I was over at a friend’s for a drink recently, when I heard a guy from the UK ask why we still use BTUs in America, adding, “I thought the standard unit of energy was Joules”. People had a few different theories about it, so, when I got home, I thought I’d verify the facts and share them with you in a post.
So, where does the BTU come from, and why do we use it?
Although standard units such as Joules and Watts have largely replaced the BTU, here in America, we still use BTUs to measure the energy used by our water heaters and HVAC systems. Possibly, this is because the BTU definition conveniently centers on heating water.
Whatever the reasons for its continued popularity, this article tells you all you need to know about BTUs, especially their connection to your water heater.
The BTU or British Thermal Unit was originally formulated as a measure for comparing the heat energy yielded by different kinds of fuel. A BTU is defined as the quantity of heat required to raise the temperature of one avoirdupois pound (1 lb.) of water by 1 °F at 1 atmospheric pressure (sea-level).
What is My Water Heater’s BTU Rating?
The BTU rating of a gas water heater is typically found on a label listing it as ‘Input’ in ‘BTU/hr.’, in addition to the product’s model and serial numbers and tank capacity. To calculate the BTU rating of an electric water heater, multiply its ‘Total Watts’ from the power rating label by 3.412.
But what does a water heater’s BTU rating really mean? As you can tell from the above definition:
The BTU rating on a water heater indicates the number of pounds of water that that water heater can heated by 1 °F in 1 hour.
For instance, the left panel in the image above is from a 40-gallon gas water heater rated 34,000 BTUs. This means that this water heater will heat 34,000 lbs. of water by 1 °F in 1 hour. More practically speaking, it means that the unit can heat 340 lbs. (roughly 41 gal.) of water by 100 °F in 1 hour.
Similarly, the right panel of the image above shows a power rating label from a 50-gallon electric heater, whose total watt rating is 4,500 W. This means that the heater’s BTU rating is:
4,500 × 3.142 = 14,139 BTU/hr
So, in 1 hour, this electric water heater can heat 14,139 lbs. of water by 1 °F. In practice, it will heat roughly 140 lbs. (about 17 gal.) by 100 °F in 1 hour.
How Many BTUs Does My Water Heater Use?
A simple way to visualize a BTU is to think of it as the amount of heat energy generated by a single, 4-inch wooden matchstick while it burns through. Thankfully, your domestic hot water supply doesn’t rely on someone burning matchsticks under your home water heater… else, there’d be a longish wait for that shower!
The amount of heat energy your water heater uses varies from one month to the next, of course. But, it is possible to estimate your home’s daily (and monthly) hot water BTU requirement.
The number of BTUs required by a water heater to provide hot water for each activity can be calculated by first multiplying the amount of water required (in gallons) by the desired rise in temperature (outlet minus inlet °F), then by 8.33 (a constant for gallon to pound conversion).
In other words:
BTUs Needed = Water Quantity (in gal.) × Desired Rise in Temperature (in °F ) × 8.33
To estimate of the amount of water used during each daily activity – brushing teeth, showering, shaving, and so on, check out this helpful post on water heater power consumption.
With those numbers in hand, all you need to do is to estimate the total amount of hot water you use in a week. Using the handy table below, you can easily figure out how many BTUs you end up using per week. Note that the table is designed keeping in mind that inlet water temperature varies significantly across seasons.
| Inlet Water Temp. | 110 °F Low Outlet Temp | 120 °F Medium Outlet Temp. | 140 °F High Outlet Temp. |
| 69 °F (21 °C) | 3415 | 4248 | 5914 |
| 59 °F (15 °C) | 4248 | 5081 | 6747 |
| 49 °F (9 °C) | 5081 | 5914 | 7580 |
| 39 °F (4 °C) | 5914 | 6747 | 8413 |
Say, you estimated that your home requires about 200 gallons of hot water each week, heated to an average temperature of 110 °F from the inlet temperature of around 59 °F. Figuring from the 10-gallon BTU value in the table, that is:
20* × 4,248 = 84,960 BTUs of heat energy in a week
To get the total monthly BTUs, you’d multiply that by 4, making 339,840 BTUs per month.
*Note that ‘200’ has been divided by 10 to account for the fact that the BTU values in the table are for every 10 gallons of hot water.
Now that you know where to look up your water heater’s BTU rating, and how to figure out your weekly (or monthly) BTUs, you might be wondering exactly how many BTUs there are in each unit of your home’s energy supply. Let me explain.
How Many BTUs Do I Get Per Unit of Energy?
The number of BTUs released by different household energy sources varies greatly. Natural gas yields the lowest BTUs per cubic foot (approx. 1000), followed by electricity, while fuels like propane and gasoline provide closer to 100,000 BTUs per unit, and wood yields about 20 million BTUs per cord.
Most homes use either electricity or piped natural gas to power their water heaters, although a significant number also use propane or heating oil, and, of course, some still use wood. The table below gives you an idea of just how many BTUs you’re likely to get out of every unit of your home’s energy supply.
| Power Source | Unit of Measurement | BTU Yield Per Unit1 |
| Natural Gas | Cubic Foot (cu ft) | 1,037 |
| Electricity | Kilowatt Hour (kWh) | 3,412 |
| Propane | Gallon (gal) | 91,452 |
| Gasoline | Gallon (gal) | 120,286 |
| Kerosene (#1 Fuel Oil) | Gallon (gal) | 136,000 |
| Home Heating Oil (#2 Fuel Oil) | Gallon (gal) | 138,500 |
| Wood | Cord = 128 cu ft | 20,000,000 |
(Note: If you’re confused by terms like “watt” and “amp”, you’re not alone! This jargon-busting post demystifies all those terms, so you’re never confused by them again.)
Now, if your water heater runs on natural gas or electricity, you might think that you’re probably spending way too much for your hot water, given the relatively low BTU yields of these energy sources. Hold on a minute…
Consider that natural gas is the cheapest energy source available to American homes.
Natural gas supplied to American homes is billed in hundreds (CCF) or thousands of cubic feet (MCF), or MMBTU (million BTU) or therms. One therm (1 thm) of natural gas equals 1.037 CCF, which is equivalent to 100,000 BTU.
The price for 1 CCF of natural gas has on average been less than $1 over the last couple of years. So, even if you’re using up 1 million BTUs to heat water every month, the part of your gas bill arising from hot water is likely to be less than $10!
If yours is an electric water heater, keep in mind that it actually takes far fewer BTUs to keep water hot in an electric compared to a gas water heater – up to 40% less in well-insulated ones! The reason is simple: gas heaters lose way more heat through ‘standby heat loss’ than do electric models.
This is reflected in the higher efficiency ratings or ‘Energy Factor’ (EF) ratings of electric compared to gas water heaters; EF ratings average 0.9 on electrics, while gas models average only 0.6.
And there you have it, folks, the A to Z of BTUs for your water heaters!
Sources
- US Energy Information Administration. 2021. Units and calculators explained. https://www.eia.gov/energyexplained/units-and-calculators/british-thermal-units.php