Solar

Utility-scale PV

Customer-sited PV

PV cost factors

Solar resources

Grid-connected PV

PV technologies

PV information

Unlike wind turbines, which typically require a 12- to 15-mile-per-hour wind before they even start turning, PVs will generate some electricity even on a heavily overcast day in winter. The more light, however, the more electricity. So, the question of whether PVs will "work" in a specific geographical location is one of economics and cost-effectiveness, not technical feasibility.

The amount of sunlight hitting PV modules varies by latitude, season, ambient pollution and humidity (cloud cover), altitude and the angle of modules installed. The farther one moves from the equator, the farther the solar radiation must travel to reach the earth's surface, which decreases its intensity. Solar intensity is, of course, strongest in the summer, and weakest in the winter and reaches an average annual intensity in the Spring and Fall. Some atmospheric conditions such as moisture and pollution can decrease solar intensity by scattering, absorbing or reflecting incoming solar radiation. Consequently, solar intensity is greater in arid climates and at high altitudes.

For the amount of peak sun hours for a variety of locations throughout the United States, see the National Solar Radiation Data Base Solar Database which contains 30 years (1961-1990) of solar radiation and supplementary meteorological data from 237 NWS sites in the U.S., plus sites in Guam and Puerto Rico.

This chart gives a range of the annual energy, in kilowatthours, produced by a 1 kW system installed in various California locations.

Source: A guide to Photovoltaic System Design and Installation

The angle and facing direction of the installed system greatly impacts the amount of available solar radiation that comes in contact with the PV modules. South-facing roof surfaces will receive the greatest amount of daily solar insolation, but PV can be installed on flat roofs or rooftops at other orientations, if less than optimal. For information on how installation factors affect the system energy output, see the California Energy Commission's report A Guide to Photovoltaic System Design and Installation.

NREL's Renewable Resource Data Center provides maps of the U.S. solar resource availability under a number of variables, including month, tilt angle, tracking systems (East-West and North South) and technology (flat plat or concentrator).

PVWATTS calculates electrical energy produced by a grid-connected photovoltaic system. Currently, PVWATTS can be used for locations within the United States and its territories.

The Renewable Energy Atlas of the West contains high-resolution maps of the wind, solar, geothermal and biomass resources in 11 western states as well as transmission, policy, land-use, and generation facility map overlays.

One of the drawbacks of consulting resource maps is that they give the impression that solar resources are really only important – or economical – in the southwestern United States This perception has caused many utilities in other areas to discount the benefits of PV technology. But straight solar intensity values are not the only means to evaluate a PV system. In many areas, a solar system's peak output periods match a utility's peak demand periods, by season and by time of day.