Solar Panel Wattage Calculator

Solar panel sizing starts with energy demand. The average US household uses about 30 kWh per day (approximately 10,950 kWh per year), though this varies widely by region, home size, and lifestyle. Your utility bill provides exact monthly usage; divide by 30 for a daily average. Peak sun hours represent the solar resource at your location. One peak sun hour equals 1 kWh of solar energy per square meter. Phoenix, Arizona averages about 6.5 peak sun hours, while Seattle, Washington averages about 3.5. This factor alone creates nearly a 2x difference in system sizing between locations. Panel wattage has increased steadily. In 2024, residential panels typically range from 350-450 watts. Higher-wattage panels (400W+) reduce the number of panels needed, saving roof space. Panel efficiency (the fraction of sunlight converted to electricity) ranges from 19-23% for mainstream panels. System efficiency accounts for all losses between the panels and your electrical panel. Inverter losses (converting DC to AC) are typically 3-5%. Wiring and connection losses add 2-3%. Soiling (dust, dirt) causes 1-5% loss. Temperature effects reduce output by 0.3-0.5% per degree C above 25C. Shading, if present, can cause 5-25% loss. A typical total system efficiency is 75-85%. The formula works backward from demand: required system kW = daily kWh / (peak sun hours * system efficiency). For a 30 kWh/day home in a 5 peak-sun-hour location with 80% efficiency: 30 / (5 * 0.8) = 7.5 kW system. With 400W panels, that requires 7500/400 = 19 panels. Roof space requirements depend on panel size. A typical residential panel is about 1.7 m^2 (18.5 sq ft). Twenty panels need about 34 m^2 (370 sq ft) of unshaded, south-facing roof area. Panels should face south (in the northern hemisphere) and tilt at approximately the latitude angle for maximum annual production. Solar economics have improved dramatically. The levelized cost of solar electricity is now $0.03-0.07/kWh for utility-scale and $0.06-0.12/kWh for residential, competitive with or cheaper than fossil fuel generation in most locations. The US federal tax credit (30% ITC through 2032) further improves residential economics.