Solar + Battery Home Calculator

This calculator sizes a home solar-plus-battery system in two independent steps, then reconciles them against your roof. First it figures out your target daily energy production: your monthly usage is annualized and divided back into a daily average, then scaled by your grid-independence percentage — targeting 80% means the system only needs to supply 80% of a typical day's electricity, not 100%. From there it works out how many standard 400W residential panels (18 sq ft each) are needed to hit that target, applying a 91% system-efficiency factor that accounts for real-world losses from the inverter, wiring, panel soiling, and gradual degradation — panels never deliver their rated output day to day. That panel count is then capped by how many will physically fit, assuming only 60% of your stated roof area is usable once you account for vents, shading, and setbacks.

Battery capacity is sized separately to cover roughly 60% of your daily target — the portion typically used in the evening and overnight when panels aren't producing. Costs use rough installed-price benchmarks (~$2.80/watt for solar, ~$800/kWh for battery), and the payback calculation nets out the 30% federal solar tax credit before dividing by annual savings. Because the roof constraint can silently cap your panel count below what's needed for your independence target, check that the reported system size actually matches your stated goal — if your roof is small, you may not reach 80% no matter what number you type in.