British Thermal Units (IT) (BTU) to Watt-hours (Wh) conversion

What is the British Thermal Unit (IT)?

The British thermal unit (IT) is a unit of energy in the imperial and US customary systems, defined as roughly the heat needed to raise the temperature of one pound of water by one degree Fahrenheit. It is widely used in heating, cooling, and the power industries.

Definition

The "IT" (International Steam Table) British thermal unit is defined exactly in terms of the SI joule.

$$1\ \text{BTU} = 1055.06\ \text{J}$$

The exact value is 1 BTU (IT) = 1055.05585262 J. This IT definition is fixed by convention (derived from 1 BTU_IT = 1055.05585262 J, i.e. based on the International Steam Table calorie of 4.1868 J). It differs slightly from the thermochemical BTU (1054.35 J) and the mean BTU, because the amount of heat to warm water depends on the starting temperature.

Origin and History

The BTU arose in the 19th century alongside the calorie as engineers sought a practical unit tied to water and everyday temperature scales. Multiple definitions emerged (39 °F, 60 °F, mean, thermochemical), and the International Steam Table conference of 1956 fixed the IT calorie, and hence the IT BTU, to a precise joule value.

Law and Notable Facts

In the United States, air conditioners and furnaces are rated in BTU per hour, and natural gas is sold in therms (1 therm = 100,000 BTU). One "ton" of cooling equals 12,000 BTU/h, historically the heat absorbed by melting one short ton of ice in 24 hours.

Real-World Examples and Conversions

• 1 BTU ≈ 1055 J ≈ 0.293071 watt-hour; a 5000 BTU/h window air conditioner draws about 1465 W of cooling.
• 1 BTU is roughly the energy released by burning one wooden match.
• 1 kWh = 3412.14 BTU, and 1 therm = 105.506 MJ.

What is the watt-hour?

Watt-hours (Wh) are a practical unit for measuring electrical energy, especially in contexts like battery capacity and energy consumption. Understanding watt-hours helps in making informed decisions about energy usage and storage.

Definition of Watt-hours

A watt-hour (Wh) is a unit of energy equal to the amount of energy consumed by a one-watt load operating for one hour. It's a straightforward way to quantify energy usage over time.

The formula for calculating energy in watt-hours is:

$$\text{Energy (Wh)} = \text{Power (W)} \times \text{Time (h)}$$

Formation of Watt-hours

Watt-hours are derived from the relationship between power (watts) and time (hours). Power, measured in watts (W), represents the rate at which energy is used or produced. One watt is defined as one joule per second.

Since watt is joules per second, we can also say following where energy is in Joules and time is in seconds:

$$\text{Energy (J)} = \text{Power (W)} \times \text{Time (s)}$$

To get watt-hours, you simply multiply the power consumption (in watts) by the duration of operation (in hours). This provides a convenient measure of total energy used.

Interesting Facts and Historical Context

While no specific law is directly named after "Watt-hours," it's fundamentally linked to the work of James Watt, the Scottish inventor who improved the steam engine. Watt's name was adopted as the unit of power in recognition of his contributions to understanding and quantifying energy.

Real-World Examples

• Battery Capacity: Batteries are often rated in watt-hours, indicating the amount of energy they can store. For example, a 100 Wh battery can theoretically power a 10-watt device for 10 hours.
• Laptop Consumption: A laptop that consumes 30 watts, when used for 4 hours, would use 120 Wh of energy.
• Electric Vehicle Range: The range of an electric vehicle is determined by the capacity of its battery pack (measured in kilowatt-hours, kWh) and the vehicle's energy consumption rate.
• Household Appliance: A refrigerator may use 500 Wh of energy per day. This information is useful for calculating energy costs.

For more on the subject, read Energy vs. Power from Energy Education website.