Choose good units No millions, billions, or trillions Make quantities comprehensible and comparable Do calculations per person, to one significant figure Energy unit: kWh Power unit: kWh per day Fluxes: W per square metre Population density: square metres per person Examples 20 mins of kettle - 1 kWh food - 3 kWh / day(*) bath - 5 kWh(*) litre of petrol - 10 kWh aluminium can - 0.6 kWh