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Economics For The
DIY Grid-Tie PV System
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System page...
This page covers the costs and energy savings for the grid-tie PV
system we put in, and compares PV to other potential renewable energy
projects.
Cost of System
The table just below shows the cost of the
system. Basically we bought a "kit" offered by
WholesaleSolar.com, and
then added a few things that were needed but not included in the kit.
There were some costs associated with building the ground mounts, and
renting a trench digger. The only new tool I had to buy for the project was a smaller
torque wrench -- it seems like a project of this size should allow one to
justify the purchase of more new tool -- bummer!
The table also shows the rebates we expect
to get from the federal government and from Montana state.
We supplied all of the labor,
|
Item |
Cost |
Cost per watt |
| |
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WholeSale 2150 watt Solar kit |
$8999 |
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10 REC 215 watt PV panels |
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10 190 watt Enphase micro inverters |
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PV mount rails and hardware |
|
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Weeb Washers |
|
|
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Addons to kit |
|
|
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Enphase EMU reporting unit |
$345 |
|
|
Disconnect Switch |
$65 |
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Placards |
$0 |
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Shipping |
$0 |
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| |
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Wire -- array to house |
$106 |
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Wire -- ground and other |
$25 |
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Conduit |
$30 |
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Array Junction Box |
$20 |
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Circuit breaker |
$9 |
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Wire nuts, lugs, wire ties |
30 |
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Trencher rental |
$54 |
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Mount Lumber |
$96 |
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Mount Hardware (plates, bolts, U ftgs) |
$50 |
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Concrete |
$130 |
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Labor |
$0 |
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Total |
$9959 |
$4.63 per STC watt |
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Rebates |
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Federal 30% |
$-2988 |
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Montana $500 |
$-500 |
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Grand Total after rebates |
$6471 |
$3.01 per STC watt after rebates |
The Wholesale Solar kit included most
of what was needed. The exceptions being the Enphase EMU reporting unit and the AC
disconnect switch, which were added on to the order. Shipping was free.
The kit approach (I think) turned out
to be a good way to go.
I think that we could have done
somewhat better on total cost by looking very hard for bargains, doing a roof
mount system, and (possibly) with a single inverter system. But, we are
all around pleased with the system and feel it was worthwhile to spend a few
more bucks in some areas.
If you use the more or less standard
rule of thumb of $8 per watt for professionally installed system, the system
would have cost $17,200 before rebates, for a Do-It-Yourself saving of $7,200. I'm
not sure that I quite believe this, as system prices have been coming down,
and the $8 per installed watt for grid-tie systems might be high. If you are thinking about doing your own system, and the
main incentive is to save money, I would definitely look around for the best you
can do with your own install, and also get some estimates from local installers
just to make sure there is really a saving in your situation.
Projected Saving in Energy
According to PVWatts, the system
should save us about 3000 KWH a year. For us, at about 8 cents per
KWH, this will save us about $250 a year.
PV Watts run for our system -- using
Billings, MT
| Station
Identification |
| City: |
Billings |
| State: |
Montana |
| Latitude: |
45.80° N |
| Longitude: |
108.53° W |
| Elevation: |
1088 m |
| PV System Specifications |
| DC Rating: |
2.2 kW |
| DC to AC Derate Factor: |
0.820 |
| AC Rating: |
1.8 kW |
| Array Type: |
Fixed Tilt |
| Array Tilt: |
45.0° |
| Array Azimuth: |
180.0° |
| Energy Specifications |
| Cost of Electricity: |
7.9 ¢/kWh |
|
|
Results |
Month |
Solar Radiation
(kWh/m2/day) |
AC Energy
(kWh) |
Energy Value
($) |
| 1 |
3.23 |
187 |
14.77 |
| 2 |
4.16 |
213 |
16.83 |
| 3 |
5.21 |
287 |
22.67 |
| 4 |
5.29 |
269 |
21.25 |
| 5 |
5.79 |
297 |
23.46 |
| 6 |
6.02 |
286 |
22.59 |
| 7 |
6.60 |
318 |
25.12 |
| 8 |
6.33 |
308 |
24.33 |
| 9 |
5.88 |
285 |
22.52 |
| 10 |
4.77 |
253 |
19.99 |
| 11 |
3.61 |
197 |
15.56 |
| 12 |
3.02 |
174 |
13.75 |
| Year |
5.00 |
3073 |
242.77 |
|
Note that the DC to AC Derate Factor
was increased from the default of 0.77 to 0.82
based on this paper from Enphase.
