A rough guide to sustainable energy No millions, billions, or trillions Make quantities comprehensible and comparable Do calculations per person, to one significant figure Energy unit: kWh Power: Population density: square metres per person Power per unit area: W per square metre Examples 20 mins of kettle - 1 kWh food - 3 kWh / day(*) bath - 5 kWh(*) litre of petrol - 10 kWh