# Units

### Understanding Units

- Units of measurement are vital in science to provide clarity and consistency.
**Energy**is measured in**joules (J)**. It’s worth noting that 1 joule is equivalent to 1 newton meter.**Power**is measured in**watts (W)**. This can be conceived as rate of energy use or production.- Kilowatt-hours (
**kWh**) is an alternative unit often used for substantial amounts of energy, particularly in the context of electricity usage. - The specific heat capacity, defining the amount of energy needed to raise the temperature of 1kg of a substance by 1°C, is measured in
**joules per kilogram per degree Celsius (J/kg°C)**. - The efficiency of energy transfer is expressed as a
**percentage (%)**. It’s a ratio of useful energy output to total energy input.

### Derived Units

- The Energy transfer can be calculated by multiplying
**power (P)**by**time (t)**. Hence, its units become a multiplication of the units of power and time, i.e.,**watt-hours (Wh)**or**joules (J)**. - The Heat gained or lost is calculated by the product of
**mass (m)**,**specific heat capacity (c)**, and**change in temperature (ΔT)**. Therefore, heat is measured in**joules (J)**.

### Conversions between Units

- Remember, 1 watt-hour equals
**3600 joules**. Hence, when converting from watt-hours to joules, multiply by 3600. - Conversely, when converting from joules to watt-hours, divide by 3600.
- To convert from kilowatt-hours to joules, multiply by
**3.6 million**(3.6 × 10⁶ J), and vice versa.

### Importance of Proper Units

- It’s key to ensure that all numbers used in a calculation are in the
**same type of unit**. If they’re not, convert them before starting the calculation. - Keep an eye on the unit the question wants the answer in, especially when dealing with derived units.
- Often, efficiency is a ratio of two energy or power values, so has
**no units**but is expressed as a percentage. - Always check your workings, and make sure that your final answer is
**reasonable**and has the**correct units**. Errors in unit conversion are a common source of mistakes.