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Portable solar system to run electric fans

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fratermus:
> most 60 watt panels will only output 50 watts

The difference usually has more to do with voltage suppression due to cell temp above lab conditions (25F, about 0F ambient).

>  with the charge controller

Even without a controller.  A panel attached to a battery the panel will be run at battery voltage.  This is why PWM is said to "waste power" and the secret to how MPPT controllers maniupulate panel voltage (and therefore power).


> as the panel is usually rated at maximum current TIMES Maximum voltage

The panel is rated at Vmpp:  voltage at which the panel puts out max power under lab conditions.  This is usually far away from Vmax (max voltage, open circuit) but generally in the neighborhood of Imax (max current) because current is usually pretty flat across the power curve.


> and as we use 12 volt batteries with 20 volt panels

Hence MPPT. 


>  ,,,the math will always be FUZZY....  they still don't add up.

A look at the panel's power curve will clarify the situation greatly. 

OP:  non-controller or pwm controller scenarios work best when the panel's Vmpp is close to Vbatt (battery voltage).  A mono panel (higher than usual voltage) and lithium pack (lower than usual voltage) are not matched very well.   This is one of those situations where more expensive (or more mindshare) is actually counterproductive.*

related info: 

* matching panels to PWM (or no controller)
* Is my solar working?

* that and AGM charged by solar, but don't get me started


fratermus:

--- Quote ---This time of year you should get about 4.5 hours of optimum sunlight per day in Maine.

--- End quote ---

This idea is correct;  the unit of measurement is Hours of Full Sun Equivalent (FSE).   It's the amount of light a flat-mounted panel will receive n a given location at a given time of year on average.  IOW, the amount of sun received throughout the day will be equivalent to 4.5hrs of sun under perfect/lab conditions. 

I point this out because it might help people predict the amount of average sun available where they are.  It also allows us to see how much gain we can get from tilting/tracking. Here's one example of FSE data by month:

http://solarinsolation.org/wp-content/uploads/2012/01/Solar_insolation.jpg <-- having troubles inlining images from this browser



--- Quote ---say a 60W 12 volt panel 60/12=5 Amps so roughly 5 amps per hour [of course it is not that clear cut usually it will be round 4 amps per hour with a 60 watt panel

--- End quote ---

Temperature derating can be calculated here. 

Controller losses will be:

* ~5% for MPPT (see product specs)
* variable for PWM.  Tiny losses under the right conditions (poly panels, high ambient temps, low altitude, Absorption stage), huge losses under the wrong ones (mono panels, low ambient temps, high altitude, Bulk stage while deeply discharged). 

This means that in special cases PWM can actually put more power into the bank than MPPT.   

I've started a gentle intro to solar for beginers on this wiki page.  It's proving harder than I thought.  I welcome corrections and additions. 

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