Solar Savings Calculator

Photovoltaic output is estimated from nameplate capacity, local solar resource and a derate factor. A system rated in kilowatts of direct current produces, over a year, approximately its capacity multiplied by peak sun hours, by 365, and by an efficiency factor near 0.75 to 0.85. Peak sun hours are not daylight hours but the equivalent number of hours at 1,000 watts per square meter, the standard test irradiance, and range from about 3 to 4 in the Pacific Northwest to 6 to 7 in Arizona and New Mexico. The derate absorbs inverter conversion loss, wiring and connection resistance, soiling, module mismatch and the temperature coefficient, since silicon output falls roughly 0.3 to 0.4 percent for each degree C above the 25 degrees C rating condition. Financial results hinge less on the physics than on tariff design. Savings equal displaced kilowatt-hours multiplied by the rate they displace, so the value of exported energy depends entirely on whether the utility offers full retail net metering, net billing at an avoided-cost rate of perhaps 3 to 6 cents per kWh, or time-of-use compensation that pays little at midday. California move to its NEM 3.0 export structure in April 2023 cut export credits sharply and lengthened payback for systems without battery storage. On the hardware side, modules degrade about 0.3 to 0.5 percent annually under 25-year performance warranties, while string inverters generally need replacement at 10 to 15 years, a 1,000 to 2,500 dollar expense that simple payback calculations often omit.

The practical silicon solar cell was announced by Bell Telephone Laboratories on 25 April 1954, the work of Daryl Chapin, Calvin Fuller and Gerald Pearson, who reached about 6 percent conversion efficiency, a large jump over the selenium cells that preceded it. Cost confined the technology to spacecraft for two decades, beginning with the Vanguard 1 satellite in 1958. The oil shocks of the 1970s prompted the first serious terrestrial programs, including Department of Energy block purchases that drove module prices down through volume, and the Energy Tax Act of 1978 introduced the first federal residential solar tax credit. Modern residential economics rest on two policy inventions. Net metering was pioneered at state level, with Minnesota enacting the first statewide law in 1983 and California following in 1995, establishing the principle that a customer meter could effectively run backward at retail value. The federal Investment Tax Credit for solar was created by the Energy Policy Act of 2005 at 30 percent, extended repeatedly through the 2010s, and restored to 30 percent through 2032 by the Inflation Reduction Act of August 2022, with scheduled step-downs to 26 percent in 2033 and 22 percent in 2034. The National Renewable Energy Laboratory PVWatts model, released publicly in the late 1990s, standardized the production estimating method that underlies nearly every consumer solar quote written today.