Tasman Energy
Multicrystalline solar array

About Solar Panels

A solar cell is a pretty nifty thing! Basically, all you do is add light and voila, you have electricity! Invented by a Frenchman; Alexandre-Edmond Becquerel (24/3/1820 - 11/5/1891), the solar cell was first seen as a novelty because of the miniscule amount of electricity generated. The first commercial use of solar cells was powering light meters in cameras in the early 20th century.

The use of a solar cell as a light meter gives us the first important piece of information about solar cells: The output of a solar cell is directly proportional to the amount of light falling on it!

A solar panel is a bunch of solar cells connected together. The number and size of cells and the method of connecting them determines the solar panels rated output. Whether you get the rated output in real world conditions will be determined by the amount of light that shines on it.

On the market today are three types of solar panels, mono-crystalline, multi-crystalline and amorphous. Mono and Multi-crystalline panels use silicone, the former has its cells cut from a single crystal silicone ingot, the later has its cells cut from a silicone ingot made from multiple shards of silicone all compressed and blended into a single bar. The panels pictured above are multi crystalline. Amorphous panels use silicone layered onto the panel substrate. Mono and multi-crystalline panels have the cells under glass. Amorphous panels have the silicone on top of a substrate that is usually a thin sheet of stainless steel.

The efficiency of a solar panel is based on its ability to convert sunlight to electricity and will be between 12 and 18 odd percent. This conversion factor is of little importance, apart from the fact that a high efficiency panel may be slightly smaller than a low efficiency panel. Efficiency is often a solar panel manufacturer's way of saying, look at how good our cells are. Given that when you buy a solar panel the output will be provided at an industry standard rating, the panel output is what really matters far more than the efficiency or method of construction.

Here is what matters when you buy a solar panel!

What you want when you buy a solar panel are several things to consider:

  • You want value for money; that is, you want the highest output in watts for the least amount of bucks. Always convert the price of solar panels into a dollar per watt sum. It goes without saying that you also want a quality product.
  • You want a panel that will stand the test of time. Exposure to sunlight for long periods of time is hard on anything. A panel sits in the sun all day, year in, year out. At the risk of repeating myself, I reiterate, you also want it in the sunniest place you can find.
  • You want a panel that has a good manufacturer warranty.
  • You want a panel that is robust. This is especially true if you live in an area that gets heavy hail occasionally or you are putting panels on the roof of a mobile vehicle.

Solar Panel Power Output

Once upon a time just about all solar panels on the market were designed for charging batteries. Times have changed. The vast majority of solar panels on the market today are designed to generate power to contribute to the electricity grid. Panels that were designed for charging batteries had an output voltage suited to batteries, like 12, 24 or even 48 volts. Panels designed for feeding power into an electricity grid have voltages unrelated to charging batteries.

Solar panels will all come with a specification sheet and a placard that will define the panel's output according to industry established criteria.

Stuffed somewhere on this page is a photo of such a placard, one from a battery charging panel called a KC120.

Solar panel placard

Here we can see some interesting stuff.

  • Pmax 120 watts
  • Vpmax 16.9 volts
  • Ipmax 7.1 A
  • VOC 21.5
  • Isc 7.45 A

The placard goes on to state this stuff at a lower irradiance then the (optimistic) industry standard of 1000 watts per square metre.

Let's look at these figures one by one.

Pmax

120 watts, speaks for itself, this is the rating of the panel and what you should get from it in perfect conditions

Vpmax (Volts at maximum power)

Here they are telling us that the maximum power is obtained at 16.9 volts

Ipmax (Current in amps at maximum power)

Here they are telling us 7.1 amps is the current to expect at Vmax. If we grab a calculator and multiply volts by current we get the following:
7.1 x 16.9. No surprises here, the answer is 120 watts! (119.99 if you want to be precise)

Voc (Voltage "open circuit")

If you put the panel in the sun with nothing connected to it and measured the voltage across the positive and negative terminals with an accurate voltmeter this is what you should see. This panel is designed to charge a 12 volt battery. In order for a battery to charge, the supply voltage of the charging device must be higher than the battery being charged. Here the Voc and the Vpmax are both above the fully charged voltage of a 12 volt battery.

Isc (Current in a short circuit)

If you take this panel and put it in the bright sunshine with nothing connected and then connect an ammeter between the positive and negative terminals this is the current that should flow through the ammeter. By the way, you can run a solar panel in a short circuit (meaning with the positive terminal connected to the negative terminal) indefinitely, it is actually a very low load on the panel. A solar panel is one of the only electricity producing devices that can run happily in a short circuit.

Panels for Battery Charging

If you want the best value, easiest to get and decent sized solar panels you no longer buy battery charging panels, the only exception being if the solar installation you are contemplating is very small and only needs 150 watts of solar output or less to keep things charged.

To charge anything other than something small like a few lights, you are probably best off buying grid feed solar panels. I say probably, you work it out, remember to convert all panel prices you get into a dollar per watt sum. What also must be taken into a account is that a panel designed for grid feeding will probably require a more expensive solar regulator than its battery charging counterpart would require.

The downside of using grid feed solar panels to charge batteries is that you will need a specialised solar regulator to interface between the panels and the batteries if you want to get the maximum amount of power out of your solar array. A device called a maximising regulator will do the trick just fine.