What is a BTU?
The British Thermal Unit, or BTU, is an energy unit. It is approximately the energy needed to heat one pound of water by 1 degree Fahrenheit. 1 BTU = 1,055 joules, 252 calories, 0.293 watt-hours, or the energy released by burning one match. 1 watt is approximately 3.412 BTU per hour.
BTU is often used as a point of reference for comparing different fuels. Even though they're physical commodities and are quantified accordingly, such as by volume or barrels, they can be converted to BTUs depending on the energy or heat content inherent in each quantity. BTU as a unit of measurement is more useful than physical quantity because of fuel's intrinsic value as an energy source.
BTU can also be used pragmatically as a point of reference for the amount of heat that an appliance generates; the higher the BTU rating of an appliance, the greater the heating capacity. As for air conditioning in homes, even though ACs are meant to cool homes, BTUs on the technical label refer to how much heat the air conditioner can remove from their respective surrounding air.
Size and Ceiling Height
Unsurprisingly, a smaller room requires fewer BTUs to cool or heat. Typically, BTU usage is measured based on the volume of the space. The following is a rough estimation of the cooling capacity a cooling system would need to effectively cool a room or house based only on the square footage, as provided by EnergyStar.gov.
| Area To Be Cooled (square feet) | Capacity Needed (BTUs per hour) |
|---|---|
| 100 to 150 | 5,000 |
| 150 to 250 | 6,000 |
| 250 to 300 | 7,000 |
| 300 to 350 | 8,000 |
| 350 to 400 | 9,000 |
| 400 to 450 | 10,000 |
| 450 to 550 | 12,000 |
| 550 to 700 | 14,000 |
| 700 to 1,000 | 18,000 |
| 1,000 to 1,200 | 21,000 |
| 1,200 to 1,400 | 23,000 |
| 1,400 to 1,500 | 24,000 |
| 1,500 to 2,000 | 30,000 |
| 2,000 to 2,500 | 34,000 |
Insulation Condition
Thermal insulation is defined as the reduction of heat transfer between objects in thermal contact or in the range of radiative influence. The importance of insulation lies in its ability to lower BTU usage by managing the loss of heat due to its entropic nature – heat tends to flow from areas of warmer air to cooler air until there is no longer a difference in temperature.
Generally, newer homes have better insulating ability than older homes due to technological advances as well as stricter building codes. Thermal resistance, which is a measure of a material's resistance to heat flow, is indicated by a material's R-value. The higher the R-value of a certain material, the more resistant it is to heat transfer.
When deciding on the proper input for the "insulation condition" field in the calculator, use generalized assumptions. A beach bungalow built in the 1800s with no renovations should probably be classified as poor. A 3-year-old home inside a newly developed community most likely deserves a good rating. Windows normally have poorer thermal resistance than walls. Therefore, a room with lots of windows normally means poor insulation.
Desired Temperature Increase or Decrease
To find the desired change in temperature to input into the calculator, find the difference between the unaltered outdoor temperature and the desired temperature. As a general rule of thumb, a temperature between 70 and 80°F is comfortable for most people.
For example, a homeowner in Atlanta might want to determine their BTU usage during winter. Atlanta winters tend to hover around 45°F and temperatures may fall as low as 30°F occasionally. Given that the desired temperature of the residents is 75°F, the desired temperature increase would be 75°F - 30°F = 45°F.
Other Factors Affecting BTU Requirements
Along with the factors discussed above and their effect on BTU usage, there are other variables to keep in mind:
- The number of residents: A person's body dissipates heat into the surrounding atmosphere, so the more people there are, the more BTUs required to cool the room, and the fewer BTUs required to warm the room.
- Location of the condenser: Try to place the air conditioner condenser on the shadiest side of the house (typically north or east). The more the condenser is exposed to direct sunlight, the harder it must work due to the higher surrounding air temperature.
- Size of the condenser: Units too big cool homes too rapidly. Therefore, they don't go through the intended cycles they were designed for, which may shorten the lifespan of the air conditioner. If the unit is too small, it will run too often throughout the day.
- Ceiling fan usage: Ceiling fans can help lower BTU usage by improving air circulation and eliminating "dead spots" where air stagnates.
- The color of the roof: A darker surface absorbs more radiant energy than a lighter one.
- Decreasing efficiency: Like most appliances, the efficiency of a heater or air conditioner decreases with usage and age.
- Shape of the home: A long narrow house has more walls than a square house with the same square footage, which means more surface area for heat loss.