After the 1973 oil embargo, there was a resurgence of interest in solar energy. Faced with a possibility of scarce oil resources, the United States government allocated $400 million per year, from a mere $1 million per year, for solar energy research. People, in turn, use this stored solar energy through fuels such as wood, alcohol, and methane that are extracted from the plant life (biomass). Fossil fuels such as oil and coal are derived from geologically ancient plant life. One hour of full sun provides 1000 watts per hour – representing the solar energy received in one hour on a cloudless summer day on a one-square meter surface directed towards the sun. To put this in some other perspective, the United States Department of Energy indicates the amount of solar energy that hits the surface of the earth every +/- hour is greater than the total amount of energy that the entire human population requires in a year.
Nevertheless, if you were unable to charge the case's power storage and you're out under sunlight anyway, you can always connect the phone to it and directly charge the iPhone on solar energy. The case also has a status indicator for visual information of the charging process. Most consist of a flat-plate absorber to intercept and absorb solar energy; a transparent cover to allow solar energy to pass through; a heat-transport fluid flowing through tubes to remove heat from the absorber; and a heat-insulating backing. Solar thermal systems are also being used for utility-scale power generation plants in California and other states. When the sun is near the horizon and the sky is overcast, the solar energy at ground level can be negligible. It also varies from one point to another on the Earth's surface.
