So how many PV cells do I need in my panels, anyway? by Richard Perez Solar modules are made with between 32 and 44 series cells for 12 Volt use. How many cells are enough? How many are too much? What is the optimum number of cells to put in a panel for 12 Volt use? Well, as usual, it depends on our specific application. The Single PV Cell In order to understand why there are differing numbers of PV cells in modules, let's first examine the single cell. This little marvel converts light directly into DC electricity. It does this job within very specific limits. These limits are, according to the quantum mechanics among us, built into the structure of our Universe. The limits of the single PV cell determine the operation of the collection of cells we call a module or panel. The electrical power generated by the PV cell has two components: voltage (E) and current (I). The output power (Watts or P) that the cell produces is the product of cell's output current times its output voltage. P=IE. The voltage output of the PV remains fairly constant over a wide range of input lighting, just as long as there is some light. The current, however, varies in direct proportion to the amount of light entering the PV cell. The more light entering the cell, the more current it produces. The cell's voltage remains the same from dim to bright lighting. For the purposes of discussion here, consider a 100mm X 100mm (4 in. by 4 in.) multicrystal silicon PV cell. Monocrystal or amorphous silicon cells will differ slightly. The absolute value of the voltage information will differ, but the general performance trends remain the same for all types of silicon PV cells. This example cell is rated using the standard AM 1.5 Solar Input of 100 milliWatts per square centimeter, about the amount of sunshine you receive on a sunny noontime. PV Cell Voltage This multicrystal 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 load and is not moving any current. Under load, the output voltage of the individual cell drops to 0.46 Volts at 25¡C. It will remain around this 0.46 V level regardless on the sun's intensity or the amount of current the cell produces. This decrease in voltage is caused by resistance losses within the cell's structure and the metallic conductors deposited on the cell's surfaces. Temperature affects the PV's cell's voltage. The higher the temperature is, the lower the cell's output voltage becomes. The PV cell's output voltage falls about 5% for every 25¡C. increase over 25¡C. PV Cell Current While the voltage of a PV cell is very reliable, its current output is one big, fat variable. The cell's current depends on how intense the light is, and most importantly for this discussion, the voltage difference between the cell (or collection of cells) and the load (in most cases a battery). Under operating conditions this cell is rated at 2.87 Amperes of current by its manufacturer. I have measured the current output of this type of cell at 4.2 Amperes on a very cold, very clear, very bright & very snowy Winter's noon. Altitude is a factor that affects the cell's output current. The Earth's atmosphere is absorbs sunlight. The higher you are, the less atmosphere there is above you, and the more sunlight you receive. Expect to see current gains of about 5% for every 5,000 feet above sea level. Cells into Modules When PV cells are assembled into modules they are wired in series. The positive pole of the one cell is connected to the negative pole of the next cell, and so on until all the cells in the module are connected in a series string. This series wiring is done to raise the voltage of the module. A single cell has a voltage potential of 0.46 Volts. This is not enough voltage to do any usable work in a 12 Volt system. But if we add the Voltage of say 36 cells by series wiring them, then we have a working voltage 16.7 Volts, and that's enough to charge a 12 Volt battery. The operational voltage range of a lead acid battery is between 11.6 and 16 volts. The battery's exact voltage depends on state of charge, temperature, and whether the battery is being charged or discharged at the time. It is this battery voltage curve that the modules are designed to fit. After losses in the blocking diode and the wiring are subtracted, the module MUST provide greater voltage than the battery possesses. If PV module cannot do this, then it cannot transfer electrons to the battery. It cannot recharge the battery. The current produced by the module remains the same as the current produced by a single cell, about 3 Amperes. The series wiring technique causes the voltages to be added, but the current remains the same. We could parallel connect the 36 cells. This would add their currents rather than their voltages. The result would be a module that produces 108 Amperes, but at only 0.46 Volts. Hardly a useful item. So How Many Cells? PV module manufacturers make 12 Volt modules with 32, 36, or 44 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 modules is one of voltage. The question for us to answer is how their output voltages relate to the voltages we require for our system. 