# Excel - Calculating Payback period

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?
Microsoft Excel

Last Comment
gh_user
Saqib Husain

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
gh_user

Hi Saqib. I came up with the formula.  Might not be right.
Alexandre Michel

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Hi GH

Cheers

Alexandre
gh_user

Thanks Alexandre.
It was great...well explained and tested!!
You are welcome!
If this answered your question, can close the question and rate the answer when you have 2 minutes
Thanks
gh_user

Hi Alexandre.  I cant seem to close question.  (I have been able to rate your answer as best solution).  I think the problem is due to question being abandoned due to my delay in responding.  I have put in request to get rectified.
gh_user

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Microsoft Excel

Microsoft Excel topics include formulas, formatting, VBA macros and user-defined functions, and everything else related to the spreadsheet user interface, including error messages.

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