# 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.