Solar Panel System Calculator

Sizing a photovoltaic array starts from an energy balance rather than a power balance: the array must generate as many kilowatt-hours per day as the building consumes. Peak sun hours are the bridge between the two. One peak sun hour means one hour of irradiance at 1000 watts per square meter, the Standard Test Condition at which panels are rated, along with a 25 degrees C cell temperature and an AM1.5 spectrum. A site receiving 5.4 kWh per square meter per day therefore has 5.4 peak sun hours, and a 400 W panel there nominally yields 2.16 kWh. Dividing daily consumption by peak sun hours and by one minus the loss fraction gives the required DC array size in kilowatts. The loss term carries most of the uncertainty. A typical derate of 15 to 25 percent bundles inverter conversion losses of 3 to 5 percent, DC and AC wiring drops of 1 to 3 percent, soiling of 2 to 5 percent, module mismatch, and temperature derating that runs about 0.3 to 0.4 percent per degree C above 25 for crystalline silicon, so a cell at 55 degrees C loses roughly 10 percent. Panels also degrade about 0.5 percent per year, which is why 25-year warranties typically guarantee only 80 to 85 percent of nameplate output at year 25. The panel count this method produces is an annual-average figure: it does not guarantee winter self-sufficiency, and it says nothing about roof orientation, shading, or whether the utility will credit surplus generation.

The photovoltaic effect was observed by Edmond Becquerel in 1839, who found that light striking electrodes in an electrolyte produced a current. Charles Fritts built the first solid-state solar cell around 1883 using selenium coated with gold leaf, but its efficiency was well under one percent. The practical era began at Bell Telephone Laboratories in 1954, when Daryl Chapin, Calvin Fuller, and Gerald Pearson demonstrated a diffused-junction silicon cell of about 6 percent efficiency. Cost initially confined it to space: the Vanguard 1 satellite launched in 1958 carried silicon cells and ran on them for years. Terrestrial sizing methods developed after the 1973 oil embargo, when the US federal solar program that became the Solar Energy Research Institute in 1977, renamed the National Renewable Energy Laboratory in 1991, began publishing insolation data for locations across the country. The peak sun hour convention and the single derate-factor approach used here were formalized in NREL's PVWatts model, first released publicly in the late 1990s, which packaged typical meteorological year weather data with one overall system loss term. Module prices fell by roughly two orders of magnitude between 1980 and the 2020s, which shifted the practical sizing question away from how few panels can be afforded toward how many the roof area and the utility interconnection rules will permit.