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Many solar panels are watt-rated. The generated power depends on lighting conditions, so either the current and/or voltage is variable. Which one is it?

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  • \$\begingroup\$ Depending on the load, right? \$\endgroup\$ – Eugene Sh. May 19 '17 at 20:48
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    \$\begingroup\$ What makes you think it's one or the other? \$\endgroup\$ – Trevor_G May 19 '17 at 20:50
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    \$\begingroup\$ en.wikipedia.org/wiki/Photovoltaic_effect \$\endgroup\$ – Trevor_G May 19 '17 at 20:53
  • \$\begingroup\$ @Trevor , I said "and/or", but really, I’m not sure. This question apparently involves a voltage-rated panel. Most panels I see online are watt-rated. This website shows a current vs. voltage curve, but its second paragraph doesn’t explain how its example arrives at a 3.0 A for a 50W panel working at 13V. \$\endgroup\$ – CodeBricks May 19 '17 at 20:58
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    \$\begingroup\$ Both. Look up the I-V curve of a PV cell. \$\endgroup\$ – winny May 19 '17 at 21:02
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Solar cells are a PV junction, basically a diode so they have similar characteristics. The voltage is dependent on the amount of energy received from sunlight and the amount of current drawn, so it is load dependent.

enter image description here Source: MPPT tracking

Many solar panels are watt-rated. The generated power depends on lighting conditions, so either the current and/or voltage is variable. Which one is it?

This image better answers your question, you can see how drawing more current drops the voltage. At a certain point when the load impedance is equal to the source you will get maximum power transfer (shown in the smaller lines on the graph). The different light levels are represented with different colors (W/m^2 is how many watts of sunlight is striking 1 m^2 of area), and lower light levels generate different amounts of power. The problem is there are three variables voltage, current (which are dependent on the load) and the amount of power received by the cell.

So, you need a circuit that can track the maximum peak power point (MPP Tracking or MPPT) to get the best efficiency from the solar cell. Either that or make sure you aren't drawing too much power from the cell to keep the voltage level high.

enter image description here

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Primarily current. Photons release electrons, which flow in the form of current. That current divides itself between two current sinks in parallel: a weakly forward-biased diode (i.e. the solar cell itself) and the external load on the terminals.

That means the voltage you see on the terminals depends on three things : the characteristics of the external load, the characteristics of the diode, and the incident photon flux.

If the external load is a short circuit, you see essentially all the current flowing in it (so you CAN generate current without significant voltage)

If the external load is an open circuit, the current flows through the diode, and you see the forward voltage of the diode at this current (a bit less than 0.6V, maybe 0.55V).

If the external load is variable, you can adjust it to get the maximum output current without much reduction in that voltage, to maximise the useful output power (MPPT).

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Both. The data sheet will show an expected current-voltage curve, but even that will vary somewhat depending on solarization, temperature, age, and the like.

Solar panels are designed to conform with the physics which govern them, and are built to maximize watts per cost... not to output a convenient regulated supply (though you can get that if you really want it, at lower watts per dollar obviously.) Figuring how to get max out of the solar panels is a hard problem, and is what MPPT controllers are all about.

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