You need to know the intrest rate on the money to buy the solar array (even
if the customer is now borrowing there is a time value to money). Lets say
the customer borrows $20,000 @ 8% on Jan 1 2010 to purchase the array. Now
assume that he normally pays his electric bill on Jan 1 each year (money he
now gets to save). You can use XNPV (must install the analysis toolpack) to
compute a savings of $3,618.23

here is the source data with 8% in A1 and the table starting a A3.
My formula is
=XNPV(A1, B3:B33, A3:A33)

8%

01/01/10 -20000 3618.230141
01/01/11 1242.823529
01/01/12 1304.964706
01/01/13 1370.212941
01/01/14 1438.723588
01/01/15 1510.659768
01/01/16 1586.192756
01/01/17 1665.502394
01/01/18 1748.777514
01/01/19 1836.216389
01/01/20 1928.027209
01/01/21 2024.428569
01/01/22 2125.649998
01/01/23 2231.932497
01/01/24 2343.529122
01/01/25 2460.705578
01/01/26 2583.740857
01/01/27 2712.9279
01/01/28 2848.574295
01/01/29 2991.00301
01/01/30 3140.55316
01/01/31 3297.580818
01/01/32 3462.459859
01/01/33 3635.582852
01/01/34 3817.361995
01/01/35 4008.230095
01/01/36 4208.641599
01/01/37 4419.073679
01/01/38 4640.027363
01/01/39 4872.028732
01/01/40 5115.630168

--
HTH...

Jim Thomlinson


"Kyle P." wrote:

Hello,
I need to determine how much money a solar array will save one of my
customers over the next thirty years.

I'd like to know the formula to calculate this.

Inputing: 17 Cents per kWh.
21,128 kWh's per year
a 5% increase in the cost of electricity per year. (17cents
per kWh increasing by 5% every year).
over 30 years.

Help Please! Thanks.