Many individual silicon solar cells tend to have an open-circuit voltage of approximately 0.5 volts and a short-circuit output current limited to approximately 3 amps, therefore it is necessary to combine these individual solar cells together in either series and parallel combinations to obtain higher voltages and currents. But how many solar cells do I need to construct a PV panel.
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A commercially available photovoltaic panel is constructed using between 32 and 48 individual solar cells in series to give a panel capable of charging a 12V DC battery. But how many solar cells are in a solar panel, and how many solar cells do I need?. Well, as usual, it depends on your specific application.
The electrical power generated by a photovoltaic cell, ( PV ) has two components: Voltage ( V ) and Current ( I ). The output power generated by the PV cell is measured in Watts, ( P ) that the cell produces is the product of the cell’s output current times its output voltage. In other words, Power (P) = Volts (V) x Amps (I).
The voltage output of the photovoltaic cell remains fairly constant over a wide range of input light intensities because of the cells photovoltaic effect, just as long as there is some light. The output current, however, varies in direct proportion to the amount of sunlight entering the PV cell. The more light entering the cell, the more current it produces up to its maximum. The solar cell’s output voltage remains fairly stable from low to bright sunlight.
For the purposes of this tutorial here, we will consider a standard 4″ by 4″ (100mm X 100mm) poly-crystalline silicon photovoltaic cell. Mono-crystalline or amorphous silicon cells are available.
The absolute value of the voltage information will differ slightly, but their general performance tends to remain the same for all types of silicon PV cells for the amount of sunshine it receives on a sunny day. So how does a solar cell work.
A poly-crystalline silicon solar cell has an open circuit voltage of about 0.57 Volts at 25°C. Open circuit voltage means that the cell is not connected to any electrical load and is therefore not generating any current.
When connected to a load, for example a battery, the output voltage of the individual cell will drop to about 0.46 Volts at 25°C as the generated current flows. It will remain around this 0.46 V level regardless of the sun’s intensity or the amount of current the cell produces.
This decrease in output voltage is caused by internal resistance losses within the cell’s structure as well as voltage drops across the metallic conductors deposited on the cell’s surface to collect the current. Ambient temperature also has an affect on the PV’s cell’s voltage. The higher the temperature is, the lower the cell’s output voltage becomes as it heats up, which is strange seeing that they spend all day sat in the sun.
While the voltage produced by a silicon photovoltaic cell is fairly constant, its output current on the other hand varies considerably. The amount of usable output current that a cell generates depends on how intense the sunlight is shinning onto the cell’s surface, and also the voltage difference between the cell and the load.
Under normal operating conditions a poly-crystalline cell is rated at about 2.87 Amperes of current. This value can increase considerably on a very cold, very clear, very bright and very snowy winter’s afternoon. Also altitude is another factor that affects the PV cell’s output current. The higher you are, the less atmospheric conditions there is above and the more sunlight the cell will receive, assuming no clouds or snow. So expect to see current gains if used well above sea level.
When individual photovoltaic cells are assembled together into modules or panels they are generally wired in series. That is the positive connection or pole of one PV cell is connected to the negative connection or pole of the next cell, and so on until all the cells in the panel are connected together in what is called a series string.
When individual photovoltaic cells are assembled together into modules or panels they are generally wired in series. That is the positive connection or pole of one PV cell is connected to the negative connection or pole of the next cell, and so on until all the cells in the panel are connected together in what is called a series string.
This series wiring is done to raise the voltage of the panel. We said earlier that a single cell has a voltage potential of about 0.46 Volts. This is not enough voltage to do any usable work in a 12 Volt system. But if we add the voltages together of say 36 cells by series wiring them, then we have a working voltage 16.7 Volts, and that’s more than enough to charge a 12 Volt battery.
The operational voltage of a typical 12 Volt lead acid battery ranges from between 10.5 volts to 14 volts. The battery’s exact voltage depends on its state of charge, ambient temperature, and whether the battery is being charged or discharged at the time. It is this battery voltage curve that the PV panels are designed to fit and so MUST provide a greater voltage than the battery possesses. If the PV panel cannot do this, then it cannot transfer electrons to the battery and therefore it cannot recharge the battery.
