Solar Panel Calculator

Divide your daily use (monthly kWh ÷ 30) by the peak sun hours times a loss factor (~80%, for the inverter, wiring and heat). For 900 kWh/month and 4.5 sun hours: 30 ÷ (4.5 × 0.8) ≈ 8.3 kW. With 400 W panels that is about 21 panels, needing roughly 375 sq ft of roof.

Peak sun hours measure how much usable sunlight your area gets per day, standardized against a reference intensity of 1,000 W/m² — it is not simply "hours of daylight." US averages run roughly 3 in the cloudy Pacific Northwest to 6 or more in the desert Southwest, a two-fold spread that changes system size directly: less sun means proportionally more panels are needed to produce the same energy. Local solar maps and installer sizing tools use location-specific averages rather than a single national figure, which is worth pulling before treating this calculator's default as your actual local number.

That derate is not one loss but several, stacked together. The inverter, which converts the panels' DC output into the AC power a home actually uses, is not perfectly efficient. Wiring resistance loses a little more between the panels and the panel board. Temperature cuts into output too — panels are rated at a standard test temperature, and real panels lose several tenths of a percent of output for every degree above that as they heat up in direct sun, which is precisely when they're generating the most. Dust, pollen and partial shading — even a chimney shadow crossing part of an array for part of the day — trim output further. None of these is large individually, but stacked together they commonly land in the 15–20% range this calculator's factor accounts for.

This gives the ballpark size. A real design factors in the exact irradiance for your specific coordinates, your roof's orientation and tilt (a south-facing roof in the Northern Hemisphere outperforms one facing east or west), shading from trees and neighboring structures at different times of day and year, the specific inverter model, and structural questions like roof condition and age. Get a site-specific assessment and quote from a licensed installer before buying — this calculator answers "roughly how big," not "exactly what to install."

Sizing a system to match your usage does not automatically mean your bill drops to zero, and how much it drops depends heavily on net metering — the policy that credits power your system sends back to the grid when it generates more than the home is using at that moment. Rules vary enormously by state and utility: some credit exported power at close to the full retail rate, others at a lower wholesale-like rate, and many bills keep a fixed grid-connection charge regardless of how much solar you generate. This calculator sizes the system from your energy use; what that system is worth financially, and on what terms, is a separate question that depends entirely on your specific utility's current policy.