Solar planning
Solar Panel Size Calculator
Estimate the solar array size in kilowatts and exact number of solar panels needed to offset your home electricity usage using location-based PVWatts V8 solar yield data.
Size a solar array
How to Calculate How Many Solar Panels You Need
- Enter Annual Electricity Target (kWh): Check your electric utility bill for annual kilowatt-hour usage (typical US homes use 9,000 to 12,000 kWh/yr).
- Select Individual Panel Wattage: Standard modern residential monocrystalline panels range from 380W to 440W (400W default).
- Apply Design Margin: Add a sizing margin (e.g. 10% to 20%) to provide a buffer for future electrification (EVs or heat pumps) or unmodeled localized losses.
- Review Approximate Module Footprint: Check total estimated square footage of module area required.
Solar Panel System Sizing Guide by Household Electricity Usage
Typical solar array sizes and panel counts needed to offset household electricity targets (assuming 400W panels, 1,450 kWh/kW-yr specific solar yield, and a 10% design margin):
| Monthly Electricity Use | Annual Energy Target | Required DC Target | 400W Panel Count | Installed Capacity & Footprint |
|---|---|---|---|---|
| 300 kWh/mo (Small Home / Apartment) | 3,600 kWh/yr | ~2.73 kW | 7 panels | 2.8 kW (~140 sq ft / 13 m²) |
| 600 kWh/mo (Energy-Efficient Home) | 7,200 kWh/yr | ~5.46 kW | 14 panels | 5.6 kW (~280 sq ft / 26 m²) |
| 900 kWh/mo (Illustrative U.S. Household) | 10,800 kWh/yr | ~8.19 kW | 21 panels | 8.4 kW (~420 sq ft / 39 m²) |
| 1,200 kWh/mo (Large Home + Central AC) | 14,400 kWh/yr | ~10.92 kW | 28 panels | 11.2 kW (~560 sq ft / 52 m²) |
| 1,500 kWh/mo+ (Large Home + EV + Heat Pump) | 18,000 kWh/yr | ~13.66 kW | 35 panels | 14.0 kW (~700 sq ft / 65 m²) |
Module footprints represent approximate net panel surface area. Actual roof area requirements depend on row spacing, setbacks, roof pitch, and obstructions.
Calculation Formulas & Mathematical Methodology
Calculates the required photovoltaic DC nameplate capacity, whole-number module count, and resulting installed DC capacity to generate your target annual electrical energy.
Variable Definitions
Annual_kWh_TargetAnnual Energy Target(kWh/yr)- Total electricity energy requirement for the year.
MarginDesign Margin(fraction)- Sizing allowance applied to the annual target (e.g., 0.10 for 10%).
Specific_YieldSpecific Solar Yield(kWh/kW-yr)- Annual AC kWh generated per installed DC kW per year from location or PVWatts model.
Panel_kWIndividual Module Rating(kW)- Rated peak DC power of a single panel in kilowatts (e.g., 0.400 kW for 400W).
Panel_CountModule Count(count)- Whole number of panels calculated using ceiling rounding.
Installed_kWInstalled DC Capacity(kW DC)- Actual total installed array DC capacity after panel rounding.
Calculation Notes
- Installed DC capacity = Panel Count × Panel Wattage ÷ 1,000.
- Approximate module footprint ≈ Panel Count × 20 sq ft (1.85 m²). Actual roof area requirements depend on row spacing, setbacks, roof pitch, and obstructions.
- PVWatts V8 specific yield already models AC inverter conversion, temperature derating, tilt/azimuth geometry, and DC system losses.
Technical References & Model Basis
PowerLab implements a deterministic photovoltaic sizing model based on location-specific solar irradiance modeling and whole-number module discretization. The following technical references provide context for performance modeling and module ratings:
National Renewable Energy Laboratory photovoltaic performance model, simulating grid-connected solar array energy production accounting for solar irradiance, system losses, and inverter conversion.
International Electrotechnical Commission standard for design qualification and type approval of terrestrial crystalline silicon photovoltaic modules under standard test conditions (STC: 1,000 W/m², 25°C cell temperature).