Heating & Cooling Terminology
What is a kW? (power)
The kilowatt is a standard unit of electrical power equal to 1000 watts, 1.34 horsepower, or an energy consumption rate of 1000 joules per second. It measures a rate of energy use or production. A typical household incandescent light bulb uses electrical energy at a rate of 25 to 100 watts, while compact fluorescent lights typically consume 5 to 30 watts.
What is a kWh? (energy)
A kilowatt hour (kWh) is a measure of how much energy you’re using.
It doesn’t mean the number of kilowatts you’re using per hour. It is simply a unit of measurement that equals the amount of energy you would use if you kept a 1,000 watt appliance running for an hour:
So if you switched on a 100 watt light bulb, it would take 10 hours to rack up 1 kWh of energy.
Or a 2,000 watt appliance would use 1 kWh in just half an hour.
While a 50 watt item could stay on for 20 hours before it used 1 kWh.
What is the difference between a kW (power) and a kWhr (energy)?
kW stands for kilowatt. A kilowatt is simply 1,000 watts, which is a measure of power. So, for example, the 10,000 watt electric shower in the top bullet point above could also be called a 10 kilowatt shower.
A kilowatt hour (kWh) is a measure of energy.
So a 1,000 watt drill needs 1,000 watts (1 kW) of power to make it work, and uses 1 kWh of energy in an hour.
That’s why, if you leave a TV or computer on standby, it is still using power and creating a kWh cost on your energy bill.
What is a Btu?
A British Thermal Unit (Btu) is a commonly used unit of measure for energy use in heating and cooling equipment. It is the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. One kilowatthour (kWh) of electricity contains 3,412 Btu.
What is an mmBtu?
The unit MBTU was defined as one thousand BTU presumably from the Roman numeral system where "M" stands for one thousand (1,000). This is easily confused with the SI mega (M) prefix, which adds a factor of one million (1,000,000). To avoid confusion many companies and engineers use MMBTU to represent one million BTU. In natural gas, by convention 1 MMBtu (1 million BTU, sometimes written "mmBTU") = 1.054615 GJ. Conversely, 1 gigajoule is equivalent to 26.8 m3 of natural gas at defined temperature and pressure. So, 1 MMBtu = 28.263682 m3 of natural gas at defined temperature and pressure.
One Btu is approximately…
1 054 – 1 060 J (joules)
2.931 ×104 kWh (kilowatt hours)
252 – 253 cal (calories, or "little calories")
0.25 kcal (kilocalories, "large calories", or "food calories") the heat given off from one match stick burned end to end.
What is a THERM
The therm (symbol thm) is a nonSI unit of heat energy equal to 100,000 British thermal units (BTU). It is approximately the energy equivalent of burning 100 cubic feet (often referred to as 1 CCF) of natural gas.
Since natural gas meters measure volume and not energy content, a therm factor is used by (Natural) gas companies to convert the volume of gas used to its heat equivalent, and thus calculate the actual energy use. The therm factor is usually in the units therms/CCF. It will vary with the mix of hydrocarbons in the natural gas. Natural gas with a higher than average concentration of ethane, propane or butane will have a higher therm factor. Impurities, such as carbon dioxide or nitrogen, lower the therm factor.
The volume of the gas is calculated as if measured at standard temperature and pressure (STP). The heat content of natural gas is solely dependent on the composition of the gas, and is independent of temperature and pressure.
One therm is equal to about 105.5 megajoules, 25,200 kilocalories or 29.3 kilowatthours. One therm can also be provided by about 96.7 cubic feet (2.74 m3) of natural gas. The therm sometimes has been confused with the thermie (see below). The names of both units come from the Greek word for heat.
What is cooling ton?
1 ton = 12,000 Btu's per hour of heat being moved out of the space being cooled. Most residential air conditioning systems are in the range of 1 to 5 tons. But where did the term come from? The term 'ton' actually originates from the days when ice was used for cooling. It turn's out 1 ton of ice (a 3 ft cube) absorbs about 288,000 Btu's of heat every 24 hours which is works out to be 12,000 Btu's and hour.
What Is Relative Humidity?
Relative humidity compares the actual amount of water vapor in the air at a given temperature with the total amount of water vapor the air can hold at that temperature. Warm air has more capacity to hold water vapor than cold air. Relative humidity is stated as a percentage, with a 0% to 60% range common for indoor environments.
What is (EUI) Energy Use Intensity?
A Building's EUI is made up of its KBTU per square foot. The KBTU is basically all of the building's energy use converted to a common unit and stands for 1,000 British Thermal Units. This can be electricity (KWH), natural gas, (Therms, MCF, CCF), propane (Gallons), or others that can be converted to KBTU, but nonenergy items such as water and trash cannot. By combining all of the energy units to one common unit, you can compare one building's EUI to another, like sized building.
How are all the units related?
1 kWhr = 3.4 mBtu = 0.284 tonhr
1 kW = 3.4 mBtu/hr = 0.284 tonhr
