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

bob evinger wd9eka at evinger.com
Tue Sep 9 22:45:29 UTC 2008

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 

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.

On Tue, 9 Sep 2008, Mark Cheavens wrote:

> Neither of those numbers tell us anything. You need to know the
> output in WATTS (or amps @ a given voltage).
> Put a variable load (carbon pile) across the leads and measure the
> amp output @ 13volts while in direct sunlight. While you are doing
> that you can also measure the output at different angles (less than
> ideal) from direct sunlight. Most start to drop significantly @ about
> 15degrees in any direction.
> Mark
> At 03:15 PM 9/9/2008, you wrote:
>> I am getting closer to setting up my APRS weather and 6 meter
>> PropNET stations far enough from the house to make AC power
>> impractical.  I have three old solar panels (cost me about $100 used
>> 15 years ago) that I want to put into service.  In full sunlight,
>> two of them in parallel put out  19.88v open circuit  and  4.78a
>> short circuit.  How does that translate to power available at 13.2v
>> nominal for running the rigs and charging the battery?
>> -- 73 --
>> J. Gary Bender, WS5N
>> DM54rp
>> Fence Lake, New Mexico USA
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