PV Gap

Albert Said:

Solar Panel load resistors?

We Answered:

A characteristic curve of a photovoltaic module (that is the technical name for a solar panel that produces electricity) in general looks like the following:

http://www.scielo.br/img/fbpe/jbsms/v24n…

Look at the graph data that looks like a rotated "L". That is what voltage as a function of current characteristically behaves.

The highest voltage value is called open circuit voltage (Voc).
The current at lowest voltage value is called short circuit current (Isc).

If you notice, a second curve is plotted that looks like a hill. That curve is power output as a function of voltage. At the peak of said hill, you get the most utility of your PV module, as you output the maximum power possible, and that is the nameplate value for the PV module's power rating.

The voltage that produces maximum power output is the maximum power voltage or operating voltage (Vmp).
The corresponding current is called maximum power current or operating current (Imp).



Now...as you can see...the voltage is a function of how much load is attached to the module (btw: the complete panel is a module...cells are small units of the entire module, usually about 60 cells per module).

If you originally had an open circuit set-up, that is a high resistance across the potted block terminals....nearly infinite (usually on the order of megaohms in reality).

If you attach a smaller resistance...you draw out more current and decrease the voltage output. Current gets closer to Isc...voltage gets closer to zero.

As the resistance increases, you tune the output voltage more toward Voc, and current gets closer to zero.

Tina Said:

Do Solar Panels Make sense?

We Answered:

I wouldn't actually worry TOO much about the amount of sunlight - you're approximately the same latitude as northern Germany and they have the highest per-capita use of solar in the world. What I WOULD be concerned about is cost. Solar is usually the last step in making a house efficient and cheap. Before you start thinking about solar you should work on other areas of efficiency. $40,000 will go a lot further towards energy efficiency and cost savings if used for insulation, solar water heating, energy efficient appliances, etc. than it will if spent on solar.

Lawrence Said:

how do i know what size charge controler i need for my solar panels?

We Answered:

Does each solar panel have the same output voltage, that is does each panel have the same number of cells, if so then you can hook each panel in parallel which will add the current from each panel together. You then multiply the voltage of the panels times the combined current to get the wattage or power capability of your panels. The output voltage of your panels should be about 14.5 volts DC to be able to charge a 12 volt battery. Make sure the charge controller can handle the combined output current of your panels. When doing design work, I always used a safety factor of 2 times, if your panels current output is 10 amps then your charge controller should be able to handle 20 amps.