In Mad Max: Fury Road, why didn't they harness the solar power to run their automobile? We already know that the solar rays were still powerful as the plants in the citadel were exposed in a rotation to obtain the fresh produce.
There are various aspect to this. But lets first start with some possible in-universe explanations:
First of all gasoline (or "guzzoline") is heavily ingrained in the culture of the War Boys. While it is more the raw power of cars they are about (with things like the valhalla spray, character names like "Organic Mechanic" for the doctor and imagery like Max being used as a living radiator mascot), this "horse power machismo" just doesn't work as well without fossil fuel.
They might not even have the technology or know-how to harness solar energy anymore, as their technology seems largely old and scavenged (okay, bionic arms notwithstanding ;-)). But even then their cars are also largely reassembled scrapyard cars and it might very well be that they just didn't have the ability or willingness to completely redesign their engines for solar electricity.
While they seem to be quite inventive, there might also not be much of a motivation for that either, since fuel does actually seem to exist in sufficient amount. Now it's true the smaller tribes (who in turn might lack solar technology) are fighting for every last drop of fuel, but it's also apparent that Joe's War Boys seem to have a good economy going with the guys over at Gas Town and there could very well be a resistance to overthrow that established economy and the other political advantages it brings (not all too dissimilar to the economic resistance the abandoning of fossil fuels is facing in our present time).
And this also leads to a larger aspect of all this. There generally seems to be a resistance to abandon the old ways, the ways that ultimately led to the downfall of the world as it is in Fury Road (or earlier films too). This whole "who killed the world" mantra and the archaic War Boy culture of Immortan Joe seems to be somewhat of a statement against the warlord machismo that supposedly made people destroy the world with their fights over fossil fuels. Granted, this is all only hinted in the film (elaborated a little in this related question and its answers), but the fight over fuel is a major aspect of many of the Mad Max films and it doesn't take much to take that as a parable of the state of our world and the way it's headed. It's no accident that Wolfgang M. Schmitt Jr. even compares the imagery of the world as presented in Fury Road to Werner Herzog's Lessons of Darkness (about the Kuwaiti oil fires) in his analysis of the film (unfortunately not available in English).
This, last but not least, brings us directly to the massive role that gasoline and its associated themes plays for the entire Mad Max mythos. Those films are about motion, cars and raw horse power as much as (or even more than) they are about Max himself. From the very first film that has the police battle gangs in muscle cars, over the 2nd one centred around a small enclave of humans trying to protect their only gas truck as a symbol of hope for the future. Though, the 3rd did actually reference a shift towards a replacement for gas (still, not electric rather than just bio-generated carbon fuel). The 2nd and 3rd film also end with a massive multi-car chase, a concept that Fury Road not only continued but basically extended to the entire movie.
Now you could say you could as well do car chases with Tesla Roadsters, but tell that to a man who grew up in the 80s (and Max Rockatansky is pretty much a product of the 80s, even if not diegetically) as driver of "The Last of the V8s"! These films are about raw fossil fuel force. In fact Fury Road drives this obsession to new heights with the above mentioned aspects of War Boy culture. Those films and the worlds they are depicting are obsessed with gasoline despite and because this obsession led to those worlds.
Same reason we don't do more green energy today:
We tend to think of solar panels as things you might put on your roof to generate power for direct use but the real "off the grid" application of solar (and wind) has it going into a bank of batteries. The solar charges the batteries; the batteries run your stuff. The bigger your solar array and the more batteries you can hook up, the better.
Lead-acid batteries are pretty simple and should be within the ability of Mad Max people to create and maintain but they are heavy and their energy density is far lower than lithium batteries. Lithium batteries are complex and probably outside the technology level of Mad Max colonies (if you're familiar with the Tesla Wall, that's lithium-ion based -- high tech stuff). You can certainly run electric vehicles off of lead-acid batteries (golf carts, for instance) but the weight to power ratio of lead-acid makes them not well suited to long ranges, heavy loads or lots of fast acceleration.
Additional topics of research if you Want To Know More: battery types & electric cars; battery manufacturing; why we still use diesel for our 18-wheelers.
In short, it's easy to imagine a clean, green, electric powered colony in the Mad Max world. It's also easy to imagine them getting smashed by the War Boys because when it comes to low tech energy production, it's hard to beat gasoline. For moving heavy things or smashing an up-armored bus into your neighbor's gate after a 100 mile drive, gasoline is the way to go.
Just a few more interesting numbers when thinking about directly running a car on solar power: modern solar panels produce about 150-200 watts per square meter on a sunny day. Figure a vehicle could fit, I dunno, let's say you can fit 5 square meters, so 1000 watts of power. By comparison, a Chevy Volt electric motor can use 111,000 watts to generate 149 horsepower. Probably not up to War Boy standards to run on pure solar.
(Just as a related footnote, batteries are quieter than nuclear reactors which is why electric submarines are great. Why not switch to all electric subs? We probably would, if we could get batteries with higher energy density. Nuclear submarines can stay down much longer than electric subs, which eventually need to surface to run their diesal engines and recharge their batteries.)