BTU stands for British Thermal Unit and is a unit that helps us measure heat or energy. While there are other units used for the same purpose like the metric calories or watts, the non-metric BTU is generally more widely used in the field of heating products. By definition, one BTU equals the amount of heat that is required to raise the temperature of one pound of water by one degree Fahrenheit. This amount of energy equals to around 252 calories, or 1055.1 joules.
When you read that a tankless water heater is able to yield a heating output of 100,000 BTUs for example, you should consider what this means from a practical perspective and if that suits your needs. To figure this out, you will need to consider the maximum water flow rate that you will need, which determines how much water you want to get passed and heated through the heater during a period of time. If for example the required flow rate is 10 gallons per minute, then we convert this to ninety pounds of water per minute. A typical cold water temperature is 50 Fahrenheit, while a typical hot one is 150 Fahrenheit so we have a difference of 100 degrees. This translates to 100 x 90 pounds = 9000 BTUs per minute. For the winter time, the cold water temperature may be much lower, so the BTU requirement may become higher due to the greater cold-hot water temperature difference.
During these calculations, you should also consider the nominal efficiency of the tankless water heater. If the manufacturer claims an efficiency of 78% for a product that yields 900,00 BTUs, this means that the amount of energy that can be passed to your water is about 70200 BTUs. The more BTUs, the more gas or electricity your heater will consume, so keep in mind that 1000 BTUs equals to around 1 cubic foot of natural gas. This doesn’t mean that a heater always burns on its maximum rating. Most of the higher quality models offer a range of BTU thermal output like 15,000 to 200,000 BTUs, so you can set it to your needs and not waste too much energy for no reason. When selecting a model based on its BTU, make sure that its minimum setting is within your usability possibilities, and that its maximum setting meets and surpasses your maximum needs. If you figure out that you need around 30,000 BTUs per hour, do not pick a 150,000 BTUs heater, as you will never use its full potential and you will be wasting money on it when you could choose a cheaper and smaller model.