# Sizing a Off-Grid battery Solar Pv System

Posted by JoeyIsOnDrugs on February 8, 2011, 9:49 pm

Hey,
I need help sizing a Stand-Alone Battery system that will be powered
and charged by a Solar array.
So here are my AC loads:
2 loads @ 120Vac & 16 Amps (2*120v*16a = 3840 watts)
2 loads @ 240Vac & 30Amps (2*240v*30a = 14000 watts)

Total Watts = 18,240
And I need that power for 4 hours/day and 7days/week

Posted by vaughn on February 8, 2011, 11:22 pm

If you can truly afford a system of that size, you CAN'T afford to NOT hire a
professional to properly design and specify your system.  However, to give you
an idea of what you would be getting into, there are various calculators on the
web.  Creative Goggling will easily find them.

Hint: One of them is here:
http://www.altestore.com/store/calculators/off_grid_calculator/

One basic parameter you will need to decide on is days of autonomy.  There are
some days where you simply won't get much output from your solar system, you
must size your system to deal with that, and/or possibly add a standby
generator.

Vaughn

Posted by Ron Rosenfeld on February 9, 2011, 3:48 am
wrote:

That will be one huge system.  And note that 2*240*30,400, not 14,000

You don't really supply enough information, and, as Vaughn writes, you should
get a real professional to help, but as a starter:

Your total daily AC load is 18,240 * 4  or 72,960 Wh (per day).

You need to divide that by inverter efficiency since the solar panels generate
DC and you need AC to power your loads.

You will also need to add in a factor for system losses, battery charging
losses, gradual degradation of the panels and so forth.  20% is a rough guess.

To obtain the array size, you need to know how many hours of sunlight you have
at your location, during the worst month, and divide the above number by that.
You may also need to know average temperatures, as PV array outputs will vary
with temperature.

Finally, to get the size of your battery bank, although you generally pick a
number for "days of autonomy", and work from there, if this is a
mission-critical system, you might want to make that computation based on, for
example, the worst period over the last 20-30 years.  This number might vary
from as low as five days, if you are in a sunny area in latitudes near the
equator, to as much as 15 or more days if you are closer to the poles.  You also
have to figure in how much you can discharge your battery bank, and this depends
on the battery system you are using.  So you might, for example, need a battery
bank that is

But a system such as you describe will probably cost \$50,000 - \$,000,000 USD,
not counting the space for the array or the building to house the battery bank
and inverter(s), and both will be substantial in area.  The parts might not cost
that much, but a good installation probably will.

Good luck!

Posted by Jim Wilkins on February 9, 2011, 12:17 pm

Why do you think "JoeyIsOnDrugs" needs that much unmetered power?

Posted by Ron Rosenfeld on February 9, 2011, 3:04 pm

I'm not a mind-reader so I don't know; and I'm not prone to idle speculation.