What is a good efficiency rating for solar panels?

Mainstream solar panels have efficiencies of 19–22%. Premium monocrystalline panels (like SunPower or REC) reach 22–24%. Standard residential panels are typically 19–21% efficient. Higher efficiency matters most when roof space is limited — a 22% efficient panel produces the same power as a 19% panel in about 14% less space.

How does panel efficiency affect energy production?

Panel efficiency determines how much electricity is generated per square foot of panel. A 22% efficient 400W panel produces the same power as a 19% efficient 400W panel — the difference is physical size. Higher efficiency becomes critical when roof area is constrained. For most homes with ample roof space, mid-efficiency panels offer the best value per watt.

What is the most efficient solar panel available?

As of 2025, the most efficient commercially available residential solar panels are from SunPower (Maxeon 7) at 22.8% efficiency, followed by REC, Panasonic, and LG. Standard budget panels average 19–21% efficiency. Higher efficiency means more power per square foot, which matters when roof space is limited — otherwise panel count simply adjusts to achieve the target system size.

Does solar panel efficiency degrade over time?

Yes. All solar panels slowly lose efficiency through a process called degradation. Premium panels degrade 0.3–0.5% per year; budget panels may degrade 0.7–1.0% per year. After 25 years at 0.5% annual degradation, a panel rated at 400W still produces about 352W — 88% of original rated output. Manufacturer power warranties guarantee a minimum output level at year 25.

Solar Panel Efficiency Calculator

Calculate a solar panel's efficiency rating as a percentage, its power density per square foot, and how much energy a single panel produces daily, monthly, and annually under real-world conditions.

Panel Wattage (W)

Panel Area (sq ft)

Irradiance (kWh/m²/day)

About This Calculator

Solar panel efficiency measures how effectively a panel converts sunlight into electricity, expressed as a percentage of incident solar energy that becomes usable power. A 400W panel measuring 21.5 sq ft (about 2 m²) has an efficiency of roughly 19–22% depending on exact dimensions. This calculator derives that efficiency from panel wattage and physical area, then applies real-world irradiance and system loss factors to show actual energy output per panel — the number that matters most for production planning and system sizing.

Modern residential solar panels range from about 17% efficiency (budget polycrystalline) to 23%+ (premium monocrystalline PERC or heterojunction technology). Higher efficiency panels produce more power per square foot, which is valuable when roof space is limited — but they cost more per watt. The efficiency percentage also determines how many panels fit on a given roof area. System losses (typically 15–25%) account for inverter conversion losses, wiring resistance, temperature derating, and soiling — these reduce real-world output below the nameplate watt rating.

To find accurate panel area, look up the panel's datasheet — every manufacturer publishes dimensions in millimeters or inches. Convert to square feet: multiply length (inches) × width (inches) and divide by 144. Irradiance in kWh/m²/day for your location is available from NREL's PVWatts tool or the Global Solar Atlas. Use the average annual value rather than peak summer values for most accurate annual output estimates. System losses of 20% are a reasonable default for a well-installed rooftop system without significant shading.

Panel efficiency matters most when evaluating competing quotes with different panel brands. A salesperson quoting a system using 350W panels at 18% efficiency will require more panels and more roof space than a system using 420W panels at 21% efficiency to achieve the same output. Understanding efficiency lets you compare apples to apples across different equipment proposals and make a more informed purchasing decision.

Calculations based on NREL solar modeling data and industry-standard assumptions, built and maintained by the independent SolarToolsOnline research team.

Estimates only — not financial, tax, or legal advice. Verify important results with a licensed solar installer or financial professional before making decisions.