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Sizing Array.

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Posted by thepixelfreak on July 24, 2008, 7:28 pm

How does one go about figuring out how large an array is required? I
know I average about 21 kWh daily, 640 kWh or so monthly. Lets assume I
want to replace all of the grid supplied with PV supplied.


Posted by dold on July 24, 2008, 11:39 pm

"replace" is an interesting word in that sentence.

You could reduce your grid-tied cost to zero by replacing about 80% of the
kWh you use.  

You could reduce your grid-tied consumption to zero by supplying an average
of 21kWh per day of PV energy.  

You could unplug from the grid and go PV-only with a larger system that can
handle the peak demands, accommodate winter production, and average 21kWh
per day.  You could reduce your usage to less than 21 kWh per day, and work
toward solar replacement of the remainder.

If you are grid-tied, you can work with averages, and ignore peaks, for the
most part.

http://rredc.nrel.gov/solar/codes_algs/PVWATTS/version1/  might give you an
estimate of how big an array would produce 7665 kWh for the year in an area
near you.  That would be 5.3 KW DC in San Francisco.  That would replace
all of your kWh.  Replacing all of your utility dollars would be something
smaller.  I have a 4.5 KW DC array, I consumed 30kWh per day on average,
and had a PG&E bill of around $00 for the year.  Some of that is immutable
costs, some is due to summer air conditioning.  My A/C draws about 6KW,
which exceeds the maximum output of my PV panels.

There are a lot of cost calculators on the web.  Most start with your
monthly utility bill dollar average.
Provides feedback about the financial viability of a system.
(leave zip blank and pick a city to select TOU rates.)


Clarence A Dold - Hidden Valley Lake, CA, USA  GPS: 38.8,-122.5

Posted by thepixelfreak on July 25, 2008, 3:34 pm
 On 2008-07-24 16:39:46 -0700, dold@93.usenet.us.com said:

Thanks for the info. I found some links yesterday that helped as well.
One stated...


One Determine your kWh electric consumption. Then match PV production
to your electric consumption. For example, you consume 600
kilowatt-hours per month (kWh/month) and want to produce 100% of your
electricity with a PV system with no battery backup. 600 kWh/month x 12
months equals 7,200 kWh/year or approximately 20 kWh/day. Most of the
U.S. has 3.5 to 5 Sun Hours of solar input. This means that a 1
kilowatt AC PV system in a 4.5 Sun Hour region will produce 4.5
kilowatt-hours per day. 20 kWh/day divided by 4.5 sun hours equals 4.4
kW AC. Go to PVWATTS at
http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/  and enter
your location, 4.4 kW, your roof tilt and orientation. See your monthly
and annual estimated PV production for a 4.4 kW system or any other
size system. To determine the number of solar modules you require, take
your PV system kW AC x 70% to 80% to factor in DC-to-AC conversion (65%
for battery systems). We consume approximately 6,000 kWh/year and our
PV system supplies over 75% of our energy.
Two Match your PV array size to your roof space. You need full sun on
your solar array all day (from at least 9am to 4pm). Trees, chimneys,
vents and other buildings can block the sun or make array installation
difficult. Our office and south-facing garage roofs had enough space
for our 225 sq. ft. array (32 70-watt modules). Our home south-facing
roof had more than enough space for the 140 sq. ft. array (20 75-watt
Three Match your PV system cost to your budget. PV modules are about
half of the total system cost. Our 1998 system costs $0k. Our 2,240
watt array costs $1k. The other equipment you may need depending on
your system configuration are mounting hardware, combiner boxes,
disconnect switches, power center, charge controller, inverter, battery
bank and wiring. Our 2007 batteryless PV system addition cost $4.3k.
Both systems were eligible for state solar incentives and federal tax



Posted by jb on July 30, 2008, 11:15 pm
Hey pixel freak, this is simple.
First of all lets say you are using 21kWh a day in juice, that is very
energy efficient! Good for you.
The first thing you do is walk outside and look up on your roof, climb
on the roof if possible. The point is to determine what available area
you have for installing an array. Many people dont recognize this but
if you want a solar system on the roof then you are first and foremost
limited to the sun facing space on your roof.
Now once on your roof try to determine how many square feet of clear
mountable area you have that will be exposed to the sun the longest
throughout the year. Keep in mind that the sun is high overhead in the
summer and low in the winter so look at adjacent properties,
structures, trees, ect...., and also think about the future, will your
neighbor be building a three story building in the future? There is an
excellent device out there called a solar pathfinder to do this
resource assesment, and sometimes you can contact a local solar guy
who already has one to come out and do a site study for about 250
At any rate one you have determined the location of where the solar
array will site and the available area you can then size your solar
The rule of thumb here is you will get about 1000Watt of solar array
in 100 square feet of available roof area.
So if you have say 300 square feet of available area on your roof for
solar then you will be specing out a 3000W or 3kW array.
A note: it is also helpful to know where you will feed the array back
to your main breaker panel, kinda a conceptual thing but the nearer to
the breaker panel the cheaper to install.
Now the next rule of thumb is to figure a complete labor and materials
estimate of 10 $/W for the solar system (grid tie without battery
backup, add another 3 $/Watt for that), so in the 3000W solar array
above you are looking at a completely installed cost of $0,000
dollars. Now in reality the price could be as low as 7 $/W but
depending on where you live and the availability of products and
contractors it varies greatly.
Okay lastly, lets say you did all this and bought and had installed
the 3kW solar array for between 25 and 30k. How much does it put out.
Well this is simple. Go find on the internet the progam called
PVwatts. Click on the Version One, then selcect your location on the
map or closest to you, and lastly input the size of the array (3kW for
my example) and the rate on your power bill down at the bottom. Click
calculate and then read it and weep! This is the best program out
there for a quick calc of output from an array.
Hope this helps and keep in mind there are many rebates and incentives
to justify buying a system, but the biggest one is the fact that once
you put that puppy on the roof you house value is now increased. I
figure it goes up about half the solar systems cost.
Good luck

Posted by busdweller on July 31, 2008, 11:50 am
What state are you in? I do PV installes all the time maybe we can
work some thing out>

Peace along the way
Dennis the bus dweller N.Y.

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