Solar System Sizing??

I have used several different on-line sizing calculators and come up with different answers to my question.
My requirement is 12 volts DC at 1.5 amps 24/ 7. I'm in SE Iowa in level 4 for sun and need 2-3 days reserve. However my application is not critical but I don't want the battery(s) to go dead and freeze in the winter either.
Could someone suggest an on-line calculator or another way to know what size panel and battery to buy?

On Feb 1, 7:58 am, please_reply@this_newgroup.com wrote:

I have used several different on-line sizing calculators and come up with different answers to my question.
My requirement is 12 volts DC at 1.5 amps 24/ 7. I'm in SE Iowa in level 4 for sun and need 2-3 days reserve. However my application is not critical but I don't want the battery(s) to go dead and freeze in the winter either.
Could someone suggest an on-line calculator or another way to know what size panel and battery to buy?

I been using a 75 watt 12 vdc panel and 100 amp deep cycle battery with charge controller to run a street light that has 132 leds. No problem so far.
Bob

please_reply@this_newgroup.com wrote:

I have used several different on-line sizing calculators and come up with different answers to my question.
My requirement is 12 volts DC at 1.5 amps 24/ 7. I'm in SE Iowa in level 4 for sun and need 2-3 days reserve. However my application is not critical but I don't want the battery(s) to go dead and freeze in the winter either.
Could someone suggest an on-line calculator or another way to know what size panel and battery to buy?

Ok 1.5A 24/7 is 435 Wh/day
1.5A x 12V = 18W
18W X 24h = 432Wh/day
For five days autonomy you will need 250Ah capacity battery and 2 - 80W panels with 4.5 Peak Sun Hours
Your daily depth of discharge will be 14%
The total depth of discharge after five days with zero input will be 70%.
-- Posted via a free Usenet account from http://www.teranews.com

please_reply@this_newgroup.com wrote:

I have used several different on-line sizing calculators and come up with different answers to my question.
My requirement is 12 volts DC at 1.5 amps 24/ 7. I'm in SE Iowa in level 4 for sun and need 2-3 days reserve. However my application is not critical but I don't want the battery(s) to go dead and freeze in the winter either.
Could someone suggest an on-line calculator or another way to know what size panel and battery to buy?

Well, back of the envelope time again... 12V @ 1.5A for 24H is (12 x 1.5 x 24) 432 watt-hours (wh) You need 3 days reserve so that's (432 x 3) 1296 wh. Say you don't want to discharge your battery more than 50% (some will go as far as 80%) then you'll need twice that in battery storage, (1296 x 2) 2592 wh, (2592/12) 216 amp-hours (ah).
You can find golf cart batteries in this size but you'll probably need two of them in series because their typically 6V.
How much solar PV panel would you need? Well, NREL might help. Since you want it to work even in the worst months then you need to size the panels based on your worst solar month. Their PVWATTS calculator might help. http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/US/Iowa/Waterloo.html
This seems to indicate that Waterloo gets 3 kwh/m^2/day (sun-hours) on average in December, the worst month. If we figure that, worst case, the PV panels will only produce 80% of their STC rating and that the battery only returns 80% of the energy that goes in and that you'll need at least 432 wh a day then it's around (432 wh / 3h / .8 / .8) 225 watts of PV panels required for a fixed mount.
Can we do better if we tilt the panels more vertical to maximize their winter performance? Adding 15 deg tilt makes the minimum 3.2 sun-hours in November. This changes the calculation to (432/3.2/.8/.8) 211 watts. Adding a tracker doesn't do much in the winter, it only increases the sun-hours (insolation) to 3.6 (188 watts PV required + tracker).
Anthony

On Wed, 31 Jan 2007 18:58:17 -0600, please_reply@this_newgroup.com wrote:

I have used several different on-line sizing calculators and come up with different answers to my question.
My requirement is 12 volts DC at 1.5 amps 24/ 7. I'm in SE Iowa in level 4 for sun and need 2-3 days reserve. However my application is not critical but I don't want the battery(s) to go dead and freeze in the winter either.
Could someone suggest an on-line calculator or another way to know what size panel and battery to buy?

I've not seen the terminology "Level 4 for sun" so I don't really know what you mean by that.
On my charts, however, most of southern Iowa has an insolation of about 3.0, although the extreme SE corner is at 2.5 during the worst month of the year. Insolation can also be described as the equivalent number of "full sun hours". It takes into account the fact that you don't get as much energy from the sun at sunrise as you do at noon.
The value of 3.0 also assumes that your panels are at optimum tilt and orientation for best year round exposure. You would probably due better to tilt them a bit steeper, so as to improve winter values (at the expense of summer values). Optimum tilt is usually equal to your latitude in degrees; and for winter optimization, add about 15� to that value. You might be able to get away with a *slightly* small panel, but probably not enough of a difference to make a difference.
Using 3.0:
DC load in watt-hours/day 432 Total load 432.00 System nominal DC voltage 12 Total load in ampere-hours 36.00 System losses/safety factor multiplier 1.2 Adjusted total daily load 43.20 Worst month daily average insolation 3 Required array current 14.40 Current at maximum power (Imp) of selected module 7.2 (BP 3125 125W) Number of parallel modules required 2.00 Rounded up to next whole number 2 Number of series modules required 1.0
Total modules in array 2
So you could use a pair of BP Solar BP 3125 solar panels to generate your daily requirement during your worst month. You'd obviously have excess during the rest of the year.
Selected days of reserve 3 Adjustment for cycling capacity 0.5 Adjusted total daily load 43.20 ampere-hours Recommended nominal battery capacity 259 Ah at 12 volts or 3.11 kWh
There are a number of batteries available in that current capacity. You'd need two of them in series as most come as 6V batteries. If you obtained a high quality battery, you could probably use 80% of the capacity, and would have more reserve.
Note that George's calculations are similar, *except* that he used 4.5 effective sun hours, whereas Anthony and I used 3.0 ESH.
--ron

please_reply@this_newgroup.com wrote:

I have used several different on-line sizing calculators and come up with different answers to my question.
My requirement is 12 volts DC at 1.5 amps 24/ 7. I'm in SE Iowa in level 4 for sun and need 2-3 days reserve. However my application is not critical but I don't want the battery(s) to go dead and freeze in the winter either.
Could someone suggest an on-line calculator or another way to know what size panel and battery to buy?

Thanks to all that responded!! That was the information I was looking for!!
I appreciate the time you took to do the math for me. I would have been very happy if someone had simply listed a URL for a solar calculator but you guys went overboard for me, thanks!
Next I would like to know if there is a favorite solar equipment on-line the group would recommend?
Thanks again, Randy