Food was rationed because there was a finite amount of it, and it was evenly distributed. Since there isn't an infinite amount of food, food must be rationed under communism, and under a resource-orientated rather than wealth-driven economy, food must be rationed.
You say that solar/wind etc is a cheap energy serum for wiping away all of our problems, but on a cost-per-watt basis, that isn't the case, or if you use the generalization that cost is approximately proportional to the amount of human work involved in
setting something up, society would progress slower under a solar based economy and would be less efficient than under an oil based one. This is trivially the case because capitalism drives human work down by optimising for cost, which is a function of human
effort. They're clearly not hiding special free energy machines (as many conspiracy theorists would have you believe) because with free energy comes free money, and capitalism would drive them to exploit such technology.
Now you might (legitimately) argue that we should close down the sectors of the economy that are non-essential and gain productivity in this direction - so we'd stop producing sports cars and expensive clothes and TVs and shut down the non-essential parts
of the software industry; and using the productivity gained from those areas we could pump large amounts of effort into other areas of the economy such as agriculture, energy or technology -- an argument expressed by Marx himself no less. The problem with
this view is that at some stage or other you end up with a group of people deciding somewhere what and who should have access to which resources, and dividing them up on an arbitrary, potentially ill-informed and low-granular way, whereas under capitalism
the fast trading of resources and the inherent interest in all of the various parties to obtain the best "price" for their good or service means that agreements have to be made, and where large numbers of parties are involved in the agreement process, an optimum
agreement up to price would be formed -- thus forming an optimum agreement up to resource consumption.
It's not that a resource-based economy is _bad_ per se, it's that it's too ideal and it fails to take into account that the people managing it won't manage it with the same competence or high-granularity of distribution that capitalism can achieve.