The output current generated by a solar panel of 36 cells in total remains the same as the current produced by one single cell, about 3 Amperes. The series wiring technique causes the voltages to be added together, but the current remains the same. We could parallel connect all the 36 cells but this would add their currents together rather than their voltages. The result of this would be a solar panel that produces 108 Amperes of electric current, (36 x 3) but at only 0.46 Volts, too low.
Most photovoltaic (PV) panel manufacturers make 12 Volt solar panels for battery charging applications with 32, 36, or 48 cells in the series string. They are all rated at about the same current, being composed of the same basic cell. The difference between these panels is one of voltage. The question for us to answer here is how their output voltages relate to the voltages we require for our 12V charging system.
This size of photovoltaic panel has the lowest voltage rating of only 14.7 Volts (0.46 Volts times 32 cells). This is because it has the fewest number of PV cells in its series string. This panel design closely matches the charging curve of a standard 12 Volt lead acid battery. As the battery charges-up, its terminal voltage rises.
When this battery is almost full its voltage is about the same as the PV cell’s at around 14.7 volts. The 32 cell module simply hasn’t enough voltage to continue charging the battery when its full so cannot overcharge the average, small, lead acid battery.
The applications suitable for these small 32 cell solar panels are in RV’s, boats, garden lighting and summer cabins. These applications are characterized by their intermittent use and relatively small battery charging capacity. In these these types of low power applications, a 32 cell panel can be used with or without a charge current regulator as the batteries will not become overcharged if left connect to the panel during long periods of non-use.
This size of photovoltaic panel has an output voltage of about 16.7 Volts (0.46 times 36 cells). This is enough output voltage to be able to continue to charge a lead acid battery even though it may be already fully recharged. The 36 cell panel is suitable for a home based 12 Volt alternative energy system with high battery capacities as it has the higher output voltage necessary to recharge deep cycle lead acid batteries.
However, a 36 cell solar panel will require some form of charge regulation to prevent overcharging the battery during periods of high solar intensities or when battery usage is at its lowest.
A 36 cell solar panel tends to be more cost effective in a typical home power application because it can produce a good amount of current or high voltages at elevated temperatures. The higher voltage produced by the 36 series wired cells will more effectively recharges a large deep cycle lead acid batteries.
High ambient temperatures will cause the voltage of any PV panel to reduce slightly, but the 36 cell panel has more than enough voltage surplus to still be an effective battery charger even at high ambient temperatures.
A 48 cell panel is the big daddy of the PV industry. 48 individual photovoltaic cells connected in series produces an output voltage of about 22 volts. These large PV panels have sufficient output current capacity to charge a 12 Volt system, regardless of the battery’s voltage or high temperature.
However, these large panels do require some form of charge regulation in just about every application. They have the sufficient voltage necessary to raise a solar system’s voltage, while charging full batteries, to well over 16 volts. This over voltage is high enough to ruin any electronic equipment rated at 12 VDC so some form of protection is needed.
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Generally, a 48 cell solar module has very specific applications where high power and currents are required such as in pumping water or are combined together with other 48 cell panels to produce a photovoltaic array. Solar arrays can combine many panels together in various combinations for increased power output.
Another disadvantage of this PV panel is its physical size and additional cost compared to 32 and 36 PV cell panels. 48 cell panels are larger so take up more roof space. On the plus side, a 48 cell panel will perform better in very hot areas and areas with very low levels of sunlight throughout the year.
If you're looking to switch to solar, you may wonder if you have enough space to install the panels. This is a valid concern - solar panels are pretty big! Most home solar panels are about 5.5 feet x 3 feet and weigh roughly 40 pounds each.
Most of the time, you won't see the size of solar panels expressed in feet. Instead, you'll see it's listed as the number of solar cells within a panel, with the most common solar panel sizes being 60-cell and 72-cell.
