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# Excel - Calculating Payback period

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Im trying to use excel to assess a solar panel quote given to me.
They have given me the cost of the panel.  I can calculate the savings after 1 year based on the current tariff (electricity price per kwh usage).  The tariff is expected to increase per year (based on index)
A need a excel formula that can calculate the number of years until the total savings match the upfront cost of the panel.
Its based on cost = savings/year (based on current tariff) x (1 + %annual increase in tariff) to the power of number of years)
Currently the variables are:
C = Cost = \$7000
S = Savings/year (based on current tariff) = \$1100
I = Index based on %annual increase in tariff = 3%

But my formula No. of years = LOG(C/S,I) is getting strange answer of 63.

Any suggestions?
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Commented:

1.684488685 using 3

and

-0.527754004 using 3%

Can you show me where you got this formula from?

You can see details of Discounted Payback Period at

http://accountingexplained.com/managerial/capital-budgeting/discounted-payback-period

Commented:
Hi Saqib. I came up with the formula.  Might not be right.
Manager; IT Consultant
Commented:
Hi GH

The formula is Log((1+i*R)/(1+i))/log(1+i)
where i= 0.03
and R = 7000/1100 = 6.36
In this case, the answer is 4.9 years

I decided to test it by doing the calculations one step at a time

To make sure my formula was correct, we can check how long we would need to repay a system costing \$9781.57 ... The answer should be 7 years

Attached is the spreadsheet if you want to play with the values

Hope this is what you were after

Alex
Calculations.xlsx
Manager; IT Consultant

Commented:
Hi GH

Cheers

Alexandre

Commented:
Thanks Alexandre.