Solar Installation Calculator

Size the DC array from annual consumption, target offset, peak sun hours, and system efficiency. The core formula is System Size (kW) = Annual Consumption × Offset% ÷ (Peak Sun Hours × 365 × Efficiency). Panel count = ⌈System kW × 1000 ÷ Panel Wattage⌉. Typical system efficiency is 78%, accounting for inverter, wiring, temperature, soiling, and mismatch losses.

Estimate annual generation with Annual Generation = System kW × Peak Sun Hours × 365 × Efficiency. Size roof area using about 17.5 sq ft per standard 60-cell panel, or roughly 50 sq ft per kW including walkways and row spacing. Adjust PSH by location: US average is 4.5 hr/day, Southwest USA 6.0–6.5 hr/day.

Check DC/AC ratio (ILR) against industry benchmarks: most US systems run 1.13–1.30, residential target 1.15–1.25, and utility-scale can reach up to 1.34. Verify string voltage against NEC 690.7 limits — 600 V residential, 1,000 V commercial — and confirm the string voltage window stays inside the inverter MPPT range at both cold-temperature Voc and hot-temperature Vmp.

Estimate total installed cost from $/W benchmarks: residential $1.80–$3.50/W and commercial $0.80–$2.00/W in 2025–2026. Apply the 30% federal ITC (Section 48E for commercial, Section 25D residential through 2025), plus state credits and rebates. Net Cost = System Cost × (1 − ITC%) − State Credits − Rebates.

Calculate simple payback with Payback = Net Cost ÷ Year 1 Annual Savings, where Year 1 Savings = Annual kWh × Utility Rate. Model 25-year cash flow with 0.5%/year panel degradation and 2–4%/year utility rate escalation. Review NPV and IRR for investor-grade analysis, and compare incentive scenarios side by side.

Generate branded PDF proposals showing consumption inputs, system size, panel count, roof area, annual generation, inverter ratio, string configuration, gross cost, incentives, net cost, payback, and 25-year savings. Export to CSV for financial models. All calculations run on-device and work fully offline.