In my sustainable agriculture course, I read that modern grain varieties put 60% of their photosynthesized energy into growing the fruit (the edible portion of the grain). Wild varieties put more like 3% into the fruit. There's your 20x difference in yield. But it comes at the expense of every other survival mechanism of the plant: modern grain varieties yield so much grain because they don't have working systems for anything, be it support, storm resistance, disease resistance, or general hardiness. They give us food because we supply them with a good amount of fossil-fuel derived work.
It's somehow similar venture capital. It's a resource produced in older times that allows you to get temporary advantage compared to somebody that uses just in-time generated resources.
This, among other reasons, is why grain ethanol was such a scam.
The worst mechanical processes, by contrast, usually get at least 10-15% efficiency. That's what photovoltaics can do, for example. Gasoline engines have a thermal efficiency of about 30%, diesel about 45%. Granted, you lose energy in the extraction/refining/transportation steps, but it's still an order of magnitude more efficient than photosynthesis.
The primary [photosynthetic] reactions have close to 100% quantum efficiency (i.e., one quantum of light leads to one electron transfer); and under most ideal conditions, the overall energy efficiency can reach 35%. Due to losses at all steps in biochemistry, one has been able to get only about 1 to 2% energy efficiency in most crop plants. Sugarcane is an exception as it can have almost 8% efficiency.
This machine was not invented yet at the time, but could have been made by any decent medieval carpenter. I could probably do it with an axe and a saw. It would have been quite valuable at the time, and could make you rich if you played your cards right.
Of course, that doesn't help solve the problem of immediate survival in a world where you don't speak anybody's language...
Yes this is all stuff in my house at the moment.
If I had more time I'd buy a 9mm revolver, load up a Kindle with reference books from demonoid and start working out.
The thing I'd be most worried about is dental hygiene. How do you look after yourself if you don't have a toothbrush and floss? I suppose there'd be less white sugar, which would help. What food you could get would taste bland and generally be awful.
First I'd go to the local book store and buy as many survival guides as I can get my hands on.
Then I'd buy some high quality tools - knives, leathermans, nail clippers, matches, first aid kits, etc..... Many of these would be priceless for that era.
I'd try to buy a small waterproof tent. Maybe a sleeping bag if it's not too large to carry. If not a sleeping bag, at least pack some warm water-resistant clothes.
If there's time I'd print out some Wikipedia articles about agriculture techniques, subsistence farming, natural medicines and fertilizers. Maybe find some information about edible insects and plants. Basically anything that helps you survive in the beginning.