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: Examples one lightbulb for 24 h - 1 kWh food - 3 kWh / day(*) bath - 5 kWh(*) litre of petrol - 10 kWh aluminium can - 0.6 kWh