﻿ Energy - definitions and units

Definitions and Units

In keeping with the standards of the US, this website uses the archaic British units rather than the more widely used metric ones. Apologies to anyone reading this from outside the US...someday metric conversions will be added (after I become familiar with them again).

U value - the fundamental unit of heat flow, given in BTU/hr/ft2/°F.  The metric equivalent is W/m2/°K.  To convert to metric, multiply by .17611, and from metric multiply by 5.5782.  Note that the U value generally refers to the entire piece of material in question, while the related K value (conductance) is per unit thickness (for imperial measure, that would be per inch).  While the K value is commonly used in engineering, in building energy modeling, you are more likely to see the same thing referred to as an "R value per inch".

Materials with greater insulating values have lower U-values. See also "R value", below.

R value - the inverse of U value.  Materials of greater insulating values, have higher R values.

Note that R values (and the corresponding U value) are measured in a specific steady state condition, maintained over a relatively long time period.  When real world conditions are different, the effectively R-value might be different from the measured one.  For more info, see the longer discussion under R-values.

BTU - the amount of heat necessary to raise 1Lb of water, 1 degree F.  Btu's are a total amount of heat, not a flow rate.   The metric equivalent is watt-hours, or kilowatt hours.  To convert BTU to kilowatt hours, divide by 3413.

BTU/hr - a heat flow rate.  The metric equivalent is watts, where 1 watt=3.413 Btu/hr.

Kilowatt - a rate of energy flow, equal to 1000 watts.  In the US, electricity is the only energy measured in these units.

Killowatt-hour (Kwh), is a total amount of energy used.  A light bulb is measured in watts (flow rate it uses electric), but your bill is in kilowatt-hours, which is a total amount of electric used, accounting for how long the light bulb is turned on for.  In the scientific world, total energy is often expressed as Joules, which is one watt used for one second, the Joule is too small a unit, so Kwh is used.

Therm - one therm is equal to 100,000 BTU.  Natural gas companies in the US sell gas by the therm, even though the meter reads in cubic feet. For natural gas, a hundred cubic feet (1 ccf) is approximately equal to one therm (in fact 1ccf is 3-5% greater than 1 therm).

Degree-day - this typically refers to a heating degree day (HDD), which is calculated as the  difference between the average daily average outside temperature and an inside temperature of 65F, summed up over all the days of the heating season.  Only days (or the part of the day) where the temperature is colder than 65 outside are counted. These values are calculated from historical weather data, and so are averages.  In any given year, the HDD value can vary by 10% or so.   If the average outside temperature on a given day is 40F, then that day represents 25HDD. If there were 100 days in the heating season at this temperature, this climate would have 2500HDD.  Typical values: LA is around 2000HDD, Seattle around 4500HDD, NYC is around 5000HDD, and Fargo is around 10000HDD.

Degree days are used to estimate annual heating/cooling energy use, and have a built-in assumption of how big the base-load energy is.  This base-load energy represents heat entering the building via other than the heating system, ie typically electrical appliances. The value of 65F is used under the assumption that base-load provides 5F worth of heating.  If you're looking to calculate total heat loss, including base-load, HDD won't give you the right value.

You would think that this HDD65 value would give the correct total heat loss for a building whose inside temperature was actually kept at 65F, but alas, even this isn't correct because when the daily average temperature is close to 65, the daily average temperature stops being a reasonable estimate of the daily heat loss, and instead you have to look at it on an hour by hour basis.  Further, things like night thermostat setback and solar gain mean that the interior temperature isn't actually a constant.

Cooling degree days (CDD), work the same way, but for the cooling season.  Alas, they also use a base temperature of 65F, which means the values are even less realistic, especially since most people now keep the interior temperature in the suggested 75-78F range.

CFM -Cubic feet per minute. This is a rate of airflow, and is how exhaust fans and a home's air leakage are often measured. To calculate heat loss due to air flow you need to know how many cubic feet of warm air escapes, how much heat a cubic foot holds per degree of temperature (.018BTU) and the difference in temperature between the indoor air, and the cold outdoor air that will be replacing it.

ACH - Air Changes per Hour. This tells you how many times per house all the air in a house leaks out. Like CFM, this is a rate of air flow, but rather than a measured rate, it is relative to the volume of the house. You can convert one to the other using the volume of the house:
ACH = (CFM*60)/Volume.

Pascal - a unit of pressure (in metric units).  It is equal to 1 Newton per square meter. It is a relatively small unit of pressure.  One PSI, the unit often used for tire pressure, is equivalent to 6,894 Pascals.  Building air leakage is typically measured at 50 Pascals of pressure, which is approximately the equivalent of a 20mph wind blowing against all sides.

Radiant Temperature - this is the temperature that is visible thru the air, as opposed to the temperature of the air itself, and represents heat transfer by radiation from one object to another (and to some degree the heat transfer from an object to the air itself by radiation).  The obvious example is standing in the sun on a cold day, in which case the radiant temperature on the surface the sun hits is much warmer than the surrounding air temperature.  In the reverse case, clear night skies can be much colder than the surrounding air.

Radiant temperature is measured with an infrared thermometer, which is typically an electronic device that uses a infrared sensor, the sensor being similar to that used in digital cameras, but tuned to the infrared band instead of visible light.

Mean Radiant Temperature - this is the average of all the radiant temperatures visible to an object.