Choosing solar panels

February 16, 2010
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.

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)
If you’re viewing a solar panel product page and can’t find Voc, Vpm, Isc, and Ipm, you should be suspicious!

(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)
If you multiply Vpm and Ipm and don’t get something close to the advertised wattage, you should be suspicious.

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