A small current is going to flow internally, but nothing else happens. It's quite normal to have solar panels running with zero load in regions with lots of PV - reason being, that the carriers need to keep their electricity nets stable and have to carefully balance electricity entering and leaving the net.

At least in Germany, every PV installation of certain size (> 30kW peak) is mandated to be able to be shutdown remotely by the carrier if you supply electricity for the net and aren't just using it for yourself. (You get paid the same during shutdowns, just like it were running. Otherwise it would be quite damaging and likely reduce adoption of PV)

Point being: no, it doesn't hurt the panels and is a regular ocurence.

The generated power will be dissipated through the panel as heat, AFAIK.

Which means that in winter, probably nothing, because it's cold, but on a hot summer day with peak sun, the heat might start damaging the cells. How much exactly you'd have to look at studies.

My guess is the output will permanently degrade by a few % per year if the panel is not connected. Might go down to 80% way quicker than normal (25-yr)

> The generated power will be dissipated through the panel as heat, AFAIK.

Solar panels are not constant-power devices. In an open circuit, they will generate their open circuit voltage at nearly zero current (except minor internal leakage), and thus nearly zero power. In a short circuit, they will generate nearly zero voltage, and thus also nearly zero power. To get maximum power out of a solar panel requires maximum power-point tracking (MPPT), where the load is adjusted such that the product of voltage and current (that is, power) is optimized for the current conditions; while significant power can be delivered to a fixed load, there's no real power being generated without a load.

Where's the power going to then? Either it's heating the panels or it's reflected back, no?

The thermal power of the sun will heat the panel, to the extent that it is not reflected. But no electrical power (or any power) is being "generated" by the panel, the panel is just absorbing photons like anything else with low albedo.

That's just nitpicking. There is thermal power generated from the electromagnetic power from the sun and GP is right that a solar panel turned off will be hotter than one turned on.

And if that was the original poster's intent, I apologize for nitpicking. My impression was that the use of "generating power" in the given (open circuit) context suggested a fundamental misunderstanding of the behavior of solar cells in this situation on behalf of the poster, and thought I might clarify; and perhaps even if the original poster understood this already, someone else learned something.

Think of what happens to a normal roof tile in the sun: it absorbs solar energy as heat and also reflects some and radiates some away. A solar panel is the same (though a bit more reflective), but when a load is connected some of the solar energy is converted to current instead of being absorbed as heat. Therefore the panel is a little cooler when a load is applied.

There is no power. Power is IV, current times voltage. The voltage will be the rated voltage of the panel at that sunlight level, but the current is 0 (minus some very, very small (microamp) internal currents).

Alternately, power can be expressed as V^2/R. But in an open circuit R is infinite, so again, zero power.

There is ~1000W/M2 of electromagnetic power from the sun, and if it's not turned into electrical power it will be turned into thermal power, thus heating up the panel.

But as noted by sibling comment, that heated panel will then begin to transfer that heat back into its environment, as a function of its temperature difference with its environment.

So as long as manufacturers engineer their panels to be tolerant of the maximum heat at a site (i.e. full sun, maximum temperature), the panels won't be harmed in any meaningful way. They'll just heat up a bit faster than if they were providing current.

Is ther a difference between "electromagnetic power" and "thermal power" here? If a panel is not connected, there is no conversion and the surface is warmed - just like any other surface exposed to the sun gets warmed.

PV panels are just like charged capacitor or a chemical battery with no loads: just holding unused potential differences with no damage to the unit.

The sunlight heats the panel a bit more than usual but there isn't really generation/dissipation going on.

And since heat radiates away at temperature to the fourth power, the increase shouldn't be particularly much.

Given ~20% efficiency it's almost negligible amount of heat.