Solar Panel Calculator
Calculates residential rooftop solar photovoltaic (PV) array size in kilowatts (kW-DC), number of high-efficiency 400W monocrystalline panels, required unshaded roof square footage, annual kWh production based on peak sun hours, and 30% Section 25D federal tax credits.
Formula
Annual kWh = (Bill ÷ $0.16/kWh) × 12; System kW = Annual kWh ÷ (Sun Hours × 365 × 0.86); Panels = Ceil((System kW × 1000) ÷ Watts)
1. Estimated Monthly kWh = Monthly Bill $ ÷ $0.16/kWh (US average rate) 2. Annual Target kWh = Monthly kWh × 12 3. Daily kWh Production Needed = Annual Target kWh ÷ 365 4. Required DC Array Size (kW) = Daily kWh ÷ (Peak Sun Hours × 0.86 system derate factor) 5. Number of Solar Panels = Ceil((System kW × 1,000) ÷ Panel Wattage) 6. Required Roof Area Sq Ft = Panel Count × 21.5 sq ft per panel 7. Gross Cost = System kW × 1,000 Watts × $2.85/Watt installed 8. 30% Federal Tax Credit (Section 25D) = Gross Cost × 0.30
Inputs
- Average Monthly Electric Bill ($)
- Daily Peak Sun Hours
- Solar Panel Power Rating
Worked example
Designing a grid-tied solar system to offset 100% of an electric bill in a 4.5 sun-hour zone.
Outcome: Household consumes ~12,000 kWh/year. Sized to an 8.5 kW-DC array requiring 22 panels (400W each), occupying ~473 sq ft of south-facing roof. Gross turnkey cost: ~$24,225. After the 30% federal tax credit ($7,267), net cost is $16,958.
What is the difference between string inverters and microinverters?
A central string inverter connects all solar panels in series, meaning shade on one single panel reduces the entire string's output. Microinverters (like Enphase) attach behind each individual panel, allowing every panel to produce maximum independent power.