Choosing solar panels
February 16, 2010
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There’s a lot of different solar panels out there, and with so many choices, it would be helpful to know what to look for when buying one. I’m going to explain some of that technical jargon. This way you can better understand how or what panels to use with a given system.
(As we discuss these terms, keep in mind that voltage and current measurements are usually taken under a consistent test light source. The results should be comparable to good sunlight.)
This may seem trivial and basic for some of you, but the point is that you need to know what is being advertized. What is a “12 volt” solar panel? Is it 12 Voc with about 9 Vpm? Or is it 12 Vpm with 18 Voc? I admit, it’s usually the latter, but you need to make sure.
Open circuit voltage, or Voc is the voltage measured when the panel isn’t connected, hence “open circuit.” Short Circuit Current, or Isc is the current (amps) measured when the panel is completely shorted, positive to negative.
Understanding the Spec./Data Sheet
Let’s say you want to check out a specifications sheet for a solar panel, like this one or this one. As you start looking at the tables, you’ll see terms like:- Polycrystalline silicon (or Multi-crystalline silicon)
- Monocrystalline silicon
- Open Circuit Voltage (Voc)
- Maximum Power Voltage (Vpm) (or Optimum Operating Voltage)
- Short Circuit Current (Isc)
- Maximum Power Current (Ipm) (or Optimum Operating Current)
(As we discuss these terms, keep in mind that voltage and current measurements are usually taken under a consistent test light source. The results should be comparable to good sunlight.)
Vpm and Ipm
These are the most important aspects of the solar panel because they determine power. Both Vpm and Ipm are the ideal maximums at which the solar panel functions. Looking at Ohm’s Law, we know that:- Vpm * Ipm = Power (Watts)
This may seem trivial and basic for some of you, but the point is that you need to know what is being advertized. What is a “12 volt” solar panel? Is it 12 Voc with about 9 Vpm? Or is it 12 Vpm with 18 Voc? I admit, it’s usually the latter, but you need to make sure.
MPPT - Maximum Power Point Tracking
Both Vpm and Ipm establish the maximum power point. When you see a charge controller or grid tie inverter, they may advertise a MPPT (maximum power point tracking) feature. All this means is it searches for Vpm and Ipm to get the most power from your solar panel(s).Voc and Isc
Consider Voc and Isc as the opposite extremes of Vpm and Ipm. Generally, you don’t really need to worry about these. You won’t get any power output at these extremes.Open circuit voltage, or Voc is the voltage measured when the panel isn’t connected, hence “open circuit.” Short Circuit Current, or Isc is the current (amps) measured when the panel is completely shorted, positive to negative.
Poly-crystalline vs. Mono-crystalline Silicon
This makes up the composition of the silicon used in the solar cells of a panel. Silicon is the same material that make up computer chips. You don’t really need to worry about the raw materials that make up a solar panel. But for your interest, here are some key differences:Composition | Cost | Efficiency | Appearance |
Poly-crystalline | less | less | usually rectangular or square without rounded corners |
Mono-crystalline | slightly more | slightly more | usually square with rounded corners |