How is the "Amperage Rating" on Photovoltaic Panels derived?

Question

Please be clear and thorough, I only have a basic understanding of electrical engineering but I try my best to truly understand concepts.

I've really scoured the Google on this one. How can a solar panel (photovoltaic panel) be rated at 24V, AND 5A? Furthermore, how can these numbers be combined to give (24x5=120) 120W?

Here's where my confusion comes from: if current is the voltage divided by resistance (I = P/R edit: I = V/R) then surely the panels can only be rated for their voltage, since the delivered current will depend on the resistance within the load. So since the load is unpredictable, there shouldn't be any "standard" current to be delivered right? So how is the amperage rating determined?

My best guess at the answer was "the amperage rating is determined as the maximum current delivered under a short circuit, I.E. it is derived by taking the maximum output voltage, and dividing it by the internal resistance of the panel (I = P/R edit: I = V/R)". However, I can't find any sort of clarification or confirmation of this, so I'm still lost as to how the amperage rating is found, let alone applied as a label on PVs, nor anything else that isn't a source of steady current.

Answer 1

You're misapplying Ohm's Law. First, current is not power over resistance, it's the square root of power over resistance. When you are finding current through a resistor, or resistive load, then \$I=\sqrt{P/R}\$ gives you the current if you already know the amount of power it is dissipating (not what power it is rated for) and its resistance. For example, if you had a 3 Ω resistor and knew it was dissipating 27 W, the current would be calculated to be 3 A. (\$3 = \sqrt{27/3}\$)

A photovoltaic panel will be rated for how much power it can deliver in full sun, usually in watts. There will also usually be ratings for open-circuit voltage, voltage at maximum power, current at maximum power, and short-circuit current. The 24 V rating you provide might be its open-circuit voltage (meaning no load is attached). Once it is connected to a load, the produced voltage may be lower. Unless, of course, that specification is its max power voltage, in which case its open-circuit voltage would be higher.

Its current rating at max power should give you an idea of the voltage it will produce. Let's say that its current at max power is indeed 5 A, then if the panel is rated for 120 W, the voltage at max power should be 24 V. As the load varies, the panel voltage will as well (all else being the same). For example, if you only draw 60 W, its voltage will actually be a bit higher, but the current it provides will mathematically be the product of the voltage and current at that load. For example, if its voltage is 26 V at half its rated power (60 W), the current would be 2.3 A.

You said:

...surely the panels can only be rated for their voltage, since the delivered current will depend on the resistance within the load.

You're sort of on the right track. In reality, both their voltage and current vary, depending on both solar radiation and the load they are connected to. This is why you usually will see the following specifications for a PV panel:

• \$V_{OC}\$: Voltage (open circuit): The maximum voltage the panel can produce with no load.
• \$V_{MP}\$: Voltage (max power): The voltage expected when panel is providing its maximum power.
• \$I_{SC}\$: Current (short circuit): The max current through the cells when it is short-circuited; the maximum current the panel can produce.
• \$I_{MP}\$: Current (max power): The current the panel produces at its maximum power output. Larger currents may be sourced at lower voltages, but at reduced power.
• \$P_{MAX}\$: Power (maximum): The maximum power produced by the panel in ideal conditions at some combination of voltage and current which should be the MP figures, above.

In practical applications, it is generally desirable to vary the voltage and current drawn by a load somewhat to "find" the maximum power for conditions. For example, on a cloudy day, there may be some combination of voltage and current which produces the maximum power that is a percentage (<100 %) of max power but isn't the \$V_{MP}\$ or \$I_{MP}\$. This is why maximum power point tracking (MPPT) controllers are frequently used in solar installations.

Finally, if you're unclear on current ratings for power supplies in general (including photovoltaic), you may want to review Choosing power supply, how to get the voltage and current ratings?.

Answer 2

How can a solar panel (photovoltaic panel) be rated at 24V, AND 5A?

The rating gives that maximum current that can be delivered while maintaining the rated voltage.

You are right that the current is demanded by the load. Any current over 5A will cause the output voltage to drop below 24V.

The solar panel will heat internally for currents greater than the rated current.

Open circuit (no load current) the panel will output a voltage higher than 24V.

The 120 watt rating is the product of the rated voltage and the rated maximum current. It is the maximum power that can be delivered to the load.

Answer 3

FYI: Ohm's Law really is only valid for resistors.*

A rough model of a single cell within a solar panel looks like this:

^{simulate this circuit – Schematic created using CircuitLab}

The I1 component is a current source. It behaves nothing like Ohm's Law. The current through a current source is constant. It is completely independent of the voltage. \$I_1\$ is the photo current that is induced in the solar panel by sunlight, and its value depends only on the brightness of the light.

R1 Represents "leakage." R1 effectively has a very high value, and we can ignore it here.

R2 represents internal resistance. Everything has internal resistance. We will assume that the value of R2 is low enough that we can ignore it also.

D1 represents the fact that a solar cell literally is a giant silicon PN diode. A diode's behavior also is nothing like Ohm's Law.

If the solar cell is not connected to anything, then the only path for the \$I_1\$ current is through the diode, D1. (Remember, we're pretending that the value of R1 is high enough that we can ignore it.) The voltage between Out+ and Out- in that case will be the forward voltage drop of the diode which, for a typical solar cell, is around 0.5V.† That is to say, the "open circuit voltage" of a single cell is around 0.5V.

No current flows through the external circuit when it's an open circuit (obviously, because there is nothing there for current to flow through), and so no power is delivered.

If the solar cell is shorted out, then the output voltage is zero.‡ No current flows through the diode in that case, and all of the current flows through the short circuit. But, again the power delivered is zero because power is current times voltage, and in this case, we've got no voltage.

Somewhere in between those two cases (open circuit or short circuit) we've got both current flowing at the output, and voltage across the output, and the cell is delivering actual power. The operating point (current and voltage) that delivers the most possible power is called the "maximum power point" (MPP). \$I_\text{mpp}\$ will be pretty close to \$I_1\$, and \$V_\text{mpp}\$ will be pretty close to 0.5V, but figuring out the exact values is where where R1 and R2 come in to play, and that's not where I'm going today.

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How is the "Amperage Rating" on Photovoltaic Panels derived?

I don't know about "amperage rating." I only know about short-circuit current (as described above.) I have not heard of any solar panel that cannot tolerate being shorted out in full sunlight.

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* Also, for conductors, which actually behave exactly like very low-value resistors.

† That's last time I checked, which is maybe a long time ago.

‡ The voltage across an ideal short circuit (like, shorted by a superconductor) is always zero.