I always wondered about the implications of free unlimited energy. Humankind as a whole would become much more powerful with all sort of potential implications. Basically, it would mean having more of everything. However, any energy changing hands means, by the second law of thermodynamics, waste heat. That is written in stone. There would be a limit for the maximum energy we can use before Earth temperature begins to increase, fixed by the capacity of Earth of dissipating heat into space. I wonder how close we are to such limit already..
Total solar irradiance of the earth by the sun is around 12*10^19 W (accounting for the average al. Total primary energy consumption is around 2*10^13 W. So there is still a factor of 6 million there. Very roughly, you would need a factor of 100 to get a 1 K increase of the temperature. So there is still a factor of 60000 until this scenario occurs. Mind, I don't think energy will ever be completely free. There is always going to be at least some maintenance cost, so there won't be that much incentive to completely waste generated energy.
I would be genuinely surprised if a multi-megawatt solar electric field produced a megawatt of waste heat. Pretty sure that's par for the course for any comparable nuclear plant. Thermodynamic cycles involving fluid-based heat transfer are just inherently hot in a way that light striking glass and semiconductor plates is not. Photons I believe do cause the atoms they strike to kick out some low-energy photons but these light rays would already do that when they strike the Earth anyways so the net heating added by solar electric production is negligible.
Technically, atoms by themselves are usually not really involved in this. Nevertheless, thermodynamics does ensure that high-energy photons tend to generate low-energy photons in the long run.
As @Timsup2nothin correctly pointed out, the net waste heat generation depends on what surface you put the solar panel over. Putting it over tarmac, will have very different results than putting it on desert. But let's make an estimation. Typically, a commercial solar module has an efficiency around 20%. So to generate 1 W of power, you need 5 W of sunlight. If we take use the average albedo of 0.3, 70% of that would have been absorbed anyway. However, for maximum efficiency, a solar panel has to be as black as possible. If we take 5% reflectivity (and I believe state-of-the-art solar panels are better than that), this means that 25% more of the sunlight is absorbed than would have been without the solar panel. 25% of 5 W are 1.25 W. This means, a solar panel put on an average surface is going to produce more waste heat than electric energy. So your multi-megawatt solar electric field is also going to produce multi-megawatt waste heat.
That said, power plants that convert thermal energy to heat (fossil fuels or nuclear) usually generate more waste heat per generated electric power than that.