Definitions Heating, Cooling, & Insulation Terms

Heat Loss, Rvalue, Uvalue, & KValue measurements:
This article defines Heat Loss, Rvalue, Uvalue, & KValue measures of heating loss rate or insulation effectiveness and provides basic building insulation and heat loss guidelines including how to measure or calculate heat loss in a building, defines thermal terms like BTU and calorie, provides measures of heat transmission in materials, gives desired building insulation design data, and shows how to calculate the heat loss in a building with R values or U values.
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When we are evaluating the quality and effectiveness of insulation in a building or the adequacy of a building heating or cooling system, we need to use measurements that permit us to describe the rate at which a building loses heat under various conditions (such as outdoor temperature, wind velocity, how leaky the building is, the area of its windows and perhaps doors, and the amount of insulation in the building walls, floors, and ceilings.
[Click to enlarge any image]
At page top: FormulaR™ and Owens Corning™ which may be visible in the page top photograph of pink Styrofoam™ insulation boards are registered trademarks of Owens Corning® and were photographed at a Home Depot® building supply center.
A few of these critical definitions is given just below, followed by some simple formulas used to calculate the heat loss in a building. Sketch at above left is provided courtesy of Carson Dunlop Associates.
AFUE is an abbreviation for Annual Fuel Utilization Efficiency. In short, the AFUE tells you, for each dollar you spend on energy for heating by gas, oil, or another fuel, just how much of your dollar shows up inside the occupied space of your building as heat. Higher AFUE is better.
See AFUE DEFINITION, RATINGS for details & examples.
a BTU is one "British Thermal Unit" which is defined as the quantity of heat that would be required to increase the temperature of one pound of water by one degree Fahrenheit.
A BTUH is defined as the number of BTU's lost (if we're talking about heat loss or air conditioning), or provided (if we're talking about providing heat for a building) in one hour. You'll often see BTUH as a number on data plates on air conditioners and on heating systems.
Also see our examples of BTU data used in air conditioning and heat pump calculations discussed at What is a BTU or British Thermal Unit? What is a Joule?for details about BTUs and various examples of BTU and BTUh calculations. There we give definitions of related terms such as latent heat, superheat, latent heat of condensation, sensible heat, and specific heat.
Complete details about the definition of BTUs and BTUH, and examples of uses & calculations using BTU numbers can be found at Definition of BTUs and Definition of BTUH
One BTU is also equal to 252 calories. So what's a calorie? (Definition of Calories)
A calorie is defined as the quantity of heat needed to raise the temperature of one gram of water by one degree Centigrade
See SEER RATINGS & OTHER DEFINITIONS where we quote:
Energy Efficiency Ratio (EER): This is a measure of the instantaneous energy efficiency of cooling equipment. EER is the steadystate rate of heat energy removal (e.g., cooling capacity) by the equipment in Btuh divided by the steadystate rate of energy input to the equipment in Watts. This ratio is expressed in Btuh per Watt (Btuh/Watt). EER is based on tests performed in accordance with ARI 210/240.
R values and heat loss: The "R" value of a material is its resistance to heat flow through the material. When buying various insulation materials you will almost always see an "R" value quoted for the material. In general, higher "R" means more resistance to heat loss and therefore lower heating or cooling bills for the building.
Mathematically, "R" is simply the reciprocal of the two measures discussed in more detail below:
"K" (R = 1/K) or "U" (R_{(whole building)} = 1/U)
As you'll read below, the heat transmission coefficient "K" measures the heat flow through an individual substance and "U" measures the overall building heat loss by adding all of the various areas and substances together.
Reader Peter J. Collins has noted in clarification of the definition of "R" value that
The Rvalue of a material is a measure of its thermal resistance.
The Uvalue (or Ufactor), more correctly called the overall heat transfer coefficient
We add that Rvalue measures the resistance of a material to transfer heat (in any direction). Higher Rvales are more resistant to heat transfer. When we are discussing building insulation, an insulation with a higher Rvalue would be expected to resist heat loss more than one of a lower Rvalue, if all other factors such as air leakage or heat radiation are the same.
