Since I have been promoting calories as a metric for energy usage, I decided to actually see how many calories I produced on the farm in 2008. The first step was to review my inventory control. What I do is to count and tally everything that comes in from the field, whether in pounds or bunches, into a notebook. I then take the figures into the house and enter them into my spreadsheet each night. The spreadsheet breaks down yield by succession planting, but I also transfer the numbers into a yield summary. I can thus track individual salad mix beds, for example, and also have a year-end summary per vegetable. The spreadsheet has columns for amount, price, total, row feet, value per row foot and value per acre. To check my inventory control, I account for all discrepancies between my actual gross receipts and the dollar value of everything that comes out of the field. This includes the value of what I give to the food bank, the value of what we eat ourselves, weight of the spoilage that goes into the compost, etc. Using these categories, I can account for all discrepancies between what I sell and what I produced, within 3.0%. In other words, I am controlling my error to within 3%, well within the 5% parameter I use for random chance.
Since I am confident in my numbers, I next researched the calories in each food item on the web. The site I used is www.caloriecount.about.com and I chose it because it seemed most comprehensive. It is also conservative, with the calories for winter squashes, for example, calculated at 153 per pound overall versus 176 on another site. (Squashes vary, with Hubbard being more calorie dense than Butternut, for example.)
After doing the research and data entry, I calculated my calorie output for each item by multiplying my yield by the calorie values and totaled the results. My calorie output for 2008, using these methods, came to 2,235,639 calories. The actual amount of land that produced this amount of calories was 1.01 acres, for an index of 2,213,504 calories per acre. I calculated my energy input in 2008 as 365,000 calories for my labor (365 days X 1000 calories per day - this is probably too high) and 775,000 calories for tiller gas (25 gallons x 31,000 calories per gallon). This means that I produced 2.21 million calories with 1.14 million calories of sweat and petrol - this comes to 1.94 calories produced for every calorie of input. The number is likely higher since I tilled and planted 2 acres last year. The extra acre was for cover crops and experiments (like my wheat). I also did not actually put 1000 calories into the farm every day last year, but I am erring on the side of caution. The upshot is that I produced enough calories to feed 2.42 people (at 2500 calories a day for 365 days).
The bottom line is that I can measure my calorie inputs and outputs and I can state with certainty that sustainable agriculture WILL produce enough food to feed AT LEAST 2.5 people per acre, using hand labor and tillers. Since I am not as productive as I want and I can actually till, plant and harvest 2 acres by myself, I can say with confidence that 1 person can feed AT LEAST 5 people using sustainable methods. For those of you who might quibble about 25 gallons of gas to work 2 acres, consider how much gas you use each week in your cars. Since we are in a Transition period, rather than a No-Gas period, minimal amounts of fossil fuels are justified. I also anticipate my tiller gas usage will continue to drop year by year as I become more efficient. The point is that I can give you production numbers, using a metric (calories) which can be used for humans, petroleum, and even horses.