Let's take a closer look at the difference between these solar panel sizes and how to pick the right one for your home.
Calculate the costs and benefits of installing solar on your homeWhen you look at a solar panel, you’ll see it’s made up of small squares. Those squares are called solar cells, and they're the part of the panels that turn sunlight into electricity.
You'll see the size of a solar panel described by how many cells it has. 60-cell panels are usually laid out in a 6 by 10 grid and are the most popular option for home solar installations. You’ll typically find that 60-cell solar panels have output ratings between 350 and 400 watts and efficiency ratings between 17% and 19%.
72-cell panels, on the other hand, are usually arranged in a 6-cell by 12-cell grid. Because 72-cell panels hold more cells, they are bigger and can produce more solar power, making them popular for commercial installations. 72-cell panel wattage usually sits around 415 to 450 watts, but they can get into the 460-watt range. Their efficiency ratings are typically between 18% and 21%.
Many people want to know the physical size of solar panels, not just how many cells the hold. The average 60-cell solar panel is about 65 inches by 39 inches, or 5.4 feet by 3.25 feet, and weighs around 40 to 50 pounds. The actual dimensions will vary from panel to panel, so we’ve listed a few for some of the most popular 60-cell panels on the market:
72-cell panels are larger, about 80 inches by 40 inches and about 1.5 inches tall. This works out to just about 6.5 feet by 3 feet for 72 cell panels, give or take a few inches. 72-cell modules generally weigh around 50 pounds. Here are the dimensions for 72-cell panels offered by top brands:
Here’s a brief breakdown of the pros and cons of each panel size.
Once solar panels are installed on your roof, you don’t have to think much about their size anymore.
But, with portable off-grid panels, solar panel size is one of the most important things to consider. Your RV might only have a small amount of roof space, or you might want something you can carry around and set up outside your campsite!
Because portable solar panels are made for travel, they tend to be smaller and have about 40 cells as opposed to 60. 200-watt portable panels are around 5 feet by 2 feet.
You could also opt for flexible solar panels, which also measure about 5 feet by 2 feet. But flexible panels can bend and are much thinner than crystalline panels, so they’re easier for storage.
The flexible panel sold by Renogy, a popular portable solar brand, has 72 cells, but they’re smaller than the ones used in typical commercial panels. The panel is also substantially lighter, weighing just 10.8 pounds!
Folding solar panel kits are also an option for portable use. Foldable panels can be small enough to fit in your backpack, or large enough to power your RV - there's a wide range of options out there.
There are many factors that you should consider before the size of your solar panels, like solar panel efficiency and solar panel warranties.
Solar panel efficiency is the percentage of light that strikes the surface of a panel that is converted into usable electricity. Modern solar panels have efficiencies that range from around 17% up to 22.8% in some premium models.
Highly efficient panels take up less space, which will allow you to fit more panels on your roof.
There are two types of warranties that cover solar panels.
The first is the product warranty. This is the period during which you can ask the manufacturer to replace the solar panel should any problem arise. Nowadays, a product warranty of 12 years is standard, but more brands are starting to offer product warranties of 25 years.
The second type of warranty is a performance guarantee. Most mainstream panels have a 25-year production warranty, guaranteeing the panel will produce a certain level of output at every stage of the panel’s life. From the date your panels are installed up until the 25-year mark, the minimum guaranteed output of your panels will gradually decrease.
You can expect an output of close to 100% for year one, gradually down to between 80 and 92 percent for year 25. This loss in output is technically known as solar panel degradation. Some solar panel manufacturers, such as Canadian Solar, even offer a 30-year performance warranty on certain panel models.
Find out if solar panels are worth it for your homeThe easiest way for homeowners to figure out the best solar panel size for their needs is to work with a dedicated solar installer. Solar installers will size your system based on your energy needs, coupled with your available roof space and the sunlight in your area. They can recommend a system size that can fit on your roof and reduce or eliminate your electric bill.