This page is a quick reference for units (and what they mean), conversion factors and any other “facts and figures” that I introduce along the way. After I’ve initially introduced a new unit or important quantity in a post, I’ll endeavour to update this page with the new information. When reading the posts, if you encounter a unit or quantity that doesn’t have any explanatory commentary, it probably means it was introduced in an earlier post. If you need a refresher, check by here first to see if it has been covered.
Metric prefixes
Before we look at units, I’ll first introduce the commonly used metric prefixes that are used in conjunction with units to specify the order of magnitude of a given quantity.
Prefix symbol | Prefix name | Order of magnitude multiplier | Meaning in “ordinary language” |
k | kilo | 10^{3} | thousand |
M | mega | 10^{6} | million |
G | giga | 10^{9} | billion |
T | tera | 10^{12} | thousand-billion |
P | peta | 10^{15} | million-billion |
E | exa | 10^{18} | billion-billion |
The International System of Units
The International System of Units (abbreviated as SI) has seven base units, from which all other units, including those related to energy, are derived. We’ll only need four of the seven four our energy-centric focus:
Dimension | Unit | Symbol |
Length | metre | m |
Mass | kilogram | kg |
Time | second | s |
Temperature | kelvin | K |
The derived SI unit for energy is the joule, J. In base units, one joule is equivalent to one kilogram-metre per second-squared, or 1 kg.m.s^{-2}
Any energy-related quantity can be measured, using the International System of Units, in joules: work done, heat transferred, chemical energy stored, electrical energy generated etc.
In addition to the quantity of energy associated with a situation of interest to us, we are often also interested in the rate at which energy is supplied or used (converted from one form to another). The rate at which energy is used is termed power. The SI unit for power is the watt, W. One watt is equal to one joule of energy used per second: 1 W = 1 J/s
The table below is intended to give some sense of scale in relation to power units (these are rough order-of-magnitude indications only):
W | kW | MW | GW | |
Supply | Sustained labour rate of a single person | Domestic photovoltaic panel | Large commercial wind turbine | Large coal-fired electricity plant |
Use | Domestic light element; laptop computer | Toaster; electric jug | Small town | Large city |
Other commonly used energy units
If, for a given situation, we know the rate of energy use in watts, and we know for how long energy has been used at that rate, then by multiplying the rate of energy use by the time, we can determine the amount of energy used. This is the basis on which electricity is metred. For this reason, electrical energy use is typically specified in units of watt-hours or Wh. One watt-hour is equivalent to 3600 joules: 1 Wh = 3600 J.
It’s conventional for household electricity use to be measured in thousands of watt-hours, or kilowatt-hours (kWh). 1 kWh = 3.6 MJ (MJ = mega joules i.e. million joules, see table above). 3.6 MJ = 3,600,000 J. Given that the kilowatt-hour is for many of us the energy unit with which we’re most familiar, and for which we have the most direct understanding of what the unit means in terms of the useful effects that a single unit provides us, this, rather than the joule, is sometimes used as the unit for aggregating energy from different sources.
Natural gas supply quantities are typically quoted in volumetric units e.g. cubic metres (m^{3}) at a specified standard temperature and pressure (for a given quantity of gas, the density and hence the volume that it occupies varies with temperature and pressure). In order to determine the nominal energy associated with a specified volumetric quantity of gas, this quantity must first be multiplied by the appropriate heating value specified in joules per unit volume at the originally specified standard temperature and pressure. The term “standard” actually has a specific meaning in this context: the International Standard Metric Conditions for measuring natural gas volume are temperature of 15^{o}C (288.15 K) and absolute pressure of 101.325 kPa (kilopascals). The reference pressure of 101.325 kPa is based on atmospheric pressure at sea level . Pressure is measured relative to the measurement device’s surrounding environment, so at sea level, an absolute pressure of 101.325 kPa will read on a typical pressure gauge—one that reads zero when it is exposed to the atmosphere—as approximately zero (allowing for natural variability in atmospheric pressure due to local conditions). So an absolute pressure of 1 atmosphere, or approximately 101.325 kPa, is also referred to as a gauge pressure of zero.
The International Energy Agency (IEA) specifies data on national annual energy supply and use in units of ktoe (kilotonnes of oil equivalent). 1 ktoe = 41.868 TJ.
Other widely used units
Crude oil volume is typically measured in units of barrels, abbreviated as bbl. 1 barrel = 158.987 litres; 1 bbl = 158.987 L
Other handy sources of information
In addition to the information that I provide here, you may also find the Useful Energy Relations page from Tom Murphy’s blog Do the Math handy. Tom’s figure titled “logarithmic scale of energy units, and useful conversions”, provides an especially good visualisation of the relative magnitude of different units (if you’re comfortable thinking in logarithmic terms—note that there’s a factor of 10^{27} difference in magnitude between the units at each end of the line i.e. the chemical energy associated with a gallon of automotive fuel is a billion-billion-billion times the kinetic energy change for an electron or proton passing through a one volt change in electrical potential).