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 hot water; here is an example:
The 30tube Apricus collector, what a normal house uses, is about 2 kW actual power, according to my experience.
There are some ephemeral situations that may reduce payback periods:
Contrary to the case for solar PV, solarhotwater systems will require some maintenance (e.g., replacement of pumps, valves, gauges, etc.). This ephemeral situation is difficult to estimate. I assume that maintenance will equal the original cost over 25 years after the first 10 years.
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):  5year loan @ 5% 

Rate Inc.rate 
$0.10 
$0.15 
$0.20 
6% 
17 
12 
9 
8% 
15 
11 
8 
10% 
13 
10 
8 
15% 
11 
9 
9 
Payback (years): 
10year loan @ 5% 

Rate Inc.rate 
$0.10 
$0.15 
$0.20 
6% 
18 
13 
7 
8% 
16 
12 
6 
10% 
14 
11 
5 
15% 
11 
7 
4 
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: 

Actual kW 
2 

Loan Period (years) 
5 

Loan rate 
5% 

Cost/peak kW 
$3,000 

Sun hours per day over year 
4.5 

Grid energy initial rate 
$0.10 

Grid rate increase 
6% 

Rebate 
30% 

SREC/kWh 
$0.30 

tax rate for 5year depreciation 
20% 
I assume that maintenance will equal the original cost over 25 years after the first 10 years.
The payback graph for these parameters is:
The payback is 0 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 solarhotwater systems. Some good references for these calculations for solar PV are, which should be similar for solarhotwater systems.:
All of these calculations agree that the energy payback and carbonemissions payback are both less than 4 years.
SolarPhotovoltaic Payback
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