Information provided by Practical Solar

When the sun is shining, each Practical Solar heliostat is providing 600 watts (0.6 kW) of thermal energy (heat) and 65,000 lux of visible light. This means that a single Practical Solar heliostat provides as much thermal energy as a space heater on a medium setting and as much visible light as forty 100-watt light bulbs concentrated on a one-square-meter surface. The amount of thermal energy a heliostat can produce over a period of a month or a year depends on the sunlight received in that geographical area. A formula for calculating this appears at the bottom of this section.

The human eye is designed for sunlight. A common unit measurement of visible light is the lumen. Lux is a measurement of the intensity of lumens spread out over a one-square-meter (1 m²) surface. The average hallway has illuminance of 80 lux, or 80 lumens per square meter. A brightly lit office can have illuminance up to 400 lux. Amazingly, sunlight produces up to 100,000 lux on a clear day, equivalent to the visible light that would be produced by sixty 100-watt light bulbs, if all of the light were concentrated on a 1 m² surface.

Energy radiated by the sun is also known as insolation, often expressed in kilowatts per square meter (kW/m²). Insolation is the sum of direct insolation, meaning sunlight coming straight from the sun to the surface, and indirect or diffuse insolation, meaning sunlight reflected by other surfaces and molecules in the air to arrive at the surface from all angles. On a cloudy day, one might not see the sun at all, but the region is still fully illuminated by indirect insolation. Over the course of a year, the percentage of direct vs. indirect insolation varies by region, but a good rule of thumb is that about 2/3 is direct insolation and 1/3 is indirect.

The intensity of insolation per square meter depends on the latitude, the weather, the season of the year, and the time of day. These variables account for large differences in available insolation at any given moment in different locations around the world. Despite the differences, insolation on a 1 m² surface perpendicular to the sun anywhere on Earth receives approximately 1000 watts (1 kW) of solar energy on a clear day. This figure of 1000 watts represents "peak" power or maximum power, undiminished by fog, smog or cloud cover. It is the equivalent of the output of an average space heater or hair dryer. Thus a 1 m² window or skylight perpendicular to the bright sun admits about 1kW of energy to the space inside. This would be more obvious if the energy was concentrated, as it is when coming out of a space heater. But the total amount of energy is the same; it has the same impact on the temperature of the space inside.

A heliostat with a 1 m² mirror that is “normal” (meaning perpendicular) to the sun likewise receives 1 kW of energy from the sun. The total of amount of energy successfully redirected to the target is diminished by three factors: (1) the imperfect reflectivity of the mirror; mirrors reflect less than 100% of the sunlight striking the surface - some of the energy is absorbed, and (2) only direct insolation will be reflected by the heliostat to the target; indirect insolation will be reflected at all angles and "miss" the target, and (3) the "cosine error", meaning the degree to which the heliostat deviates from perpendicular to bisect the angle between the sun and the target.

The amount of insolation collected at the target(s) is also a function of the number of heliostats in the array as well as the size of the mirrors. Thus a 50-heliostat array will produce approximately five times more energy than a 10-heliostat array, assuming all mirrors are the same size.

The United States government has compiled comprehensive statistics on insolation intensity at hundreds of locations. See table. These tables provide an easy and quick reference to determine the amount of insolation per square meter (expressed in kilowatt hours per square meter: kWh/m²) one might expect during an average day, month, and year usable by a CSP system. Most locations in the United States receive enough insolation year-round to generate considerable energy from a CSP system.