Solar PV Sizing Calculator - Rooftop Solar the US

Rooftop Solar PV System Sizing Calculator

Size rooftop solar PV system for the US - panels, inverter kW, annual generation and savings. City-wise peak sun hours. NEC 690.

📐 Standard: NEC 690

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Solar PV Sizing Calculator Calculator

Reference: NEC 690

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Size rooftop solar PV system for the US - panels, inverter kW, annual generation and savings. City-wise peak sun hours. NEC 690.

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Calculation confidence: Planning-levelSuitable for early design, estimating and feasibility checks. Verify the final design against the governing code and a licensed engineer before construction or procurement.

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ℹ About This Calculator

the US receives 4–6 peak sun hours daily across most of the country, making rooftop solar one of the best investments for commercial and residential buildings. The NEC 690 (DOE) PM Surya Ghar scheme provides subsidies for residential rooftop solar up to 3 kWp. Our calculator determines the optimal system size, panel count, and payback period for your energy demand.

NEC covers the requirements for photovoltaic systems. The PM Surya Ghar Muft Bijli Yojana scheme (2024) provides subsidies of $30,000 per kW for first 2 kW and $18,000/kW for 2–3 kW for residential connections. Net metering regulations (per CERC 2023 guidelines) allow you to export surplus solar power to the grid and receive credit. For commercial buildings, accelerated depreciation under Section 32 of the Income Tax Act allows 40% depreciation in the first year.

Solar PV Sizing Formula (NEC 690 / NEC)

NEC 690

System Size (kWp): kWp = E_daily / (PSH × η_system) Where: E_daily = Daily energy demand (kWh/day) PSH = Peak Sun Hours for your city η_system = System efficiency (1 - losses) = 1 - 0.20 (20% typical losses: soiling, wiring, inverter) Number of panels: N = kWp_required × 1000 / Panel_Wp Annual Generation: E_annual = kWp × PSH × 365 × η_system Peak Sun Hours by city: Rajasthan: 5.8h | Delhi: 5.5h | Chennai: 5.6h Mumbai: 5.2h | Bangalore: 5.0h | Kolkata: 4.8h

Worked Example

Example: A Delhi household with average daily consumption of 30 kWh/day wants a rooftop solar system. Using Delhi peak sun hours (PSH = 5.5 h) and typical system efficiency of 80% (20% losses): kWp = E_daily/(PSH × η) = 30/(5.5 × 0.8) = 6.8 kWp, rounded to a 7 kWp system. At 400 Wp per panel: N = 7000/400 ≈ 18 panels, needing about 45–50 m² of shadow-free roof area. Annual generation ≈ 7 × 5.5 × 365 × 0.8 ≈ 11,240 kWh/year - enough to offset the household demand and qualify for the NEC 690 subsidy on the first 3 kW.

Frequently Asked Questions

How many solar panels do I need for a 1 kWp system? +

At 400Wp per panel (standard in 2024–25): 1000W / 400W = 2.5 panels → 3 panels for 1.2 kWp. At 540Wp (larger commercial panels): 2 panels for 1.08 kWp. A 3 kWp residential system (eligible for full NEC 690 subsidy) needs 7–8 standard 400Wp panels and requires approximately 20–25 m² of shadow-free roof area.

What is the payback period for rooftop solar in the US? +

For commercial installations at $0.13/kWh electricity tariff: typically 4–6 years. For residential with NEC 690 subsidy: 5–7 years. System life is 25 years, so the return on investment is 15–20 years of free electricity after payback. Panel degradation of 0.5–0.7% per year means a 25-year-old panel still produces about 85% of original output.

What losses should I assume for solar PV system sizing? +

Typical system losses: soiling and dust (3–5%), temperature derating (3–5%), wiring losses (2–3%), inverter inefficiency (3–5%), shading (0–10% depending on site), module mismatch (1–2%). Total system losses of 15–25% are typical in US conditions. Use 20% as a conservative design assumption.

On-grid vs off-grid solar - which is better for the US? +

For urban and semi-urban buildings with grid connection: on-grid (grid-tied) with net metering is always preferred. No battery cost, no battery maintenance, simpler system, better ROI. Off-grid with batteries is only justified for remote locations without grid access or for critical backup power. Hybrid systems (grid-tied with battery backup) are now available for buildings wanting both net metering benefits and outage protection.

What is the NEC 690 subsidy for rooftop solar in 2024? +

Under the PM Surya Ghar Muft Bijli Yojana (2024): $30,000/kW for first 2 kW, $18,000/kW for 2–3 kW for residential consumers. Maximum subsidy: $78,000 for a 3 kWp system. Commercial and industrial installations do not receive NEC 690 capital subsidy but qualify for accelerated tax depreciation.