# Energy and Efficiency

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• Energy and Efficiency
• Energy can be transferred BUT cannot be CREATED or DESTROYED
• When energy is transferred not all of it is useful, some many be given off as wasted energy.
• Wasted energy transfers to the surrounding
• Sankey diagrams
• Width of arrow= amount of energy
• Arrow continuing straight= useful energy
• Arrows pointing down= wasted energy
• Pay back time
• eg. The double-glazing for a house costs £3,000 but saves £150 per year in fuel costs. What is its payback time?
• Answer; payback time = 3,000 ÷ 150 = 20 years
• Is something cost effective to install? How long will it take for me to start benefiting economically from it?
• Efficiency
• Efficiency= useful energy out / total energy in x100%
• Efficiency= useful power out / total power in x100%
• Formulas given in exam
• Transfering electrical energy
• Everyday applianes
• Lamp; electrical energy-> heat and light energy
• Kettle; electrical energy-> heat and sound energy
• TV; electrical energy-> light, sound and heat energy
• The amount of energy an appliance transfers depends on how long the appliance is switched on and its power.
• E= P x t
• E is energy transferred in kilowatt-hours, kWh.   P is power in kilowatts kW.    t is time in hours, h
• E is energy transferred in joules, J.         P is power in watts, W .          t is time in seconds, s
• How much does it cost to use a washing machine for 1.5 hours?  Power rating of washing machine = 2.0 kW cost of electricity = 6 pence per unit
• E=P x t so, E=2x1.5. E=3
• Units x cost per unit= total cost. So, 3 x 6p =18p