And as Mr. Collins elaborates,
Rvalue = resistance to the movement of heat
U = the ability to transfer heat, obviously an inverse condition, to resistance, or in other words, or to allow the transfer of heat
But as we elaborate below at Definition of U value or Ucoefficient of heat loss resistance, the NFRC (National Fenestration Council) in discussing solar heat gain at windows, describes the UFactor (U) as follows:
UFactor measures how well a product prevents heat from escaping a home or building. UFactor ratings generally fall between 0.20 and 1.20. The lower the UFactor, the better a product is at keeping heat in. UFactor is particularly important during the winter heating season. This label displays UFactor in U.S. units. Labels on products sold in markets outside the United States may display UFactor in metric units.
If you like, read below this section to see our details about "K" values, "U" values, and "R" values as measures of heat movement in buildings. Actually calculating a building's actual rate of heat loss is pretty simple  it's a "cookbook" process that uses the
following formula:
Also see HEAT LOSS in BUILDINGS
Heat Loss using "R" values:
(Building Heat Loss in BTU's per hour) =
(Building Total Surface Area in sq.ft.) / (Surface Area "R" value) x (Temperature Difference)
Temperature Difference = the difference in temperature in deg F. on the two sides of the building surface, typically indoors and outdoors
Surface Area "R" value = the "R" value of the surface area being evaluated (say an insulated wall).
Heat Loss using "U" values:
(Building Heat Loss in BTU's per hour) U = 1/R,  or in other words 
(Building Total Surface Area in sq.ft.) / (Surface Area "U" value) x (Temperature Difference)
But there's more work to do for a complete answer to building heat loss. We need to make up a simple table which will contain the total surface area of each type of material (since each will have it's own "R" value) and then plug in the area's "R" value and the temperature difference. Usually we assume the same temperature difference for all of the areas of the building though this might be a simplification since that may not be exactly true.
We're also missing, from this simple calculation, the effects of wind on a building's heat loss, though a more sophisticated version of this approach might simply adjust the temperature difference to include the wind factor.
For example, you could use a wind/temperature chart to derive the effective outdoor temperature when it's also windy. In cold conditions, adding a wind velocity will lower the effective outdoor temperature and thus it will increase the temperature difference across the building wall.
Use any "wind chill factor" chart for this data. Still more sophistication of measures of heat loss are possible by adding the effects of moisture on heat loss from a surface, but while this is important for a (sweaty) human in cold conditions it is generally ignored when considering building heat loss.
This is a perfect application for an Excel or similar spread sheet, listing each building surface type (wall, window, door), it's R, K, or U value, and its total area. Adding temperature difference across these surfaces permits a calculation of the heat loss (or gain) through each surface type. These are simply added together to represent the entire building's heat loss or gain.
You may have noticed we keep talking about heat loss and then we add "or heat gain" in the same sentences or headings. That's because heat loss analysis works just fine for both building heating and building cooling. The only differences between looking at heat loss and heat gain for a building are the direction of heat flow and the fact that we may be using different equipment with different equipment efficiencies (a heating furnace or boiler versus an air conditioner).
If we're in a heating climate and are in the heating season, heat will flow from the building interior to the outdoors.
If we're in a cooling climate and are in the cooling season, heat will flow from the outdoors to the building interior. Just remember that (according to the laws of thermodynamics), heat (or energy) always flows from the warmer (or more exited state) into the cooler (or less excited state) area of a building.
A building's K value or Kcoefficient of heat transmission is one way to express the heat loss in a building. "K" is defined as the number of BTU's of heat moving through any material with these details:
So "K" takes a lot of variables into consideration and gives us the rate of heat loss per square foot of building surface, per inch of thickness of material in that building surface, per degree of temperature difference in Fahrenheit, in BTUs per hour.
By "degree of temperature difference in Fahrenheit" we mean that we are taking into consideration the difference in temperature on the two sides of our building surface. For example, if the indoor temperature in a building is 68 deg. F. and the outdoor temperature is 48 deg. F., then we have a 20 degree temperature difference on the two sides of the building (wall or roof for example).
