National design handbook prototype on passive solar heating and natural cooling of buildings |
When studying thermal design of buildings, thermal comfort, solar energy and other related areas a number of units of measurement will be repeatedly encountered. Some will be familiar whilst others may not. To help clarify these the following information is provided.
Preferred multiples and submultiples
Prefix |
Symbol |
Factor |
Magnitude |
tera |
T |
1012 |
1000000000000 |
giga |
G |
109 |
1000 000 000 |
mega |
M |
106 |
1000 000 |
kilo |
k |
103 |
1000 |
milli |
m |
10-3 |
0.001 |
micro |
µ |
10-6 |
0.000.001 |
nano |
n |
10-9 |
0.000000001 |
pico |
p |
10-12 |
0.000000001 |
Reference: Metric Handbook (SAA-MHI)
Temperature is measured in degrees Celsius (°C); this unit replaces degrees Farenheit (°F) the conversion being:
In scientific work one may often encounter the Kelvin scale (°K). This scale has the same interval as °C but the starting point, known as absolute zero is equal to -273.15°C. i.e. 0°C = 273.15°K and a temperature difference of 5 deg C is the same as a temperature difference of 5 deg.K.
Note: The temperature of an object is written "°C" whilst the difference in temperature of two objects is written "deg.C".
The following are commonly used terms:
T = temperature difference (delta T)
Ta= ambient temperature (usually the air temperature surrounding the object concerned).
T = outside air temperature (usually dry-bulb temperature)
Ti = inside air temperature
Tei = environmental temperature
Work, heat and energy are measured in units called Joules (j)
1 joule = 1 watt for 1 second = rate of energy flow x time = a quantity
The unit of heat flow, energy flow or power Is the watt (W) as shown above.
Generally quantities of energy or heat in relation to buildings will be given in Mj, Gj or kWh. Heat or energy flow rates will be W or kW.
1 kWh=3.6MJ
(1000W x 3600 sec = 3 600 000 J = 3.6 MJ)
Many large single bar electric radiators give out heat at a rate of 1000W. Such a radiator would therefore consume 1 kWh of energy in one hour.
It is preferable to refer to quantities of heat or energy in Joules rather than watt-hours to avoid confusion with the watt.
Density of heat flow rate
If the total rate of heat flow from an identifiable unit is to be measured (such as heat loss from a given building or the radiation received from the sun) the unit of measurement is the watt (W or kW). The density of that flow rate (or intensity) is measured per unit area. Therefore the density or intensity of heat flow or solar radiation is measured in W/m².