How Many Solar Cells Do I Need

How Many Solar Cells Do I Need For My Solar Panel

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.

Photovoltaic Cell Voltage

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.

Photovoltaic Cell Current

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.

Connecting Individual Solar Cells into Modules

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.

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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.

So How Many Cells Do I Need

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.

32 Photovoltaic Cells in Series

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.

36 Photovoltaic Cells in Series

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.

48 Photovoltaic Cells in Series

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.

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.

I currently have six 100 watt 12v Renogy Mono panels with the specs below: note 36 cells per panel.

I am looking at buying some 12v Rich Mono panels with the specs below: note 33 cells per panel.

I currently use a pair of Bogart PWM charge controllers.

I want to add more panels and upgrade to a Victron MPPT system and also want the flexibility of connecting the panels in various series-parallel configurations.

The Rich panels are the only panels I can find that match the existing Renogy volts and AH, and my physical size requirements. Moreover, the Rich panels are the only panels I have found that have 12 AWG wires rather than 14 AWG wires. My old Renogy panels have 10 AWG wires.

Question: Will the 33 cell Rich panels pose a problem?

PS: I still do not know if the Rich panels have any diodes. My research is ongoing.

Thank you for any input you can provide!

For MPPT controller, if you are placing panels in series so the Vmp is moderately above battery voltage there is no problem.

Do not use a 32 or 33 cell panel, single or multiple in parallel if you intend to use an MPPT charger on a four-cell stack 12v LFP battery array. You need at least a 36 cell panel for this, or run two or more 32/33 cell panels in series.

There is roughly 0.45v to 0.5v per cell Vmp contribution depending on panel temp. MPPT controller requires some overhead voltage above battery voltage to work properly. Some of the overhead is for the DC to DC converter, some of the overhead is to allow the controller to search for MPPT point. 32 or 33 cell panel is okay for a PWM controller on 12v battery.

Most MPPT controllers will just drop into PWM mode operation if they cannot establish a reliable MPPT point but panel Voc is still above battery voltage.

For MPPT controller, if you are placing panels in series so the Vmp is moderately above battery voltage there is no problem.

Do not use a 32 or 33 cell panel, single or multiple in parallel if you intend to use an MPPT charger on a four-cell stack 12v LFP battery array. You need at least a 36 cell panel for this, or run two or more 32/33 cell panels in series.

There is roughly 0.45v to 0.5v per cell Vmp contribution depending on panel temp. MPPT controller requires some overhead voltage above battery voltage to work properly. Some of the overhead is for the DC to DC converter, some of the overhead is to allow the controller to search for MPPT point. 32 or 33 cell panel is okay for a PWM controller on 12v battery.

Most MPPT controllers will just drop into PWM mode operation if they cannot establish a reliable MPPT point but panel Voc is still above battery voltage.

Normally I would agree with you, but the panel performance values are almost a perfect match.

Normally I would agree with you, but the panel performance values are almost a perfect match.

You are comparing apples and oranges specs. One is for 25 degs C panel, one is for 47 degs C panel.

33 x 0.564v = 18.6v Vmp for a silicon mono cell is not possible unless cells are held to 20 degs C. The Rich spec claims 25 degs which is not realistic with sun panel heating. Maybe in wintertime cold temps. At 47 degs C, which is still relatively low panel temp with sun panel heating it would be 0.516v x 33 = 17.0v Vmp, not the 18.6v listed in their spec.

Renogy lists 18.6v Vmp for 36 cell panel and their spec is for 47 degs C +/-2 degs. That will have about 0.516v Vmp at 47 degs C x 36 cells is 18.576v which matches their spec pretty well.

Monocrystaline cells
OK... sorry, I have not read all of the above because of time restrictions but I do have some comments.

All so called 12V panels that I am familiar with have an open circuit voltage of about 21-22 VDC and Vmp is usually around 18V.

There are good reasons to wire panels in series and that is to minimize the voltage drop in the wires between the panels and the controller. I strongly recommend the OP avoid the pulse width modulation style controllers. MPP controllers are now affordable and allow putting panels in series. The only caution one needs is to understand the so called safe voltages are those below aobut 45V. Think old systems with 48 VDC huge batteries.

I wired my panels (33v) 2 in parallel in series with another 2 in parallel for my boat. Sure, I can get voltages above 70 but I can also get by with almost no voltage drop in the wiring between my total W panels and my Victron controller with two strings of #10 wire. Thanks for all the helpful replies.

I have an existing lead acid battery bank and believe that my Bogart PWM system was the best system for my lead acid batteries. I am now building a 560 Ah Lifepo4 battery bank and will be installing Victron charging and monitoring components.

Given that Rich solar never replied to my inquiry about whether their 33 cell panels have diodes, and given the uncertainty respecting the actual specifications vs. what Rich Solar advertises, I am going to pursue other alternatives.



Thanks again!

Given that Rich solar never replied to my inquiry about whether their 33 cell panels have diodes, and given the uncertainty respecting the actual specifications vs. what Rich Solar advertises, I am going to pursue other alternatives.

I have 4 Rich 100W poly panels that perform very well. I’m not going to pull one off to pop open the diode/junction box but I’d be shocked if they didn’t have diodes. They play well with 4 WindyNation monocrystaline 100W panels

Can you explain more what you meant by this? Thank you.

The “let-go” voltage of DC maxes out a bit over 50V by ‘definition.’ 40VDC can still kill you but not likely you’ll hold on long enough to do that, though it can burn you.
50VDC isn’t likely to immediately kill you either but it may and is probable it may immobilize you long enough to inflict severe injury- or kill you. 90VDC can shut vital systems down or un-time your heartbeat and may kill you dead on the spot or wack out your body that you collapse dead later or die that night in your sleep.

Is that true all the time? No. Maybe not even close- but the probabilities are a risk factor too great to ignore or take lightly. I can’t remember exactly now (not a joke, it’s just long ago) but 24VDC tingled me once. It was more than a tingle really but I suddenly became more respectful of DC is what I remember.