Solar Panels
Solar Panels
[A flow chart that features four questions in bubbles. Each question has yes/no options in bubbles overlain to the left and right on the question bubble. Curved arrows points from the yes and no bubbles to either the next question or the result. The result written at the bottom is not inside bubbles. The chart has two main branches, that ends up in five places using only four different results, as the middle result is shared by both branches. Above the chart, there is a caption:] Should I put solar panels on it?
Does it move around? Yes Does it have regular chances to recharge or swap batteries? Yes Probably not No When running, is it hot to the touch? No Maybe Yes Haha good luck No Is there an empty space nearby where it would be easier to put them? Yes Probably not [Uses the same sentence as the one in the first branch.] No Sure

This handy decision tree aims to help in finding out whether a given object should have solar panels installed on it.
The root question is whether the object of choice moves. If it doesn't and has no nearby empty space that would be more practical for the solar panel installation, then yes, the object should be equipped with the solar panels. If the object is static, but you could more easily install the panels somewhere else nearby, probably that's the best place. An example of this is a slanted rooftop of a house or a field on a hillside: it's certainly possible to put solar panels there, but if a flat surface, like a flat-roofed house or a level field, is available, it would generally be easier to put them on that. This way, you can select the optimal direction for the panels to face, which might not be possible on a given incline, or even have them move to track the sun . However, if the house has a side that is turned towards the sun (south in the Northern hemisphere) then a house roof could be even better than on the ground, which is why the title text says "sure" for rooftops. For another example of things where "putting next to it" instead of "on it" is generally the easier (and arguably better) option, see the "highway surfaces" of the title text.
If the object moves, the next question is whether its batteries can be recharged or swapped with ease, in which case batteries may be a better option than solar panels, if the purpose of the panels is to power the object. The idea is that solar panels on a vehicle sound like an interesting idea, but batteries can be much more easily (and economically) recharged from a fixed electrical station than using solar panels on the vehicle as a power source. It may be possible to have solar panels on the electrical station , but that is a separate device to consult the table on.
Finally, if the object moves and batteries are not an option, the last question is whether the object heats up during operation. If so, solar panels may not work well. Randall doubts it mockingly, see also the title text regarding his Haha Good luck final option. 
Solar panels can only produce electrical power equal to about 20% of the solar radiation they receive. Thus, a device that heats up during use likely consumes much more power than the amount which could be produced by solar panels covering its surface - so "good luck". Obviously, many animals are also "moving objects" fitting this condition, and installing solar panels on them is bound to be a challenge.
Moreover, solar panels do not work effectively when excessively hot [1] (solar panels are typically designed to operate in temperature ranges of 15-25 Celsius, 59-77 Fahrenheit, 288.15-298.15 Kelvin, 518.67-536.67 Rankine, 12-20 Réaumur, 15.38-20.63 Rømer, 127.5-112.5 Delisle, 4.95-8.25 Newton, 5.968 546×10⁻²¹ - 6.174 608×10⁻²¹ joules of translational kinetic energy or 37-51 Felsius ).
But if changing batteries is not an option, and heat production and power requirements are low, then solar panels can be an excellent solution on a moving object. An excellent case for this is on space probes and satellites, which are typically powered entirely by solar panels (and reliably receive sunlight, because there are no clouds to interfere). Randall is well aware of this, as shown with the comics 695: Spirit and 1504: Opportunity about the two solar-powered Mars rovers , although in this comic he seems to have only been concerned with Earthbound objects.
The flow chart, however, does not mention if the thing in question actually needs solar panels, but according to the title text it works very well, and thus Randall implies that if the answer is sure then it is relevant to put solar panels there. The more solar panels in place, the fewer fossil fuels are needed, and this is in line with Randall's general interest in reducing climate change .
The title text suggests that this flow chart is very broadly applicable to anything the Sun hits.
Rooftops are classed as "sure", and those are, indeed, an active subject of solar installation (though, if there's suitable land nearby, it might not be the most efficient).
Highway surfaces are classed as "probably not".  There have been proposals and experiments a concerning photovoltaic pavement covering roadways with solar panels , but these have proven to be impractically expensive and prone to damage.  The flow chart suggests that, since many highways are near land that could be used for solar panels, that will usually be the more viable option.
Sailboats are classed as "maybe".  Unlike boats with motors, sailboats don't consume enough power to heat up, only requiring enough power to provide electricity for whatever equipment and appliances are on board. Since some sailboats are at sea long enough that swapping or recharging batteries may be difficult, solar panels could be a viable option.
Multiple other moving objects, including jets, cars, and wild deer ends up on the haha good luck result.  While these examples seem unrelated, they all have the same limitation: they consume far more power while moving than could realistically be harnessed from solar panels (as demonstrated by the fact that they noticeably heat up).  There are some experimental solar-powered cars, but these tend to be exceptionally low power (and resultingly low-performance) vehicles. Wild deer are clearly a humorous option, as they'd have little use for the electricity from solar panels, and would likely resist any efforts to install them.  Nonetheless, Randall includes them to make the point that the chart is effective, even with ridiculous examples.
[A flow chart that features four questions in bubbles. Each question has yes/no options in bubbles overlain to the left and right on the question bubble. Curved arrows points from the yes and no bubbles to either the next question or the result. The result written at the bottom is not inside bubbles. The chart has two main branches, that ends up in five places using only four different results, as the middle result is shared by both branches. Above the chart, there is a caption:] Should I put solar panels on it?
Does it move around? Yes Does it have regular chances to recharge or swap batteries? Yes Probably not No When running, is it hot to the touch? No Maybe Yes Haha good luck No Is there an empty space nearby where it would be easier to put them? Yes Probably not [Uses the same sentence as the one in the first branch.] No Sure
