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Topic: Realistic daily net amp-hr expected from solar panels (Read 1 time) previous topic - next topic
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Realistic daily net amp-hr expected from solar panels
Yahoo Message Number: 99125
Hello all,
 How many net amp-hrs can I expect for a 200 watt (2 x 100) solar panel per day?  Assuming 12 hours of day light in south Texas.
 I know it's a lot lower than 200*12/12 or 200 amp hrs.  100 amp hrs?

Thanks,

Sam
Re: Realistic daily net amp-hr expected from solar panels
Reply #1 –
Yahoo Message Number: 99132
Quote
How many net amp-hrs can I expect for a 200 watt (2 x 100) solar panel per day?  Assuming 12 hours of day light in south Texas.
First, the power output of the panels is not rated at 12V, but at 17V to 24V, depending on the number of cells. You should be able to get the specs for maximum power current rating, a more useful figure. You can also expect a dramatic difference between winter and summer yield, even in south Texas. Maximum output of the panels assumes 1 sun irradiation, roughly noon at the equator, and a temperature of 72 degrees, obviously a set of conditions unlikely to be duplicated in the real world. Panel output drops as they heat up. If your panels are mounted flat on the roof, then you might average about 80% of rated current output in the summer for 8 hrs, with an average of half that for the remaining 4.
 However, with high-voltage panels and a 'maximum power point tracking' controller, you may improve the 80% of maximum current rating to 80% of maximum power rating, which will optimize performance. Tilting panel mounts can give even more improvement. There are many variables, and knowing the complete specs of your panels and system will allow you to compare results others have achieved, once you know their equipment and locale.

Steve
2004 FL
2013 Honda Fit
Re: Realistic daily net amp-hr expected from solar panels
Reply #2 –
Yahoo Message Number: 99138
Quote
Hello all,

How many net amp-hrs can I expect for a 200 watt (2 x 100) solar panel per day?  Assuming 12 hours of day light in south Texas.
 I know it's a lot lower than 200*12/12 or 200 amp hrs.  100 amp hrs?

Thanks, Sam
This is a tough one because it's complicated and there are lots of fuzzy variables.

You wanted data for two 100 watt panels.  Panels are rated at the maximum power they can create.  I will assume you mean Lazy Daze's solar panel system provided to LD by AM Solar. AM Solar's "100W" solar panel is rated at 4.54 Amps at 21.5 Volts. That is, it's actually a 97.6 watt panel watt panel.  See: http://www.amsolar.com/am100.html

To get the actual charging current from these specs is complicated.
Lazy Daze uses AM Solar's maximum power point tracking (MPPT)solar panel controller.  A solar panel puts out almost constant current for all voltages up to a maximum value.  Since the panel's output power is volts times amps, the panel will make more energy if the voltage is higher.  Most controllers just hook the panels to the batteries until the battery voltage indicates it's charged then diconnects.
This works fine, but it under utilized the solar panel's capabilities.

The MPPT utilizes electronic power supply circuitry to improve this situation.  It holds the panel's voltage at its maximum power value (about 21.5 Volts for the AM Solar panels) and then transforms the panel's energy to energy at the battery's voltage.  This is in the range of 12 to 14.8 Volts, varying with the state of the battery's charge.  The point of all this motion is to increases the amount of charging current available to the battery.  This conversion process is not lossless.  AM Solar has carefully never said just how efficient their system is.  Let's assume it's 90%, which is typical for similar systems.

