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nooB question about panel requirements

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Posted by Ben on October 9, 2007, 1:31 am

I'm sure this type of question gets asked here a lot so forgive me.
Interested in calculating the 5 year cost to power a water refrigeration
unit I assembled exclusively by solar panels / battery power to determine
cost effectiveness.  If the numbers work out I'd like to construct it

The unit runs on 115v, 3.0 amp.  I read that to calculate watts you multiply
the V x Amp so if that is correct the watts would be 345.  I'm expecting the
average running time of the unit to be 8 hours per day to maintain
temperature so I'm guessing that's 2760 watts per day or 2.76 KwH right?.
It will be set up in SW Missouri unobstructed from dawn to dusk.

Wondering approximate cost of a generic (or recommended setup) including
panels, batteries, wiring, etc.  How long it might last.  What items (such
as batteries) might need to be replaced and at what interval.

Also where a good place to obtain the materials might be in Springfield, MO
or Kansas City, KS area.

Hope I've put enough info out to get an assist here.  Thanks in advance for
any help.


Posted by Ron Rosenfeld on October 9, 2007, 12:57 pm

The following assumes an off-grid system with high reliability.

The most SW corner of MO gets about 3.5ESH (effective full sun hours)
during the worst month, according to a map I have from BP Solar.  So that's
the resource you need.

Your calculation for consumption is correct for your unit.  However, an
inverter will not be 100% efficient.  Depending on the ratio of load to
capacity, and quality, it *might* be better than 90% efficient, but for
safety sake, I would figure 85% efficiency which boosts your requirement to
3,247.06 watt-hours/day.

You then need to figure various system losses, and losses in your
batteries, which add about another 20% to the requirements.

Assuming a 24V system, one computes a total daily load (including all
losses and safety factors) of 162.35 ampere-hours.

Dividing that by the 3.5 ESH gives a required panel output of 46.39

The Sharpe NE-170U1 seems to have specs compatible with your requirements.
It is a nominal 24V panel which, at maximum power, will put out 4.9
amperes.  So ten of these in parallel will cover your requirements during
the worst month.

If you are putting the system together yourself, you should shop mail-order
rather than restricting yourself to just KC or STL.  I see one price of
$10 so your looking at $,100 + shipping.

The next big (expensive) item will be batteries.

Most would recommend a 5 day reserve, although if your needs are very
critical, you might want a ten day reserve.

If you purchase high quality batteries, you can get away with allowing an
80% depth of discharge (DOD).  With cheaper batteries, you should only
allow a 50% DOD so you'd have to buy more.

With your daily load of 162.35 Ah and 80% DOD you will need 1015AH capacity
at 24V (or 24.35kWh) for a five day reserve.

Surette 4KS21PS seems to be a good match at 1104Ah @ 4V.  You would need
six of these wired in series.  I see one price of $,009.49 each so the
batteries would cost $,056.94 and may include shipping.

There are a number of inverters on the market in your power range.  You'll
need to check the start-up surge on your refrigeration unit and be sure
that your inverter can handle that, as well as the running watts.  Motors
will run hotter, and draw more power, if you get other than a "sine wave"
inverter, so that needs to be taken into account, also.

In addition, you will need a controller, wiring, mounts, etc. which may run
another few thousand dollars.

You should also check to see if your state provides any rebates.

If you were going to be grid-connected, the calculations would be

The batteries I recommended come with a 10 year warantee, and might last
longer with good care.

Panel life is unknown, but they do have a 25 year warrantee and will likely
last quite a bit longer.

Good quality inverters have a long life-span; cheap ones not so long.

Posted by Ben on October 9, 2007, 9:12 pm

Thank you Ron for your very thorough and thoughtful reply.  This gives me
the information I was after.  Just using a *rough* calculation I come up
with the following total cost at an annual rate:

Panels: $,000 / 25 year life = $20 per year

Batteries: $,000 / 10 year life = $00 per year

Inverter + Misc wiring, etc: $,000 / 25 year life = $20 per year
Total Cost Per Year = $,040

This verses another *rough* estimate for grid at $00 per year so we're not
saving anything .

Will need to brain storm some more and will likely have more questions.
Once again thanks for the reply.


Posted by Ron Rosenfeld on October 10, 2007, 12:13 am

I would probably be figuring the panels at closer to 50 years, and the
batteries at 15 years.

The things that makes an off-grid solar system worthwhile from a purely
economic point of view are:
    1.  Very high electric rates
    2.  Grid extension costs
    3.  Subsidies

With a grid-tied system, you don't need as much in the way of batteries; so
if you have state subsidies, that might be an option to consider.

Posted by Eeyore on October 11, 2007, 9:40 am

Ron Rosenfeld wrote:

By which time they will be producing less output of course.

Depends heavily on battery type and typical regular depth of discharge.


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