This is a return of about 4% on the
$6,500 we invested in the system. When you consider that the "earnings"
from this system are income tax free, and that the savings will go up as the
cost of energy goes up, its really not a bad return.
Carbon Emissions Reduction
For our part of the country, about
1.5 lbs of CO2 is emitted for each kilowatt-hour of electricity used. So, the CO2
reduction per year is about (3073 KWH)(1.5 lbs/KWH) = 4,600 lbs of CO2.
This is a relatively large CO2
reduction for the amount of energy saved because electric power generation is an
inefficient process that makes use of a lot of high carbon fuel (coal).
So, saving or generating electricity is a high leverage way to reduce carbon
emissions in the US.
For example:
1 KWH of electric energy from a
coal fired power plant
2 lbs of CO2
1 KWH of electric energy from an
NG power plant 1.3 lbs of CO2
1 KWH of heat energy from NG in a
95% efficient furnace 0.5
lbs of CO2
1 KWH of heat energy from burning
wood
0 lbs of CO2
About half of our electric power in
the US comes from coal fired power plants. These plants (because they are
so high in carbon emissions) account for 80% of the CO2 emissions due to
electricity generation in the US. In spite of the rapid growth of wind and
solar, the 50% has changed little over the past few years. My personal
opinion is that the only practical path to reducing these emissions from coal
fired power plants significantly is for us (you and me) to use a lot less
electricity.
We have been
working on trying to get our carbon
emissions down, and this 4,600 lbs of CO2 reduction is a worthwhile chunk --
one of the larger single contributions. As you can see from the section
below its not nearly as effective as conservation and efficiency, but its
worthwhile.
Fun
This was a fun and challenging
project, and even if it never saved a dime, it would be worth it.
PV is a fascinating technology, and it was a real pleasure to learn something
about it and see the system take
shape.
How Does PV Compare to Other Solar
or Conservation Projects?
So, how does PV compare to other
solar or conservation or efficiency project you could do? Here are some
examples.
Note that this is a DIY oriented
site, and all of these projects assume "free" labor (your labor).
| Project |
Our Solar
PV System (1) |
Best 8 "Half"
Projects (2) |
Full
"Half"
Program (3) |
$1K Solar
Water Heater (4) |
"Solar Shed"
Space Heating (5) |
| Initial Cost |
$10,000 |
$378 |
$11,725 |
$1,000 |
$4,700 |
| Energy Saving per year (KWH/yr) |
3,000
KWH |
10,350
KWH |
59,100
KWH |
3,700
KWH |
8,900
KWH |
| CO2 reduction per year (lbs/yr) |
4,500 lbs |
10, 456 lbs |
36,100 lbs |
4,000 lbs |
4,400 lbs |
(1) Our new
grid-tie PV system as described here...
(2) The
best (most
cost effective) projects from our "Half" program... these are
simple conservation projects.
(3)
All of the
Half Program projects... this is all 23 of the conservation and solar
heating projects we did for Half.
Note: The totals for the Half program are high because they include just about
everything we spend energy on, including transportation.
(4) The
$1000
DIY Solar Water Heating project...
(5) Our
solar space heating system...
These numbers are kind of hard to
believe when you first look at them. How can a handful of simple projects
(the "Best 8") that cost less than $400 to do save 3 times as much energy as a
state of the art, $10,000 PV system? Well, it may be hard to believe, but
I can tell its true. Its also very likely that if you go find your "Best
8" projects and do them, your results will be similar.
I'm not trying to be hard on PV -- I
think its a great technology, and I love my new system, but be sure you do the
other stuff too. If you do the other stuff first, you can pay for your PV
system with the dollar savings from the other projects.
I did not include rebates for any of
the projects in the
above comparison -- they vary a lot from place to place. I also feel that we might be
better off if rebates were based on something like the actual energy saving
and/or emissions reduction accomplished. The way it is now, the renewable
technology industry (PV, wind, ...) with best lobbyist gets the biggest rebate
programs -- I think this works to the detriment of other renewable technologies
that may be superior from cost effective energy reduction point of view, but are
not as glamorous and don't lobby so well. Seems like we should be
rewarding actual results?
Back to the PV
System page...
Gary November 20, 2009