It is becoming more widely known that switchgrass [ 1] is one of the best possible sources for biofuel in the coming years. Primarily because of its incredibly high biomass yield per acre, with the advantages of being a low maintenance perennial crop, tolerance to pests and many climates, and other advantages. [ 2]
I was curious about how efficient ethanol really was, compared to some alternatives, which led me to research this and derive some figures. If you want to skip the math, my conclusions are at the bottom.
A single acre of switchgrass under research conditions has been shown to produce an average dry biomass yield of 11.5 tons per acre over 6 years, with a record high of 15 tons per acre. With ethanol production as high as 100 gallons per ton using the still-in-development cellulosic conversion process, a good switchgrass crop could produce as many as 1,500 gallons of ethanol fuel per year. [ 3] Using standard sugar alcohol conversion, sugarcane can produce about 665 gallons per acre, while corn comes in around 400. [ 4] See here for more corn vs. solar numbers.
The actual commercial yield of per acre of switchgrass is probably closer to 6-8 tons per acre [ 2], but I will give it the benefit of the doubt, and assume for the purposes of this article that it can average 10 tons per acre when proper cultivars are chosen for the growing conditions. If you wish to be more optimistic, you can multiply any figures I get for switchgrass by 1.5, to get the 15 tons per acre.
Read those numbers, and much more, after the jump
Like any plant, switchgrass requires primarily carbon dioxide, water, and sunlight for its growth. When converted to a liquid fuel then burned, the hydrogen and carbon collected by the plant over its life is re-released into the atmosphere as carbon dioxide and water, resulting in an overall 'carbon neutral' process. Plants use the energy of the sun to create what we use as fuel (albeit indirectly as cellulose), so biofuel is essentially captured solar power.
(Note: coal, crude oil, natural gas, wind, and hydroelectric power are also captured solar power, but discussion of that is for another article.)
- 1 acre of switchgrass can produce approximately 1,000 gallons of ethanol per year.
- Ethanol has an energy density of 24 MJ/l, or 90.850 MJ/gallon. 
- 1 watt-hour = 3600 joules (joule is the SI unit of energy).
- 1 acre of land is approximately 4046 square meters.
- At noon at the equator on a sunny day, the earth recieves approximately 1000 watt-hours per square meter, and at 40° latitude, that figure changes to about 600. Across the southern half of the USA, the average annual insolation (solar power absorption)  is 200 W·h/m², taking into effect the seasons, cloud cover, and night. 
- Inexpensive multicrystalline solar cells from 10 years ago are about 18 percent efficient, new three-junction concentrators are about 36 percent efficient. 
The average energy a square meter of earth receives across the southern USA: 200 W·h/m² * 3600 = 720,000 J, or 720 kJ/m² 720 kJ/m² * 4046 = 2,913,120 kJ/acre, or 2,913.120 MJ/acre
Since there are 365 * 24 = 8,760 hours in a year, and that is the average insolation per hour, the total insolation for a year would be: 8760 * 2913.120 MJ/acre = 25,518,931.2 MJ/acre, or 25,518.931 GJ/acre
Solar panels at 18 percent efficiency will give us 18 percent of that total in direct electricity: 25,518.931 GJ/acre * 0.18 = 4,593.408 GJ/acre
One acre of switchgrass cellulosic ethanol = 90.850 GJ
One acre of 18 percent efficient solar panels = 4,593.408 GJ
That is over 50 times more energy from the inexpensive solar panels than from the plants. When the price of multijunction solar panels falls, bringing 36 percent efficiency to the masses, it will be over 100 times more energy than we could ever get from ethanol.
Dividing the energy output we get from ethanol by the total solar energy that area receives, then multiplying it by a very generous efficiency of 35 percent for an ethanol-fueled vehicle, we get a sun-to-wheels efficiency of 0.125 percent for ethanol vehicles.
Doing the same for an electric vehicle, with 18 percent efficient solar panels, 80 percent efficient electricity transmission, and 80 percent efficient electric motors, we get a sun-to-wheels efficiency of 11.5 percent.
If you put 36 percent efficient solar cells on your roof, and drove a Tesla Roadster which is 90 percent efficient on average, and has an 86 percent charging efficiency, the total sun-to-wheels efficiency is 27.9 percent.
- Sun -> switchgrass -> ethanol -> ethanol vehicle's wheels: 0.125 percent efficient.
- Sun -> cheap solar -> sloppy homebuilt electric vehicle's wheels: 11.5 percent efficient.
- Sun -> high-efficiency solar -> Tesla Roadster's wheels: 27.9 percent efficient.
- Why are we trying to grow our energy when we can harvest it directly from the sun.