Like Fred said the formula is simply the NPER financial function. BUT
that use in this case is a piece of fiction. That assumes that money is
free, there will be no increase in the house insurance, no increase in
the apraisal of the property and taxes, that the electric company will
waive the customer or meter charges. Now given how all that has been
ignored, I would not be inclined to believe that the "system" would
provide all the electricity either. Actually this reminds me a lot of
all the schemes that popped up around 1980!

Eric wrote:
Ok, hopefully this is a a lot easier to understand.

Q1 Yes the customer pays for the Solar Electric System (which includes
installation) That is the value in A1

Q2 The real question is how long will it take for the customer to break
even? In other words, how many years will pass until A1 = the sum of the
amount they would have spent on elecricity, given that electricity cost
raises 4% over the previous year and the last one year they spent on
electricity (which is found in cell B1).

That should be calculated as follows

A B
1 324.65 100

Given those to values and also knowing that Electricity raises every year 4%
over the previous year, I want Excel to caclulate years of pay back.
It should look something like this:

=sum(104,108.16,112.4864)
which equals 324.65 which means that in exactly 3 years the system would pay
for itself. The returned value would therefore be 3.0

As the values in A1 & B1 change payback time number of years to calculate
for will change drastically.

Hopefully this is much easier to understand. Thank you for your patience.