From: , BuildingGreen, More from this Affiliate
Published December 21, 2007 02:45 PM

Energy Metrics: Btus, Watts, and Kilowatt-Hours

Nearly every green publication (including, unfortunately, our own) has, on occasion, confused units of electric power (kilowatts) with units of energy (kilowatt-hours). It’s an easy mistake to make for those who are not steeped in the engineering of energy flows. But the basic principles are not that complicated—and are worth getting straight. 

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We begin with the definition of energy: the capacity to do work, such as creating heat, light, or motion. Energy is typically measured in British thermal units (Btus) or, in the metric system, joules (J). Power is a measure of the rate at which energy flows, and in electrical systems it is measured in watts (W). A watt is energy flowing at a rate of one joule per second, which is approximately 3.4 Btus per hour.

What makes things confusing is that, instead of measuring electric energy in Btus or joules, we measure it in kilowatt- hours (kWh). One kWh is the energy expended when running one thousand watts (or 3,412 Btus per hour) for an hour. It would be simpler to just call it 3,412 Btus, but that’s not how our usage has evolved. In Europe, thermal energy is also measured in kWh.

In practical terms, it is useful to understand that equipment power ratings in watts describe the maximum rate at which a device uses energy. Similarly, lighting power metrics, such as the one watt per square foot allowance for office spaces per ASHRAE Standard 90.1-2004, describe the rate at which energy is used when all of the lights are turned on. Homes are typically charged only for the electricity they use, measured in kWh. But commercial and industrial facilities also pay “demand charges,” which are calculated based on their peak power draw (usually measured in megawatts, or MW), which compensates the electric utility for ensuring that it has enough power available to meet that demand.

The same principle applies to thermal energy. Boilers and furnaces are sized based on their heating power, in Btus per hour in the U.S., and in kilowatts elsewhere. A typical residential unit puts out 100,000 Btu/hr (29 kW) while commercial units tend to be much more powerful. A 100,000 Btu/hr boiler burning at full power for a day will produce 2.4 million Btus of heat (700 kWh). Overall energy consumption of buildings, including both electricity and other fuels, is typically counted in million Btus per year (using the confusing Roman-numeral-based abbreviation MMBtu).

To report energy use in way that is comparable from one building to another, we normalize by the building’s gross floor area, giving us energy numbers in thousand Btus per square foot per year (kBtu/ft2·yr). The average for office buildings in the U.S. is 91 kBtu/ft2·yr, or in metric units, 290 kWh/m2·yr. If all this feels intimidating, remember that (if you grew up in the U.S.) you’ve managed to master the entirely arbitrary system of inches, feet, and yards. These energy metrics are much simpler.

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