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How many panels ? ( to run 230 volt sprinkler pump 30 minutes a day?) - Page 7

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Posted by bealiba on July 12, 2008, 4:51 am

Sigh, The battery capacity is the minimum required. I have not
specified a battery. If you require the 30 minute rate then the chosen
battery will need a minimum of 180Ahs capacity at the 30 minute rate.
The formula requires this information from the user for the users
chosen battery. The formula is correct.

I have not made up any specifications. I have not specified a battery.

The 180 Ah capacity is the minimum required, not a battery

The formula is correct.

The formula is correct. It requires an amount of user input. Given the
correct input it will correctly size a system.

I presented the formula as it is. If you want to use it feel free, or,
you can have it as a spreadsheet, also free form me at the email
address that can be found in my profile. I am always happy to discuss
system sizing at any time so if you need any help with the formula or
Spreadsheet you can drop me an email.

Neither Tweedledee or Tweedledum have as yet proven the formula to be
incorrect in any way.

Tweedledee has said:

Plain to see that you can't have it both ways 24/7 or 30 minutes.

Use the formula or not. Take a guess or use real numbers. Get it right
or get it wrong.

Totally up to you.

Posted by Ron Rosenfeld on July 12, 2008, 12:55 pm
On Fri, 11 Jul 2008 21:51:15 -0700 (PDT), bealiba@gmail.com wrote:

The formula may be "correct", but the user inputs you provided with your 5
day of autonomy guesstimate result in a significantly oversized system.

And your sheet has no way of accurately estimating system components and
performance for a particular location.  

Your initial 1 day of autonomy was a guess.  

Your subsequent 5 days of autonomy is another guess.

   Neither are correct for the location and panel array size calculated.

Your specification of 180Ah at the 100 hr rate was wrong.  

Your input for panel voltage did not agree with that of the OP's panels.

Your input for ambient temperature was wrong.

Your technique, properly applied, may have been useful years ago, but both
data availability and computer techniques have advanced; allowing better
tailored results.

As you so properly write:

You have been taking guesses.

Many of your inputs have been wrong.

And, although your last system will work, it is significantly oversized,
with more panels and more batteries than would be required at the OP's
location.  But this is probably the best you can do with your spreadsheet.

Without doing simulations, though, there is no good way to figure this out.
And since you are the designer, you should not expect the OP to have
knowledge of ten year averages and ranges for solar insolation; required
days of autonomy for his location; temperature ranges, etc.  You are the
one that should be supplying this "user input".

Simulations supplied with pricing data can also figure out the most
economic system, and even compare it with grid extension costs and grid
power costs.

Posted by bealiba on July 12, 2008, 1:29 pm
Back to the same old same old. All you have to do is prove it, a
single set of calculations.


No. The 180Ah is the correct minimum capacity. If you choose a battery
and apply a discharge rate of C 0.5 the minimum capacity is still

It was correct.

I did not provide any information for ambient temperature. I did
provide lowest average temperature

Show us

I'm right, you are wrong

All you have to do is prove it, a single set of calculations.

Why not. All the info can be found on many sites from the local
weather to NASA. All I provided was the formula to do the sizing. It
is correct. There are two ways to get a standalone PV system - 1) Hire
someone to do it for you, in which case you don't need to come here
and ask questions. - 2) Be prepared to learn how to do it for your
self, in which case you need to apply your self to the job and chase
your own information.

Yeah, I'll go along with that. Mind you it would be a huge program
having to contain all the worlds suppliers catalogs and price lists
not to mention the complete meteorology of the world as well. Tell you
what, why don't you write it up over the weekend and post it here on

If you are not prepared to find the info you need, you are not
prepared to be in control of your own energy supply. The calculation
for system sizing is correct, as long as the info "YOU" put in is

Now you can tell us how 28 Amps of panels at 12 Volts will run a 230
Volt pump that draws 2500 Watts without batteries.

Oh dear, is that the sound of Running feet?

Posted by Ron Rosenfeld on July 12, 2008, 6:18 pm
 On Sat, 12 Jul 2008 06:29:54 -0700 (PDT), bealiba@gmail.com wrote:

If not a guess, then it was a wrong input.

I see.  So you are claiming that when you posted:

B13     Capacity of battery bank at 100 hr rate (B12 x B10) = 180

That we should understand that this is not at the 100 hr rate, but rather
at the 30 minute rate?

Or are you claiming that this battery has the same capacity at the 100 hr
rate vs the 30 minute rate?

Could you explain then, the difference between your 14V input and the OP's
17V panels?

OP:         the panels are 50 watt, 17 vdc, 2.94 amp

GG:  C8     Selected module I at 14 volts at NOCT 2.94A

It was still wrong.  The lowest average temperature for that area is
21.5C; the lowest average daily low temperature is about 17.5C

see below

I'm going to respond to this in a separate message, since it deserves more

I never said it would.  You are the only one making that preposterous
claim.  I stated that energy storage would be required. I can't help it if
your application of your spreadsheet to my data came up with a wrong


Posted by bealiba on July 13, 2008, 12:21 am
180Ah is the minimum required capacity for the job. IF the system is
expected to use the C0.5 rate or a C120 rate the minimum battery
capacity for the same load will still be 180Ahs

You must understand that this is user input and will be for the
battery you choose.

No. This come from your ability to understand user input.

14 volts is the test standard at 25C. 17 volts is what the produces
when not connected to a load. Surprised you don't know this.

Oh, I see. As we are talking here about the lowest average daily
temperature  the difference between 15C and 17.5C is not going to make
much difference and is in fact around 3Ahs

Below is empty

Here we are at the "Below"  and not a single calculation in sight.

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