Edmond Becquerel was the first to discover the photovoltaic effect. This later led to the invention in 1893 of the first solar cell by Charles Fritts. He made this by coating a sheet of the semiconductor selenium with gold. An array of such cells connected in series and parallel makes up a solar panel. Today we have vastly improved panels capable of instrumental power delivery.

Some solar panel installations

How a Solar Panel Works

A solar cell is somewhat similar to a junction diode. We have a layer of protective see-through non-reflective glass in sandwiched layers—a grid of conductor wires to collect the current. A layer of N-type semiconductor material is in contact with a layer of P-type semiconductor material, forming a semiconductor junction at the point of contact. A conductive plate then follows this to include the other terminal. Figure 1 below shows the construction.

(See my article on transistors on this site for an in-depth explanation of how a semiconductor junction works.)

In a nutshell, the free electron in the N-type material bonds with the P-type material’s free holes at the corner, forming a charge layer. At this point, no current flows. When photons from the sun’s energy strike the junction, the bond breaks and the electrons and holes are forced back into their layers towards the terminals. If you make a connection to the external terminals, the current will flow.

Figure 1: A solar panel cross-section

How to Choose a Solar Panel

For a simple system like my example in Figure 2, we start at the end and work our way back to the beginning. We want a 5V output to run a cell phone or an Arduino project. We have chosen a pair of Li-ion 14500 3.7V 800mAh batteries. This means we need to use a step-up converter module to step the 3.7V up to 5V. Note the max current we can draw from this little module is 600mA. The module is the one on the left with the big USB connector.

Next, we need to charge the battery. As this is a Li-po battery, we need to be very careful how we charge it. Fortunately, this is all taken care of by the TP4056 battery charging module, which accepts an input from 4.5V to 6V and regulates the output charge to the battery. All that remains is to choose a panel capable of putting out 6V, and the size will determine how fast we can charge.

I decided on a 6V 4.5W panel. The output current for this is 4.5/6 = 750mA. If we assume an efficiency of 85%, this will give us 640mA. As the two cells in parallel have a total capacity of 1600mA, we will need to charge them for 1600/640 = 2.5h at least.

Figure 2 shows the complete system. I added a slide switch to turn the 5V module off when not needed not to discharge the batteries.

Figure 2: A simple solar cell phone chargerClose up of the modules
Wiring the circuit up

Getting the Right Voltage/Current Panel

The above example was a trivial one, and as you can see, I based it on what components were readily available to make a charger. I didn’t care if it took two or eight hours to charge. But what if we needed something much bigger, say, to supply a house?

We can’t decide to have a panel of any voltage that suits us. We have to use standard panel voltages, typically 6, 12, and 24V. To do this, we need to understand the relationship between power voltage and current. , P = V * A. So if it turned out we needed 1200W, we could get that from a 120V panel supplying 10A or a 12V panel providing 100A. Both give 1200W.

In general, large currents need thick (more expensive) copper wiring with large installations and are likely to cause tiny volt drops along the wire. At any connections, so a better solution is a higher voltage system with lower currents given the exact power requirement. Connecting two panels in series gives you double the voltage, and then, they add.

See figure 3 below. Connecting two in parallel gives you the same voltage, but twice as much current is available (see figure 4). So it is clear that with clever use of series and parallel combinations, you can get the system voltage you require.

In a large installation, you also have to consider the input voltage requirement for the charge controller and the final battery voltage cluster to be used.

Figure 3: Panels in seriesFigure 4: Panels in parallel

Getting the Panel Requirements

Let’s assume this is for a small holiday bungalow.

You will need the following parameters, and I have suggested some typical ones:

Daily kWh usage:

  • Refrigerator, 100W, 12h on, 12h off = 50Wh
  • Lights 100w for 4h = 400Wh,
  • Microwave 800W for 1h = 800

Allow for losses and inefficiency of 30%, so multiply this by 1.3 = 1250*1.3 = 1625Wh

Average daily sun hours:

You can get this off the internet for your location. We get about five hours per day yearly in sunny South Africa, so divide this into the daily usage 1625/5 = 325W. This is the size of the panel we need assuming we are going to have a battery system. Assuming we buy 100W panels, we need a bit more than three; say four will do.

We have seen how solar panels work. We also tried connecting them and finally did a small project.

I used the image in figure 1 with credit to: https://commons.wikimedia.org/wiki/File:Silicon_solar_cell.gif