This temperature difference on the two sides of a surface, say an insulated building wall, for example, is very important in understanding how a building loses heat (in the heating season) or gains heat (in the cooling season). That's because the rate of heat transfer through a material increases exponentially as a function of the temperature difference. This is intuitively obvious and is confirmed by physicists.
For example, if the temperatures on either side of a building wall were the same, there would be no heat loss or gain through the building wall. As the temperature difference on either side of that same wall increases, say from one degree of difference to 20 degrees of difference the rate of heat transfer increases.
An interesting version of this heat transfer theory was shared with the author in a class on how to minimize building heating costs when the instructor told us that "the thermal conductivity of finned copper heating baseboard is exponentially greater at higher temperatures".
He was saying that if we ran heating water from our heating boiler through the baseboards at 200 deg.F. we would see much more efficient heat transfer from the heating baseboards into the building. There are other factors involved in heating system efficiency such as the length of boiler on cycle (longer is more efficient), so there was more to think about, but the instructor was applying classic heat transfer theory that is reflected in the "K" values of building insulation as we've discussed here.
Uvalue measures the ability to transfer heat, an inverse condition, to heat movement resistance, or in other words, or Uvalue measures the ability of a substance to allow the transfer of heat
The NFRC (National Fenestration Council) in discussing solar heat gain at windows, describes the UFactor (U) as follows:
UFactor measures how well a product prevents heat from escaping a home or building. UFactor ratings generally fall between 0.20 and 1.20. The lower the UFactor, the better a product is at keeping heat in. UFactor is particularly important during the winter heating season. This label displays UFactor in U.S. units. Labels on products sold in markets outside the United States may display UFactor in metric units.
Computing "K" values (discussed above) tells us the heat loss rate for a specific material, thickness, area, and temperature difference but while we need to be able to calculate "K" values, those alone don't tell us what's going on in an actual building. We need to be able to combine all of the rates of heat loss (or gain) across all of the types of surfaces, insulation, and building material for the whole building  at least for all of its external or perimeter surfaces including roofs, walls, and floors as well as windows and doors. That's where the "U" value makes its appearance.
A building's "U" value or Ucoefficient of resistance of heat loss is a related measure of resistance to thermal energy or heat flow out of a building (if it's warmer inside than outside) or conversely the same concept works in a warm climate where air conditioning is in use, except that we expect outside heat to be flowing into the building.
A building's "U" value is much more complete, and therefore useful than "K" values alone because a building's "U" value combines the "K" factors for all of the building's surfaces and materials. In other words, we add the effects of heat loss (or gain), still expressed in the number of BTU's per hour per square foot of area, and still expressed per degree of Fahrenheit of temperature difference and still expressed per inch of thickness of material (just as with "K" values), for all of the substantial areas and surfaces of the exterior of a building's floors, walls, windows, doors, ceilings, or roofs (if cathedral ceilings are present).
To calculate the "U" value, or overall heat loss (or gain if we're air conditioning) for a building, we need to add the "R" values for each material in the structure, and to factor in the total area of each material in the structure. We discuss this procedure in more detail below at "Calculating Heat Loss for a Building".
For details about SEER please see SEER RATINGS & OTHER DEFINITIONS from which we quote:
Seasonal Energy Efficiency Ratio (SEER): This is a measure of equipment energy efficiency over the cooling season. It represents the total cooling of a central air conditioner or heat pump (in Btu) during the normal cooling season as compared to the total electric energy input (in watthours) consumed during the same period. SEER is based on tests performed in accordance with ARI 210/240.48
The "indoor design temperature" for a building refers to the assumed target indoor temperature that the building owner or occupants want. Typically 70 deg.F. is used unless the owner specifies something different.
The "outdoor design temperature" for a building is (for heating purposes) assumed to be the average lowest recorded temperature for each month between October and March (the heating season in most climates). If we are specifying a "design temperature" for cooling climates we'd use the average outdoor highest recorded temperature during the heating season, perhaps April through September.
