’s take a look at each of them below.

Heating Capacity

The most obvious difference is in the heating capacity of both. Obviously, the 36,000 BTU water heater is not capable of heating up quite as much water in an hour as the 40,000 BTU water heater. The difference is a little more than negligible, though it depends on the setting.

For the average two-person home, a 36,000 BTU water heater will serve just fine. But in homes where shower usage is a little heavier, the 40,000 BTU water heater may do a better job of delivering hot water on a consistent basis.


The second major determining factor between these two water heaters is the physical size. Remember that, for the most part, homeowners are using the size of the tank to determine what unit that they need. They aren’t really going to go on BTU.

A 36,000 BTU water tank is likely to be in the 30- to 40- gallon range. This is the standard in most homes today. The 40,000 BTU water heater is in a 50-gallon tank size. The larger tanks can be slightly more costly to run in the long-term but come with the benefit of more consistent and longer-lasting water heating.

A Higher BTU Rating is Better

No matter how you cut it, that BTU rating is an indication of how quickly the unit recovers. The faster the recovery rate, the sooner that you can use hot water again. For homes where appliances are constantly in use, a higher BTU is definitely recommended.

Electric water heaters may be a superior option (other than tankless) for higher BTU. Electric water heaters generally have either a pair of 4,500-watt heating elements or a single 5,500-watt heating element. Having two elements is better, though, because they heat up quite a bit faster than the single higher-wattage heating element.

How to Calculate BTUs Needed

You have come to the conclusion that your water heater needs replacing. How do you know that you are choosing the right replacement, however? You do your research and determine that BTU ratings are important to you. How do you calculate BTUs?

Step 1: Find the Capacity

Start out by finding the capacity of your current water heater. Look at the sticker or tag that comes on the water heater. For the most part, this is a label on the front part of the water heater, which includes data like capacity. Know what you’re working with before deciding what to upgrade to.

Step 2: Creating an Equation

Using the information from the tag or sticker, you can make your equation. It takes a single BTU per each pound of water to elevate the temperature a single degree Fahrenheit. A standard water heater in America is 40 gallons, with each gallon weighing 8.33 pounds.

Step 3: Calculate

Now it’s time to calculate. If you currently have a 40-gallon water heater, multiply 40 by the weight of water, coming to 333.2 pounds of water. That means you need at least 333.2 BTU to raise the water by a single degree. But since most areas need more than that one degree of heat, we need more information.

Step 4: The Final Calculation

Know what the average temperature of your water is; you can typically get this from the local water company. If your water is typically 50 degrees and you want it to get to 140, you would then subtract 50 from 140. That difference is then multiplied by the weight of water and multiplied by the total number of gallons. In this instance, the answer comes out to 29,988 BTU, which means your new unit would need at least 30,000 BTU to meet your needs.

Video: How Much BTU Heat You Need

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What About Tankless Water Heaters?

There is, of course, the possibility that you make the upgrade to a tankless water heater. Tankless water heaters are far more efficient than their tank counterparts. That is because the purpose of the tankless heater is only to heat up water when it is needed. There is no tank storing water, hence the name.

Tankless water heaters use far more BTUs than their tanked counterparts. Whereas the industry standard water heater tank uses 36,000 BTU, the average tankless water heater requires anywhere from 150,000 to 200,000 BTU.

The reason is that tankless water heaters need to get the water up to temperature almost instantaneously. To do that, it requires a very high volume of gas. Before installing a tankless water heater, have a professional determine whether your gas line needs to be upsized.

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Ryan Womeldorf
Ryan Womeldorf

Ryan Womeldorf has more than a decade of experience writing. He loves to blog about construction, plumbing, and other home topics. Ryan also loves hockey and a lifelong Buffalo sports fan.

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