If the solar cell wasn't there, most of the energy would have ended up as heat anyway. The sunlight was going to hit that patch of earth whether the panel was there or not. Whereas coal that isn't burnt is avoidable energy release. Photosynthesis efficiency is approximately 3-6%. So panels in total likely release less heat than forest which has an albedo approximately 10-20%. Albeit a forest releases a bunch of the heat in water vapor which drastically decreases the temperature rise from the heat.

A high albedo surface like fresh snowpack would be optimal for avoiding heating, but I doubt panels produce more warming than the average surface they cover.

That's ok; that sunlight would have hit the earth anyways. Solar installations are optimized for minimizing price per kwh, not for optimizing use of solar energy.

1 BTU = ~1.06 Joules

Just use gigaBTUs (GBTU) to get your metric fix.

It's meaningful because charts showing "primary energy" include waste heat, so you only need to replace ~1/3 of primary energy with renewables to fully replace its use.

I think it's referring to the yellow-shaded portion between the energy contained and energy gained. The energy contained is the potential chemical energy that is released when burning in the form of heat. We can only harness so much of that energy as no system is flawless like the theoretical Carnot engine. The theoretical amount you can gain compared to amount actually captured is the efficiency, so this graph is meant to highlight how inefficient coal burning is.

And while, on paper, renewable energy is less efficient in that a smaller percentage of energy can be captured, we are also not losing something in exchange for that inefficiency since there is no fuel involved. We just get less than we'd like. Everything not captured from burning coal is not just a waste of resources, but also adds things that are a detriment to the environment like greenhouse gases. It's harder to get as much from renewable sources, but they're also not making the problem worse at anywhere near as big a scale.

The graph is poorly labeled, as it should only refer to the gap, but implies the full value of energy contained instead of the difference between energy gained and electricity obtained. But I think it's valid

Coal (anything) doesn’t "contain" energy. We can transform some things, and some transformations produce energy in some form or another.

Akstchually energy is a property of matter, or matter is a property of energy, whatever 🤓 but your point still stands