Watch out when calculating building or room heating needs. In a recent review of the number of linear feet of heating baseboard needed for a New York building addition we tried out an excellent heat loss analysis program provided by SlantFin. The program considers most of the key variables you'd want examined for an accurate and reliable heating design. But we found that our building had properties not considered by the heat loss software, including
Fortunately simpler rules of thumb analysis by consultants at our heating equipment and parts supplier indicated that the modified design would adequately heat the space.
Some building insulation designers and architects look at the number of "degree days" as an easy way to get at the average outdoor temperatures for an area and a season. A Degree Day is the daily average number of degrees Fahrenheit that the outdoor temperature is below 65 deg.F.
The number of "degree days" during a heating season is easy to obtain: call your local oil delivery company or utility company. These energy providers keep close tabs on degree days for their area since this number is used in planning for the automatic delivery of energy. It's the number of "degree days" that have occurred in a given period, combined with a building's historic rate of heating oil use, for example, that tells an oil company when to schedule that building for an automatic delivery of heating oil.
"One ton" of cooling capacity, historically, referred to the cooling capacity of a ton of ice. Restated we can define one ton of cooling capacity as the amount of heat energy absorbed in the melting of one ton of ice over a 24hour period.
One ton of cooling capacity is the same as 12,000 BTU's per hour of cooling capacity or 288,000 BTUs of cooling capacity provided over a period of 24 hours (12,000 x 24 hours = 288,000).
Tons of ice does not, however, explain an important factor in the comfort produced by air conditioning systems, reduction of indoor humidity  that is, removing water from indoor air. Cool air holds less water (in the form of water molecules or gaseous form of H2O) than warm air.
Think of the warmer air as having more space between the gas molecules for the water molecules to remain suspended. When we cool the air, we in effect are squeezing the water molecules out of the air. When an air conditioner blows warm humid building air across an evaporator coil in the air handler unit, it is not only cooling the air, it is removing water from that air. Both of these effects, cooler air and drier air, increase the comfort for building occupants. One ton of cooling capacity equals 12,000 BTU's/hour of cooling capacity.
Also see
DEW POINT CALCULATION for WALLS
DEW POINT TABLE  CONDENSATION POINT GUIDE
Note that the BTU rating of an air conditioner itself does not tell you how economically those tons of cooling capacity are being produced. For the answer to that question
see SEER RATINGS & OTHER DEFINITIONS for air conditioners and heat pumps.
What's a WattHour? Watt hours (Wh), sometimes written W.h, can measure either electrical energy produced, say by a power station, or Watts can measure the amount of electrical energy consumed (say at a light bulb or an air conditioner in our home). For air conditioners, the A/C units' total Wh is the energy used in running the air conditioning system for an hour.
See DEFINITIONS of ELECTRICAL TERMS for details about volts, watts, amps, and power factor.
Watts (W) as used in a simplified manner here and by electricians, is a measure of electrical power and is expressed by any of the formulas shown below. [All forms of power are measured in units of Watts, W, but this unit is generally reserved for real power (see definitions further below.]
DC circuits: W = V x I (this is a simplified formula and is technically exactly correct for DC circuits. For AC circuits,
AC circuits: Watts W=V*I*PF where PF = power factor
See DEFINITIONS of ELECTRICAL TERMS for details about volts, watts, amps, and power factor.
Also see AMPS & VOLTS DETERMINATION "How to estimate the electrical service ampacity and voltage entering a building".
Reader Daniel Mann adds that "Watts is correctly shown as WattsVoltage times Current times power factor. Since power factor varies all over the place,..." [W = V x I] "perpetuates misinformation". We include additional more technical explanation of power factor, real power, apparent power, complex power, and reactive power as we elaborate
at DEFINITIONS of ELECTRICAL TERMS.
Lots of electrical appliances include a label providing the appliance's wattage, and in the case of heating and air conditioning equipment, lots of other details are provided too.
See A/C DATA TAGS for details.
