## Note on units

A **Joule** is the energy delivered in 1 second by a 1 Watt power source. It is the unit of energy or work.

A **Watt** is a Joule per second, e.g. a 100 Watt lightbulb uses 100 Joules of energy every second it is on. It is the basic unit of electrical power.

A **kiloWatt hour** (kWh) is a common unit of electrical output. 1 kWh means 1000 Watts used for 1 hour, say a 1 bar electric fire for 1 hour. It is often the unit of electricity pricing e.g. 1 unit on your electricity bill = 1 kWh.

## Exponential notation

This is commonly used for very large or small numbers. For example, 1 billion tonnes is the same as 1 gigatonne (1 Gt), or 1 x 10^{9} tonnes. 1 millimeter (1 mm) is 1 x 10^{-3} meters (0.001m), etc.

- 1 trillion = tera (T) = 1 x 10
^{12} - 1 billion = giga (G) = 1 x 10
^{9} - 1 million = mega (M) = 1 x 10
^{6} - 1 thousand = kilo (k) = 1 x 10
^{3} - 1 thousandth = milli (m, same symbol for meter) = 1 x 10
^{-3} - 1 millionth = micro (μ) = 1 x 10
^{-6} - 1 billionth = nano (n) = 1 x 10
^{-9} - 1 trillionth = pico (p) = 1 x 10
^{-12}