While a wind turbine – a big one like those at the Mars Hill Wind Farm – will produce more than 1,000 kilowatt-hours per year for each square meter of wind disk, a photovoltaic array, for about the same output, will need to be 10 times this area. In Spain, such an array can generate 1,300 kwh per year; in northern Germany, one could generate about 700. Most of the grid-connected solar electricity in the U.S. is generated from solar-thermal systems. A parabolic trough is used to focus solar energy on pipe circulating oil. This oil, in turn, makes steam for a steam turbine. (One year’s data: Mars Hill produced 150 million kwh, or 1,390 kwh per square meter of wind disk – better than most.)