A BTU is a measure of heat energy, or the amount of heat given off when a unit of fuel is consumed. One BTU is the amount of heat energy we need to raise the temperature of one pound of water by one degree Fahrenheit. One BTU also is defined as 252 heat calories (this is not the same as food calories).
When talking about air conditioners or heaters, we talk about the A/C unit's BTUh capacity  the number of BTUs of cooling (lowering rather than raising temperature) it can produce in an hour of running.
When we are heating a building BTUs describe heat given off by consuming fuel or energy from some source (electricity, natural gas, LP gas, oil, etc.) of which some portion is delivered to the building occupied space (see AFUE and HSPF).
When we are cooling a building, or when we are describing an air conditioner or heat pump's rated capacity (in BTUs), we are describing the removal of a quantity of heat from the building  or really from the building's air.
Complete details about the definition of BTUs and BTUH, and examples of uses & calculations using BTU numbers can be found at Definition of BTUs and Definition of BTUH
Also see DEFINITION of HEATING, COOLING & INSULATION TERMS where we further discuss and define BTUs, Calories, and other energy measures.
BTUH: Based on the definition of BTUs above, BTUH describes the number of BTUs of energy produced (as heat) or removed (by air conditioning) in one hour.
Latent heat is defined as the amount of heat absorbed by a substance with no change in a temperature  such as when a substance changes state (from water to steam, for example)
In other words, heat that is absorbed by a substance with no change in temperature is latent heat. For example when a substance changes state (liquid to gas) latent heat is involved.
Definition of Superheat: The latent heat of vaporization is defined as the number of BTUs to raise one pound of liquid to a pound of vapor (to a varying degree per BTU depending on the type of vapor  this is "superheat"). Our Sketch explaining latent heat of vaporization shown at left is provided courtesy of Carson Dunlop Associates.
The latent heat of condensation is defined as the number of BTUs necessary to change a state back from a vapor to a liquid
The latent heat of solidification is defined as the amount of energy (or number of BTUs) needed to change a liquid to a solid (such as water to ice) while the temperature remains unchanged (at sea level, 32 °F).
Sensible heat is defined as the amount of heat that we can sense or feel or measure.
When an air conditioner system is working, the larger diameter tubing on the lowside of the system combined with the effects of the refrigerant metering device (cap tube or thermostatic expansion valve) results in a reduced pressure on the low side (compared with high side pressure). The reduced pressure causes vaporization of the liquid refrigerant inside the cooling coil, which in turn means that sensible heat is absorbed by the cooling coil.
When the same air conditioner system is working, the smaller diameter tubing on the high side reduces available volume so that (along with the effect of the compressor itself) we increase the pressure and temperature of the refrigerant so that sensible heat can be transferred to ambient outdoor air.
Specific heat is defined as the amount of heat required to raise the temperature of a given substance by one unit of temperature (in our examples by one °F.) Specific heat is also defined as the amount of heat (in calories) to increase the temperature of one gram of a substance by one deg C (Celsius).
The specific heat of water is defined as a constant and = 1
The specific heat of ice is is 5
A definition of one BTU is the energy required to raise one pound of water by one degree F (sensible heat).
In which direction does heat flow: heat energy always flows from the warmer substance to the cooler substance, down to 460 °F where all molecular movement stops.
A neat fact is that the heat flows more rapidly (efficiently) between two substances when there is a greater temperature difference between them. That's why the thermal conductivity of finned copper tubing heating baseboard is exponentially greater at higher degrees of heating water temperature, and that's why we like to run our heating boiler at a higher rather than a lower upper limit temperature.
Also see DEFINITION of HEATING, COOLING & INSULATION TERMS
Note: Outside of the U.S. and some other places, BTUs is being replaced with the SI unit of energy, the Joule. (J). The English have beaten out the Scots by James Prescott Joule who defined this value. since there are 3600 seconds in an hour) the following formulas equating Watts, Joules, and Newton meters can be written:
1 Watt second (Ws) = 1 joule (J) = 1 newton meter 1 Watt hour (Wh) = 3600 Joules
1 kilowatt hour = 3.6 x 10^{6} Joules, since there are 1000 watts in a kilowatt.
