One with lots of small cells. The smaller the cells, the faster the charging time.
The grid: Coal fired plants, nukes, PV, dish Stirling, geo thermal, wind . . . whatever they got that will put out some juice. Polar bears, tropical frogs and hurricanes are not the issue.
Plug in hybrids can go 40 miles between charges. A tractor may require 20X higher hp than a Volt but a tractor only needs to go 1 mile between charges.
If a 400 hp articulated tractor moves 10 mph it's only running 6 minutes between charges and will require 3X 400 hp or ~ 1 MW -- 1/6th the juice drawn by an electric locomotive.
In 4 years diesel will be $15 / gallon and electrification will amount to hundreds of dollars/hour in savings, much more money than necessary to hire someone willing to sit at the end of the field for a few minutes reading a magazine charging up between furrows.
It's only a half mile so a fat high current low voltage wire might be desirable for safety reasons.
Regardless of the size, from the articulated tractor sucking down 22 gallons of diesel/hour [$110/hr and spiraling] to a 15 hp garden tractor, the battery + electric motor system will always have a higher specific power than any IC engine + fuel tank system.
The reason is a tractor just doesn't need to go very far to work a field.
Same as the batteries in plug in hybrids which require an even larger battery / hp.
In as little as 8 years a lot of the big wells will give out rather quickly leaving the world with 2/3rds of current production according to a recent _London Times_ interview with a former BushCo aid. On top of that China seems to have a permanent double digit growth rate while, according to Soros, the U. S. will be in a decades long recession so the size of China's economy will surpass America's in as little as 6 years. This will leave the great majority of Americans to try to get by on a couple percent of the oil we consume today.
So here are 3 plausible scenarios:
1. electrify the fields & tractors, or,
2. return to plowing fields with oxen, or,
If you cannot come up with any ideas, cheap or otherwise, for a plausible 4th scenario then I say we electrify the fields.
"Every idea is the product of a single mind."
-- Bishop Richard Cumberland