We assumed that at maximum sun light, the power from the AM Solar panel 21.5V x 4.54A = 97.6 watts.  This goes into the solar panel controller and, at a 90% efficiency, 87.8 watts comes out.  The battery charging voltage varies but lets assume it's constant at 14.0 volts.  Therefore, the maximum output current is 87.8/14.0 = 6.3 amps peak per panel or 12.6 amps peak total for both panels.
 I created a spread sheet that calculates the daily amp-hours a panel will create for a given date and a given latitude assuming an absolutely clear sky (e-mail me privately if you want a copy).
Assuming that south Texas is 29 degrees north, on Dec 22nd, a solar panel laying flat (like on a LD's roof) will give about 3.3 A-H of total charge per day for each ampere of solar panel current rating.
On March 22nd and Sept 22nd it will give about 6.2 A-H, and on June 22 it will give about 8.0.  This variation is caused both by the variation in the length of day and the height of the sun in the sky.
Tilting the panels toward the sun in the two months on either side of Dec 22nd will increase this value slightly (up to about 15%) but if you do so, the lower edge of the panels must face due south within just a few degrees.  Tilting during the other eight months doesn't help much.
 So, multiplying these A-H values by the 12.4 amps of peak solar panel current we arrive at a maximum of charge of 41 A-H per day on the 22nd of December, a maximum of 78 A-H per day on the 22nd of March, and a maximum of 101 A-H per day on the 22nd of June.  And of course, ANY cloud in the sky or ANY shade on ANY part of the solar panel will greatly diminish these values.  And, keep your panels clean!

Every assumption used above is questionable, so one must take these values as the roughest sort of data; that is, having them is only slightly better than having nothing thing at all.  On a given day, you could see anything from 10% more A-H because the panels were cold (this increases their output voltage capability), your battery voltage was low and the day was exceptionally clear and sunny, to perhaps 90% less A-H because the day was dark and cloudy, the panels were hot and your battery voltage was high.

Note that not all of the amp-hours made by the solar panels during the day end up in the battery.  Even if you use no electricity at all, the refrigerator and the propane detector consume power.  The propane detector draws about 1 to 2 A-H per day.  The refrig's draw is about 1 to 3 A-H per day depending on how much the gas valve is held open.

Linley
Re: Realistic daily net amp-hr expected from solar panels
Reply #4 –
Yahoo Message Number: 99154
Quote
So, multiplying these A-H values by the 12.4 amps of peak solar panel current we arrive at a maximum of charge of 41 A-H per day on the 22nd of December, a maximum of 78 A-H per day on the 22nd of March, and a maximum of 101 A-H per day on the 22nd of June.  And of
course,
 
Quote
ANY cloud in the sky or ANY shade on ANY part of the solar panel will greatly diminish these values.  And, keep your panels clean!

Every assumption used above is questionable, so one must take these values as the roughest sort of data; that is, having them is only slightly better than having nothing thing at all.  On a given day, you could see anything from 10% more A-H because the panels were cold (this increases their output voltage capability), your battery voltage was low and the day was exceptionally clear and sunny, to perhaps 90% less A-H because the day was dark and cloudy, the panels were hot and your battery voltage was high.

Note that not all of the amp-hours made by the solar panels during the day end up in the battery.  Even if you use no electricity at all, the refrigerator and the propane detector consume power.  The propane detector draws about 1 to 2 A-H per day.  The refrig's draw is about 1 to 3 A-H per day depending on how much the gas valve is held open.

Linley
Thank you very much, Linley.  That's so detailed and so easy to understand.  Much obliged.

Sam
 
Re: Realistic daily net amp-hr expected from solar panels
Reply #5 –
Yahoo Message Number: 99158
Linley,

That pretty much mirrors what I'm seeing. I have a "flat mounted" 400 watt array with a MPPT controller. I typically see 4 to 8 amps in the early morning after sunrise. This will go up to as much as 25 amps (my controller's limit) at noon in the summer - - if the batteries are low enough to accept that much current or if I put a heavy enough load on them (i.e.running the fridge on AC from the inverter pulls 30 amps from the battery banks).
 Typically my batteries are only down 90 amp hours in the morning. They are fully charged way before noon.
 So, I would guess that a 200 watt array might see something on the order of 13 maybe 15 amps peak. Something up to 100 amp hours  per day may be doable, but you won't see that under all conditions. That's just a wild guess due to all the variables.
 With solar, it's best to have more than what you need on a sunny day in order to make up for all the cloudy ones. Also, as Andy and others have said, it doesn't make a lot of sense to add solar panels unless you have someplace to store the energy. A rule of thumb is something on the order of 1 watt of PV panel to each amp hour of battery capacity.

bumper Yonder

[Non-text portions of this message have been removed]
bumper
"Yonder" '05 MB
"WLDBLU" glider trailer