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DIY PV System --
How Big to Make it?
Back to the PV
System page...
Deciding How Big to Make Your
Grid-Tie PV System
This page goes over determining what
size of PV system you want to buy.
In actual fact, a grid-tie system can
be almost any size -- there is no "exactly right" size.
The factors that determine what size
system you install are:
- What your current power usage
is.
- How much money you have to
spend on the system.
- How much you can cut your
current power use through conservation and efficiency.
- How much of your electric usage
you want the grid-tie system to cover.
- How much space you have to
install a system.
Since the grid is always there to
cover what your PV system does not produce, you are not going to "run out" of
electricity if your PV system falls short of your power demand. So,
this is really a decision about how much you want to cut your power bill and
your CO2 emissions.
But, I do think its important to
understand your current electricity usage, and how much of that usage the PV
system is actually going to offset. You don't want to spend a lot of
money on a PV system, and then find your electric bill has not gone down nearly as much as you
expected.
In our case, when we moved into this
house in 2000, our power usage was about 1000 KWH per month. This is
obscenely high, but close to the US household average. Through
conservation and efficiency, we were able to cut this down to just about 500 KWH
per month. This conservation was a part of our
"Half" program... All of the projects we used to
reduce electricity
consumption are listed along with what they cost and what they saved.
One thing that leaps out of the numbers immediately is that conservation and
efficiency are much more cost effective at saving electrical energy and carbon
than the PV system is. In our case, cutting electricity use from 1000 KWH
to 500 KWH per month cost a bit over $1,000. Installing the PV system,
will save us just 250 KWH a month, but cost about $6,500 after rebates. So,
the advice that everyone gives you about conservation and efficiency first, and
then PV is absolutely true!
So, after taking a cut at
conservation and efficiency, we are down to 500 KWH per month. How much of
that do we want to address with the new PV system?
We decided to go after about half of
the 500 KWH with the new PV system. The logic for this decision is:
- Our power company only pays for
the PV power until your net usage to zero over a year. If you use 6000
KWH a year, and you generate 7000 KWH a year, they only pay for the first
6000KWH. They will not let your bill on a yearly basis go below zero.
So, this argues for not having a
system larger than your actual needs.
Each utility has their own net
metering arrangement, so check on yours.
There are a few places that
actually pay a premium for solar generated power, and will pay for as much
as you want to produce -- if you have this arrangement, you may want to put
in a larger system.
- We feel that we can still save
some additional electricity through conservation and efficiency -- we wanted
the PV system to be small enough so that we still have some incentive to
save additional power through conservation.
- The cost of a system this size
matched pretty well with what we are willing to spend at this time.
- The grid-tie systems, and the
one we chose in particular, allow fairly easy expansion of the system at a
later time if we decide we went too small.
So, having figured out how many KWH
per month we wanted to save, how do we figure out what size PV system that will
take?
The answer is
PVWatts. This is the very nice PV system sizing calculator
developed by NREL. Its a good calculator -- its honest, takes into
account your local weather, all of the important PV system parameters, and is
easy to use. There are two versions, creatively named Version 1 and
Version 2. I think that for this kind of calculation, Version 1 is easier
to use, and the link above points to Version 1.
Just pick your location from the map.
Then pick a trial size for the "DC Rating" in kilowatts, and click on
"Calculate". If the system turns out to be too small or too large, just
adjust the "DC Rating" input until you get to the size you want.
Here is a sample run for a 2.0 KW PV
system in Billings, MT:
 |
* * * |
AC Energy
&
Cost Savings |
|
 |
| Station
Identification |
| City: |
Billings |
| State: |
Montana |
| Latitude: |
45.80° N |
| Longitude: |
108.53° W |
| Elevation: |
1088 m |
| PV System Specifications |
| DC Rating: |
2.0 kW |
| DC to AC Derate Factor: |
0.820 |
| AC Rating: |
1.6 kW |
| Array Type: |
Fixed Tilt |
| Array Tilt: |
45.8° |
| Array Azimuth: |
180.0° |
| Energy Specifications |
| Cost of Electricity: |
10.0 ¢/kWh |
|
|
|
Results |
Month |
Solar Radiation
(kWh/m2/day) |
AC Energy
(kWh) |
Energy Value
($) |
| 1 |
3.25 |
175 |
17.50 |
| 2 |
4.17 |
199 |
19.90 |
| 3 |
5.22 |
267 |
26.70 |
| 4 |
5.27 |
250 |
25.00 |
| 5 |
5.76 |
274 |
27.40 |
| 6 |
5.99 |
264 |
26.40 |
| 7 |
6.56 |
294 |
29.40 |
| 8 |
6.31 |
286 |
28.60 |
| 9 |
5.88 |
265 |
26.50 |
| 10 |
4.78 |
235 |
23.50 |
| 11 |
3.62 |
184 |
18.40 |
| 12 |
3.03 |
163 |
16.30 |
| Year |
4.99 |
2856 |
285.60 |
|
The trial system I put in here with a 2.0 KW DC rating generates
2856 KWH per year. This is just a bit short of our goal of 3000 KWH a year
-- we ended up installing a 2.15 KW DC rating system.
What PVWatts calls the DC Rating is
the total STC rating of the PV panels -- this is what almost all systems
advertise as their system size. So, if you see a system advertised as a
2000 watt PV system, they very likely mean a system with a DC Rating and an STC
rating of 2 KW or 2000 watts.
Note that PVWatts has other inputs
that you can adjust to match the system you are going to put in. For
example:
- You might want to adjust panel
tilt and orientation if you know what that is going to be (for example to
match your roof).
- If you plan to use a tracking
system so that the panels track the sun's movements, you can select a
tracking system.
- You can adjust the factors that
go into the Derate Factor. For example, some of the Derate factors do
not apply to the system we bought, so I adjusted accordingly.
- You can enter what you actually
pay for one KWH of electricity.
But, I'd say, don't sweat it too much.
Your life will not change much if you miss the system size by 10 or 20%.
If the economics of the PV system is
a major factor for you, than have a look at the
economics page...
Gary November 20, 2009