32 Cells in Series This module has the lowest voltage rating of 14.7 Volts (0.46 Volts times 32 cells). This is because it has the fewest cells in its series string. This module is designed to very closely follow the charge curve of a 12 Volt lead acid battery. As the battery fills, its voltage climbs. When this battery is almost full its voltage is around 15 volts. The 32 cell module simply hasn't enough voltage to continue recharging the battery when its full. These 32 cell modules are commonly called "self regulating" because they lack the voltage to overcharge the average, small, lead acid battery. The applications suitable for the 32 cell module are RVs, boats, and summer cabins. These applications are characterized by intermittent use and relatively small battery capacity. In these applications, the 32 cell module can be used without a regulator and the batteries will not be overcharged during periods of disuse. 36 Cells in Series This module has an output voltage of 16.7 Volts (0.46 times 36 cells). This is enough voltage to continue to charge a lead acid battery even though it may be fully recharged. The 36 cell module is the workhorse of the Home Power user. It is most suitable for 12 Volt AE systems with battery capacities over 350 Ampere-hours. It has the higher output voltage necessary to recharge high antimony, deep cycle, lead acid batteries. It does, however, require regulation in many cases to prevent overcharging the battery during periods of disuse. This type of module needs regulation in systems where the total current generated by the PVs is greater than a C/20 rate to the battery. For example, a 350 Ampere-hour battery has a C/20 rate of 17.5 Amperes (350 Ampere-hours/20 hours). At 3 Amperes per module, the 350 Ampere-hour battery will not require regulation until there are 6 modules within the system. This is true only if the system is in constant use. If a system is unused for days at time, then regulation should be added if the 36 cell modules can produce a C/50 rate or more to the battery. The 36 cell module is more cost effective in Home Power applications because of its higher current at higher voltages and temperatures. The higher voltage of 36 series wired cells more effectively recharges the large lead acid batteries. Higher temperatures cause the voltage of any module to drop. The 36 cell module has enough voltage surplus to still be effective at higher temperatures, like on a hot Summer's afternoon. 44 Cells in Series The modules are the hot rods of the PV industry. 44 cells in series yields a working output voltage of 20.3 volts. These modules do not diminish in current output into a 12 Volt system, regardless of battery's voltage or high module temperature. They WILL REQUIRE REGULATION in just about every application. They have the voltage to raise the system's voltage, while charging full batteries, to well over 16 volts. This is high enough to make any equipment on line (like an inverter) very unhappy. Over voltage can ruin electronic equipment. The 44 cell modules have very specific applications. They are designed for systems that must accept voltage losses in transferring the PV energy to its destination. Consider a low voltage pump located some 300 feet down a well. The electricity that powers this pump must travel 300 feet down the well to the pump and 300 feet back up again. This 600 foot long wire run will have appreciable voltage losses even if monster big wire (like 0 or 00 gauge) is used. In order to deliver acceptable voltage levels at the pump we can increase the voltage of the module and just eat the losses in the wire. The 44 cell module, with its 20.3 Volt operating level can stand a loss of over 6 Volts and still be effective at the pump. A word to the wise here. The cost of additional cells within the module is far greater than heavy copper wire. Be sure that it's not cheaper to use big wire in your application before you decide on the 44 cell module to solve voltage loss problems. Another side benefit of the 44 cell module is its response in high temperatures and very low levels of light. We ran two modules, each using the same cells, side by side for comparison. The only difference between the modules was one had 36 cells in series, the other 44. The 44 cell module consistently produced more useable power in three situations: 1) The system voltage was above 15 volts, 2) the ambient temperature was very hot (over 40¡C.), & 3) the ambient light was very dim (in fog or on overcast days). We tabulated the results and compared performance with price and the 36 cell module was more cost effective. Even though the 44 cell module performed better, this increase in performance was not enough to offset its higher price. If you live in a very hot area, then the additional voltage of the 44 cell module may indeed pay for itself. In A Nutshell The 32 cell module is for small and often unused 12 volt systems. Its big advantage is it doesn't need a regulator. The 36 cell module is best for most Home Power systems. It supplies the most cost effective energy to 12 volt systems using lead acid batteries. The 44 cell module is suited to 12 volt systems with voltage loss problems. Its advantages are higher output voltage and strong performance in very hot locations.