L. David Roper
http://www.roperld.com/personal/roperldavid.htm
6 April, 2016
Solar energy is an inflationprotected investment; it reduces electricity costs at the current rate instead of future rates. The economics of solar energy include:
The first thing you should do is make your house as energyefficient as possible, so that renewable energy does not have to supply so much energy.
Several assumptions have to be made to calculate payback for solar PV; here is an example:
There are some ephemeral situations that may reduce payback periods:
In the spreadsheet described below there are two worksheets; one for true cost and one for including ephemeral reductions.
There is a downloadable Excel spreadsheet for doing the calculation. (The spreadsheet may be changed in the future, so you may want to download it again if you need it in the future.) Doing the calculation for the parameters given above and varying the initial electricity rate ($0.10, $0.15, $0.20) and the average rate increase per year (6%, 8%, 10%, 15%), I get the following truecost payback periods, not including any rebates, tax deductions/reductions or SRECs::
Payback (years):  30year loan @ 5% 

Rate Inc.rate 
$0.10 
$0.15 
$0.20 
6% 
30 
22 
16 
8% 
22 
16 
10 
10% 
17 
12 
8 
15% 
11 
8 
6 
Payback (years): 
20year loan @ 5% 

Rate Inc.rate 
$0.10 
$0.15 
$0.20 
6% 
20 
20 
20 
8% 
20 
19 
15 
10% 
20 
15 
11 
15% 
13 
9 
7 
These numbers do not include any of the ephemeral situations which would reduce the payback periods:
Inclusion of these, depending on their values, can make the payback period to be zero!
Here is a specific calculation with ephemeral situations included:
Parameters: 

Peak kW 
23 

Loan Period (years) 
5 

Loan rate 
5% 

Cost/peak kW 
$5,000 

Actual kW factor: 
0.7 

PV kW decline/year 
0.80% 

Sun hours per day over year 
4.5 

Grid energy initial rate 
$0.10 

Grid rate increase 
6% 

Replace inverter loan years 
1 

Inverter cost 
$5,000 

Rebate 
30% 

SREC/kWh 
$0.30 

tax rate 
20% 
The payback graph for these parameters is:
The payback is 7 years.
A Home Power Magazine study showed that, with reasonable assumptions, investing in green infrastructure for your home and business can return as much or more money savings as investing in the stock market earned over the last twenty years. And if enough information were available to calculate the money saved by not dumping carbon into the atmosphere, the energyefficient investment would be even more favorable. This is a good time for U.S. citizens to buy products from companies to make their homes and businesses more energy efficient instead of buying stock in companies, which may go into increasing the salaries of executives. Make companies earn the money they receive from you by actually creating products you need for energy efficiency
As important as money payback are energy payback and carbonemissions payback for solar PV systems. Some good references for these calculations are:
All of these calculations agree that the energy payback and carbonemissions payback are both less than 4 years.
SolarHotWaterPayback
L. David Roper interdisciplinary studies
Roper Global Warming
L. David Roper, http://www.roperld.com/personal/roperldavid.htm
6 April, 2016
c:/science/meteorology/tornados.xlsx