Payback Time GCSE Physics

This is how to work out payback time, a lot of people forget how to work this out so I hope my notes help you! :)

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What is payback time?

Payback time is basically where, for example, you bought double glazing for your home for x amount, payback time is the time it takes to save that x amount of money (from heating bills etc) because of the change you made (double glazing).

Sounds confusing but it's quite simple really.

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Here's a question

Q1a)   This is the amount of energy an uninsulated house loses every second from different parts of the house.

  • Roof - 150J
  • Windows - 109J
  • Floor - 75J
  • Draughts - 66J
  • Walls - 200J

Q1ai)   Each year the house costs £760 to heat. How much money is being lost through heat loss through the roof?

METHOD)    150+109+75+66+200=600     You add all the energy lost amounts.

--------   x150=£190(rounded)       Your answer would be £190 

Then you divide the total amount of money spent to heat the house by the total energy lost amount. Then times that by 150J (energy lost through the roof).   

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This is the second part to the exam Q

Q1aii)    Insulating the loft would cut the heat lost through the roof by 50%. The loft insulation has a payback time of 1 1/2 years. How much did the loft insulation cost to buy?

METHOD)   £95 is the 50% saved from £190.

                   You are saving money for 1.5years.

   1.5 x £95= £142.50

So the loft insulation costed £142.50 to buy.

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The two questions before this were questions to do with payback time. This next question shows you how to work out the actual payback time.

(this is in a bit of a weird order sorry I should have put the next question first then these two but oh well... it should still make sense :L)

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Payback time Q

Q2)   It costs £3000 to put double glazing into your home. This improvement saves £150 per year on fuel bills. What is the payback time?

METHOD)     £3000
       ------------ = 20years

This is the formula:

            Cost of the improvement 
PBT = ---------------------------------------
                 Annual saving 

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