Right, for more-or-less obvious reasons (my alias translates to "Moocow") this thread is both amusing and highly interesting to me.
I'm curious about their numbers: 50 cows generate 12 kW-hr - does that mean they provide a continous power output of 12 kW? If so, we're looking at 5,76 kWh/day per cow, which seems like a lot. Let's crunch some numbers:
According to what was linked earlier, a single cow produces 64lb, or 29kg of manure per day. According to Wikipedia, 1 ton of cow dung is good for 45 m³ of Biogas, which can be treated and refined into something 99% close to natural gas. The efficency of that process of that I don't know, let's ignore it for now. So that's 29kg/1000kg * 45m³ = 1.305m³ of biogas per cow, per day. Natural gas contains about 10 kWh worth of heat energy per 1 m³, and gas turbine power plants can have efficencies of around 39%. Let us assume 35%, as a small generator near a cow farm might not be the most efficient in the world.
Which means: 1.305 m³ /day * 10 kWh / m³ * 35% = 4.5675 kWh /day. Per Cow, so you would need 18.6 cows (let's make that 19 cows) to recharge your Model S once per day. Now, we have ignored some losses.
How big are they?
According to this (German) site:
biogas.fnr.de: Faustzahlen the amount of electrical energy you can expect from one dairy cow is 1095 kWh/year, or about 3 kWh/day. Which means the above calculation isn't that far off and the missing efficiency number is around 66% (Slightly less, since they assume 38% efficiency vs. my 35%). That brings the amount of dairy cows needed to power a Model S to 28,3, but that's to charge it from empty-to-full, which is not realistic for most people.
It also means that the amount of manure required to fully charge the Model S is 820,7 kg, or 1809.33 pounds, not 22k as stated above.
All that said, the overall efficency of a Sunlight - grass - cow - dung - electricity is still far less than Sunlight - PV - electricity. But PVs don't give milk (yet)
Edit: As to their 12 kW number per cow, that might include heat energy and then that's a pretty reasonable number. If it's just for electricity, they might have discovered some secret in the secretion.