[aprssig] (Slightly OT) What is solar panel power output?

bob evinger wd9eka at evinger.com
Tue Sep 9 21:41:05 CDT 2008

Its been ages since I got deep into the theory. I think bob 
bruninga gave the better theoretical description. I was going off of what 
I remembered and the fact that I think after you breach that "knee" it is 
relative constant current.

I hang out with a lot of non-ham's on a 12 volt alternative energy list on 
one of the yahoo groups. A large percentage of the guys/gals there run 
full time off grid, I'm not quite there yet but inching closer.  I could 
run my place off grid if I had to with a little pain for the kids :)
I need to triple or more my battery capacity. The panel increase in the 
spring I now only have about 1 days reasonable reserve in the 
batteries for what I draw off them. I'm a number cruncher, I read my 
commercial kwh meter every day, record my solar generation everyday that I 
am here. Have the solar numbers going back almost 10 years now in a 
database with my solarpower webpage getting updated when I update the 
database grouped by month. So I can go back for the last 10 years and see 
my monthly totals/average/min/max ah production. Its good for comparing 
year to year. though I made a few jumps in power level over the  years.

Since the upgrade I have been averaging between 3 and 5kwh/day for the 
summer. ALl the ham gear sits on it full time but if things became 
critical the aprs igate and weather station would get shutdown for the 
important stuff. Fridge/freezer/lights.

anybody wanting a picture of my rats nest can go to 
http://solarpower.evinger.com  there are some relatively recent pics of 
the power shed, panels and wind generator. I also have a webcam on my 
bogart battery monitor which stays reasonably up to date. That shows goes 
into/goes outa current in the batteries.

bob E.
marshall, IL

On Tue, 9 Sep 2008, Steve Noskowicz wrote:

> Bob,
>   Correct me if I am wrong, but aren't good solar cells closer to a (Pretty) constant voltage *UNTIL* you get close to the current limit, then they supply *near* constant current down to zero volts?
>  There must be some "softness" to this, In other words, *some* voltage decrease as the load increases, probably some IR losses.
>  Then as load increases further, you have a knee where the Voltage starts dropping faster and faster as the current increases slightly to Isc.  [or is there some foldback?]
> However,  I believe this is what you are saying.
> -- 
> 73, Steve, K9DCI
> --- On Tue, 9/9/08, bob evinger <wd9eka at evinger.com> wrote:
>> From: bob evinger <wd9eka at evinger.com>
>> Subject: Re: [aprssig] (Slightly OT) What is solar panel power output?
>> To: "TAPR APRS Mailing List" <aprssig at lists.tapr.org>
>> Date: Tuesday, September 9, 2008, 5:45 PM
>> actually those numbers tell a lot. Solar panels are current
>> sources. That
>> amp rating is short ciruit current, and the current it will
>> provide into a
>> battery with proper sized wire should be slightly less.
>> The preferred testing method of a new panel when recieved,
>> and also one of
>> the easiest ways to verify solar alignment is to disconnect
>> it fron the
>> battery and put an ammeter directly across the panels
>> leads(ammeter has to
>> be rated for greater than the peak output of the panel)
>> then adjust angle
>> for highest current at noon, assuming a fixed panel mount.
>> If the panels
>> are adjustable then it is best to do this for at least
>> summer winter. If
>> the panels are not adjustable, or in a place one cant get
>> to in the
>> winter, then the more vertical the panel for winter time
>> the better. Snow
>> will fall off quicker(if in florida disregard :) ).
>> take the current and you might as well assume 12 volts,
>> that is the power
>> available effectively if you are using a low end
>> regulator,Pulse width
>> modulated.
>> If you have enough panels, it can be worth getting an
>> MPPT(maximum power
>> point tracking) charge controller but they will cost more
>> than a PWM
>> controller. They will also harvest another 20-25% more
>> power out of the
>> panels. THis is because they are dc-dc converters and will
>> keep the panels
>> voltage at maximum power. With a PWM controller, the
>> FET's open up and the
>> batteries pull the panel voltage down. On a low battery
>> that may be 11.5
>> volts. An mppt controller will keep the panels voltage at
>> or near its
>> rated 16.5 volts and give more available power.
>> simplest example.  assume a panel capable of 10 amps
>> with a pwm and the batteries sitting at 11.5 volts there
>> will only be 115
>> watts available and 10 amps will flow into the battery.
>> MPPT controller at 16.5 volts and 10 amps going into the
>> controller you
>> will have 165 watts available, on the output side of the
>> MPPT controller
>> under these conditions there should be close to 14.3 amps
>> flowing into
>> the battery. As the battery charges the current out will
>> drop off.
>> An MPPT controller works quite well. I used PWM controllers
>> for years but
>> this spring doubled my PV capacity to almost 1000 watts of
>> panels and
>> replaced my pwm controller with an mppt.
>> I have about 54 amps of current availalbe if it was just a
>> pwm controller.
>> I have seen up to about 72 amps charging with the mppt
>> controller during
>> the 10am - 2pm prime power window. The technology helps
>> more on partly
>> overcast days and non-peak hours.
>> Sorry probably way more than most on here wanted to know.

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