Looking to buy a solar panel for your home, or maybe build your own? It can be confusing trying to sort out the actual capacity of a panel and what it will or won’t power. So here are a few facts about solar panels to get you started on your way to energy freedom.
Solar panels are composed of individual solar cells, wired in some combination of series and parallel to achieve the desired voltage and amperage that produces the wattage you need for operation. In order to charge a 12 volt battery, you need a panel that outputs 18 volts. Here is a fact you’ll want to know: a solar cell, no matter how large or small, puts out a total voltage of 0.55 volts. For the sake of making math easy, let’s call it 0.5 volts.
So in order to get 18 volts, you need to create a voltage multiplier circuit (which for those of you not following the photovoltaic course on this site means a SERIES circuit). Between 34 and 36 cells wired positive to negative will achieve this. (0.5 * 36 = 18 volts) Let’s say you have a bunch of cells that have 0.5 volts each, and 1.5 amps each. If you simply wire 36 in series, as is recommended, you will end up with a panel that puts out 18 volts with total amperage of… 1.5 amps. All together, this means you will get 27 watts (per hour of full sun).
If you’ve checked the back of any of your appliances recently, you may realize that this wattage is not enough for your needs. After all, even a compact fluorescent 12 volt bulb will probably use at least 9 or 10 of those watts each hour it operates, which is 1/3 of your total available power. (For the sake of this example we will ignore the fact that you probably wouldn’t need a light while the sun is shining!) Use three bulbs, and your power will run out with the sunshine at dusk. So how do you increase your available power?
Most photovoltaic system batteries are designed to operate at 12 volts (similar to a standard car battery), so you don’t want to up the voltage unless you want to start upping the number of batteries as well. If wattage (available power) is equal to voltage * amps, and you can’t change the voltage, then it stands to reason that upping the amps is the way to go. To do this, you wire groups of cells together in parallel. In order to wire things up in parallel, the things wired together should be the same, meaning you will be wiring together groups of 36 cells (wired in series). To double the amperage, you would wire two sets of 36 cells together, for a total of 72 cells, 18 volts, and 3 amps (1.5 amps * 2). Now, you have an available 54 watts (18 * 3) per hour of full sun. For every additional set of 36 panels in series that you add (in parallel) to your array, you get an additional 1.5 amps, and your total wattage is increased by 27 watts per hour of full sun.
There are a few other things to remember when choosing a panel for your needs. The first is kind of obvious: the sun only shines so many hours per day. In most places, you can expect between 5 and 8 hours of “full sun” (the conditions under which your panel produces its maximum output) per day. So if you have a 50 watt panel and 6 hours of full sun, you can expect around 300 watts of power production daily sent to your battery (or perhaps a little more from a few hours of partial sun production). On cloudy days, or if you live in an area of high particulate pollution such as a large city, you will get less.
Which brings me to fact #2: to get the full output from your panel, they must be kept clean. Over time, dust settles on the panels and reduces the amount of light hitting the surface. Or in the winter, in cold areas, you will have snow to contend with, and in the fall, you’ll have to keep an eye out for leaves falling atop the panel. Birds are another common culprit. We’ve all seen the ground under areas where birds congregate… it isn’t pretty. Basically, your panel will output at the level of the cell getting the LEAST sunlight. So if you have one cell in full shade (or fully covered with something), and others in the sun, your total output will be reduced more than you’d intuitively think. Save yourself some grief and locate the panels in a place easy to access for cleaning, and outside the regular path of avian neighbors.
Lastly, since solar panel setups are Direct Current (DC) systems, and since most household appliances run on Alternating Current (AC), you will usually need an inverter to convert DC to AC power your appliances can use. But there is a good reason to consider investing in some 12 volt appliances and lights. In the process of inverting the power to AC, about 15% of the power is lost. So if you have a total of 500 watts to use, and you invert it all to AC, you will have about 425 watts available for powering loads (appliances). There is an increasing variety of 12 volt appliances on the market, from light bulbs and radios to refrigerators. The more 12 V appliances you use, the more use you get for your available power.
This is by no means a complete list of considerations when buying a solar generation system, but it should be enough to help you sort through the eBay flotsam and find a panel that meets your particular needs. Stay tuned for next time, when we’ll be discussing batteries!