Calculating Payback Period as a decimal in years

[reesjonesrb]

Good Morning,
I am constructing my solar PV payback spreadsheet and need a formula to calculate how long the payback period is. I have cumulative cashflows, and can see that it is negative for 6 years and then in the 7th is politive. I am looking for a way to quantify the decimal amount of that 6th year until breakeven is met. i.e 6.4 years (or whatever it may be)

All the cumulative cashflow is in cells Q3-Q27 which also corresponds to the years in D3-D27 (year 1-year 25)

Your help would be greatly appreciated.

Thanks,
Barns

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[StephenCrump]

Perhaps something like this?

B7: =MATCH(0,B2:B5,1)-VLOOKUP(0,B2:B5,1,1)/(INDEX(B2:B5,1+MATCH(0,B2:B5,1))-VLOOKUP(0,B2:B5,1,1))

Excel 2010
	A	B
1		Cumulative
2		-30
3		-10
4		30
5		65
6
7	Period	2.25
Sheet1