There's a few tweaks that your society would make rather rapidly. First and foremost, they would choose to standardize on a long-lasting caloric source like grains. If I already have dinner tonight, and you offer me 2 steaks in exchange for my work, I'm going to say no. Why? I don't need steaks and they spoil. Spoiled food is wasted wages. If you offer me two steaks, I'm going to try to trade them in for a grain as soon as possible. Grain is an excellent store of calories in the long term. Do this for a few days, and your employer will quickly realize that they can cut the middle man out and pay you in grain directly! I'd say it'd take about 5 days for this transition to occur!
This transition will also save you from the other issue: scurvy. Well, scurvy and dozens of similar diseases brought on by malnutrition. We need a lot more than just calories to survive. Spinach has a mighty 79 calories per bunch, contrasted with potatoes which come in around 160 calories per potato. It's a lot easier to grow a potato than a bunch of spinach, so farmers will quickly start growing only potatoes. However, spinach has loads of nutrients you need that aren't in potatoes. Your entire post-apocalyptic society could starve while being well fed on potatoes!
If you have standardized on grain, rather than calories, the value of spinach is going to be much higher than its mere caloric value suggests. You have to get away from measuring worth in calories before anyone will grow spinach. If the standard is calories of grain, rather than calories in general, there's room for the value of spinach to grow beyond the value of its calories.
Speaking of which, another reality is that your system will result in 0 meat. No steaks. Why? Because their value in calories is far far below the value in calories you had to put into the animal. In most cases it's at least a 10 fold difference in calories going into the animal to meat harvested, and its typically higher. Anyone trying to feed cattle would quickly realize that they're driving themselves bankrupt with such an inefficient industry. There's a reason why, when you go to the store, a 200g steak costs quite a lot more than 3 cups of milk!
This also avoids the reality that caloric content is remarkably hard to measure short of burning the food to see what it had in it. Caloric references like we have today are really just intended to be a reference point. Your actual caloric intake may be markedly different depending on many factors.
Edit: To respond to several comments, I'm not advocating a purely vegetarian diet, especially not one consisting just of potatoes and spinach. I'm merely pointing out that the caloric value of these foods is not their only source of value, and if the prices were fixed according to their caloric value, many source of food would simply not be used. Instead, I would expect the currency to transfer from calories to some other unit of value which is more capable of expressing the value of different food products.