We can think of an air conditioner's "efficiency" as expressed either in the total operating cost for a season of use, or you may prefer to just express the air conditioner's efficiency as its operating cost to run the system for one hour.
The equation shown at page top is designed to reduce all of the parameters describing air conditioning efficiency to a single efficiency number, SEER. SEER numbers are useful when we're comparing one air conditioner with another. But suppose we want to know the actual air conditioning cost per season, or air conditioning cost per operating hour to operate our air conditioner?
To translate our air conditioners SEER rating into actual air conditioning operating costs we need to know:
One ton of air conditioning capacity produces the same cooling ability as melting one ton of ice in 24 hours. Sketch courtesy of Carson Dunlop Associates.
288,000 BTUs / 24 hours = 1 Ton of cooling
12,000 BTUs / hour = a 1ton air conditioning system
[Click to enlarge any image]
A oneton air conditioner claims to remove 12,000 BTUs of heat from the building air per hour of operation.
Or if we know the total number of BTUs at which an air conditioning system is rated, since this number is usually given in BTUH or BTUs / hour, we just divide that number by 12,000 to get the number of tons of cooling capacity.
A 36,000 BTUh air conditioner is providing 36,000 / 12,000 or 3 Tons of cooling capability per hour.
If we know the number of tons of cooling capacity that an air conditioning system is rated for, we just multiply the number of air conditioning capacity in Tons by 12,000 to get the number of BTUs of cooling capacity of the system.
A 3ton air conditioner is providing 3 x 12,0000 or 36,000 BTUs of cooling capability per hour.
To assist in choosing the right sized air conditioner, we provide a typical air conditioner chart
at AIR CONDITIONER BTU CHART.
Watch out: more is not always better. Don't buy an air conditioner that is too big: if you install a system that is too powerful (too many tons of cooling capacity) the building will be less comfortable than if you install a properlysized air conditioner. Too many tons of air conditioning mean the system will shut off on short cycles and won't run long enough to reduce the indoor humidity to a comfortable level. Details are
at DEHUMIDIFICATION PROBLEMS
How much electricity our air conditioner uses
per hour is easy to calculate.
Let's assume that the data tag on our air conditioner
says that the unit is a 5000 BTUh device with a SEER rating of 10. This means our A/C unit will produce
5000 BTUs of cooling in an hour of running. Since SEER=10 means that 10 BTUs used per Wh, then
5000 BTUh / 10 SEER = 500 Watts per hour that our A/C unit will use.
A common example we use (because the math is easy) is to assume we have 125 days of cooling season during which we run the air conditioner for eight hours per day.
8 x 125 = 1000 hours of cooling operation over a season
500 Wh (watts used per hour) x 1000 (hours per season) = 500,000 Wh per season
So we are using 500,000 Watt Hours of energy (electricity) per cooling season. We divide this by 1000 to convert to Kilowatts since that's how our electrical bill will express our electricity usage.
500,000 Wh / 1000 = 500 kWh or kilowatt hours per season of use
That's how much electricity we're using over the cooling season.
Definition of Low Side in an Air Conditioning System refers to the components on the lowtemperature and lowpressure side of the compressor unit. In an air conditioner, the low side includes the suction or intake side of the compressor unit, suction piping connected to the evaporator coil, the evaporator or cooling coil, and the outputend of the metering device or TEV.
Definition of High Side in an Air Conditioning System refers to the components on the hightemperature (above ambient air temperature) and high pressure side of the compressor unit. In an air conditioner in cooling mode these include the output or high pressure side of the compressor unit, the high pressure gas refrigerant line connected to the condensing coil, the condensing coil itself, and the inlet side of the metering device located near the evaporator coil.
These parts are named and illustrated
at AIR CONDITIONING & HEAT PUMP SYSTEMS
and
at COMPRESSOR PRESSURE READINGS we discuss air conditioner system high side and low side further.
