# Solar Power for Subs: The Panels

When it comes to solar-powered pumping systems, they all start with the panel. After all, it’s the panel that captures the sunlight needed to run the system.

How it works

The electricity to run the pump and motor in any solar-powered system resides in the property of certain types of silicon crystals to produce a small amount of DC voltage when exposed to light. This is called the photovoltaic effect, and it is often just abbreviated PV. The term photovoltaic or PV system simply refers to a solar system that generates electricity using this property.

When silicon crystals are connected together, they can generate useful amounts of electricity. One unit of these connected crystals is called a solar cell, and dozens of cells are contained in a single solar (photovoltaic) panel.

How to spec a solar panel

Several different variables are associated with solar panels, but for a pumping system, we only need to consider four values: Voc, Vmpp, Impp, and Wmpp.  Voc stands for open circuit voltage and is exactly what the name implies. That is, with no load (zero current being delivered), the array will generate this amount of DC voltage. This is similar to measuring AC voltage from the power company in a conventional water system when the motor is not running.

Unlike AC power from the power company, however, once we start to pull current (amperage) from the array (to drive a motor for example), the amount of voltage produced will start to fall off as the amount of current increases. This is remarkably similar to a pump curve. That is, Voc can be thought of as the shut-off head.

As we move down the pump curve delivering water (GPM), the pressure drops accordingly. At a point about midway on the curve, the pump will deliver its maximum horsepower. In the case of a solar cell, the amount of power being delivered will simply be the voltage multiplied by amperage:

power (in watts) = voltage X amperage

This point where the most power is delivered is denoted by Wmpp, or maximum power point watts. It’s also sometimes called just Pmax or maximum power. The voltage and current at this point are called Vmpp and Impp for maximum power point voltage and maximum power point current, respectively.

All panel manufacturers provide the values Voc, Vmpp, Impp, and Wmpp for each of their panels. For example, in the data sheet excerpt below, Vmpp is 32.1 volts and Impp is 8.42 amps. Notice that when those two are multiplied (32.1 x 8.90), it equals 270 watts.

The values above are at standard conditions. Since the amount of energy produced by a solar panel is dependent on the amount of light striking it as well as the ambient temperature, the industry has defined a standard set of conditions to ensure that different panels from different manufacturers can be compared side-by-side. In real-life conditions, the actual values will be somewhat more or less than listed by the manufacturer.

Today, dozens and dozens of companies manufacture solar panels. In most cases, your local distributor can probably make some recommendations.

What’s next?

Although the values above are for a single panel, most solar pumping systems will require more than one. The question then becomes, “for my given water requirements, how many panels do I need?” With today’s web tools, calculating this is quite easy. We’ll cover this topic in the next post of Franklin AID.