At OPERATING COST we determine the actual dollar cost of running an air conditioner either by the hour of by the season of use. It's easy to get from that data to actual air conditioning operating costs in dollars.
The AFUE (Annual Fuel Utilization Efficiency) rating number for heating equipment is a measure of heating system efficiency. AFUE tells you, for each dollar you spend on energy for heating by gas, oil, or another fuel, just how much of your dollar shows up inside the occupied space of your building as heat.
See AFUE DEFINITION, RATINGS for more details and for current required AFUE percentages for U.S. & Canadian heating equipment.
HSPF is a heating performance measurement used to evaluate the efficiency of heat pumps when in heating mode. Higher HSPF numbers mean greater heating efficiency. Currently heat pumps in the U.S. should have an HSPF of 6.8 or higher.
Also see ENERGY STAR PROGRAM and for tips on how to cut air conditioning or heat pump operating costs, see AIR CONDITIONING HEAT PUMP SAVINGS.
What's a WattHour? Watt hours (Wh), sometimes written W.h, can measure either electrical energy produced, say by a power station, or Watts can measure the amount of electrical energy consumed (say at a light bulb or an air conditioner in our home). For air conditioners, the A/C units' total Wh is the energy used in running the air conditioning system for an hour.
See DEFINITIONS of ELECTRICAL TERMS for details about volts, watts, amps, and power factor.
Watts (W) as used in a simplified manner here and by electricians, is a measure of electrical power and is expressed by any of the formulas shown below. [All forms of power are measured in units of Watts, W, but this unit is generally reserved for real power (see definitions further below.]
DC circuits: W = V x I (this is a simplified formula and is technically exactly correct for DC circuits. For AC circuits,
AC circuits: Watts W=V*I*PF where PF = power factor
See DEFINITIONS of ELECTRICAL TERMS for details about volts, watts, amps, and power factor.
Also see AMPS & VOLTS DETERMINATION "How to estimate the electrical service ampacity and voltage entering a building".
Reader Daniel Mann adds that "Watts is correctly shown as WattsVoltage times Current times power factor. Since power factor varies all over the place,..." [W = V x I] "perpetuates misinformation". We include additional more technical explanation of power factor, real power, apparent power, complex power, and reactive power as we elaborate
at DEFINITIONS of ELECTRICAL TERMS.
Lots of electrical appliances include a label providing the appliance's wattage, and in the case of heating and air conditioning equipment, lots of other details are provided too.
See A/C DATA TAGS for details.
Because no amount of insulation can keep a drafty building warm, also review ENERGY SAVINGS PRIORITIES.
...
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What does 20 amps actually mean  Mary Riccio
In a residential electrical circuit, a 20amp rated circuit means that the electrical wire and it's overcurrent protection (fuse or circuit breaker) are rated for a total load or total current draw of 20 amps. The sum of all of the electrical current drawn by everything connected to that circuit must be 20Amps or less  else the circuit breaker will trip or fuse will blow to prevent overheating of the wire (and a fire hazard).
The definition of AMPS or ampacity and other electrical terms can be found at InspectAPedia either by using the search box found at the top and bottom of our pages, or by clicking on this link
to DEFINITIONS of ELECTRICAL TERMS.
Quoting from that article's "Definition of AMPS Electrical Current",
Amperage or Amps provided by an electrical service is the flow rate of "electrical current" that is available. Mathematically, Amps = Watts / Volts. (Amps = Watts divided by Volts)
Speaking practically, the voltage level provided by an electrical service, combined with the ampacity rating of the service panel determine how much electrical demand, or in another sense how many electrical devices can be run at one time in the building.
Depending on what you are looking at, 20Amps may be the rating of a particular piece of equipment. For example a 20Amp circuit breaker means that that safety device will limit to 20 Amps the current drawn on the circuit that it is protecting. If you plug three 10amp electric heaters into that circuit and they all are running simultaneously, their combined current draw (3 x 10 = 30Amps) should trip the circuit breaker and turn off the circuit to protect it from overheating and a possible fire.
...
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