When I was shopping panels, every salesperson mentioned efficiency within the first two minutes. “Our panels are 22.8% efficient.” “This line is 20.9%.” “Tier-1 efficiency.” The numbers were everywhere. Nobody explained what they meant.
Efficiency sounds like a quality metric — higher is better. That’s partially true. But the way efficiency actually affects your system depends on your roof, and for many homeowners, a 2-point efficiency difference changes almost nothing about the financial outcome.
Here’s what efficiency actually means and when it matters.
What the Percentage Actually Measures
Solar panel efficiency is the percentage of sunlight energy hitting the panel’s surface that gets converted into electricity. A panel rated at 20% efficiency converts 20% of the solar energy it receives into usable power. The remaining 80% is lost as heat and reflection.
This is measured under Standard Test Conditions (STC): 1,000 watts per square meter of irradiance, 25°C cell temperature, a specific air mass coefficient. Real-world conditions differ from STC — it’s hotter in summer, panels get dusty, irradiance varies with angle and cloud cover. But STC gives a standardized basis for comparison.
What Efficiency Determines in Practice
Efficiency determines how much power you get per square foot of panel. A 400W panel at 22% efficiency is physically smaller than a 400W panel at 19% efficiency. Both produce 400W — the higher-efficiency panel just does it in less space.
That’s the entire practical implication of efficiency: space. Higher efficiency = more watts per square foot.
If your roof has ample usable space, efficiency doesn’t change your outcome. You can fit more lower-efficiency panels and reach the same system size. A 10kW system built from 19%-efficient panels costs less than the same 10kW system built from 22%-efficient panels — and produces the same annual electricity.
If your roof has limited usable space — small south-facing section, complex roofline, shading that limits where panels can go — higher efficiency lets you fit more watts in less area. This is where efficiency earns its premium.
The Temperature Coefficient: The Number Nobody Talks About
Every panel spec sheet includes a temperature coefficient, expressed as %/°C. This is how much the panel’s output decreases for every degree Celsius above 25°C (the STC test temperature).
A typical silicon panel has a temperature coefficient of -0.35 to -0.40%/°C. On a 95°F (35°C) summer day, the panels reach 50–60°C. At -0.37%/°C and 55°C operating temperature (30°C above STC):
Output loss = 30 × 0.37% = 11.1% reduction in peak output
A premium panel (SunPower Maxeon, REC Alpha) has a temperature coefficient of -0.24 to -0.27%/°C. Same 55°C day:
Output loss = 30 × 0.25% = 7.5% reduction
That 3.6-percentage-point difference in hot-weather output is compounding across every hot summer day. For hot-climate homeowners in Texas, Arizona, and Florida, the temperature coefficient matters more than the headline efficiency rating — because summer is when you’re running AC and need production most.
Efficiency vs. Wattage: What You’re Actually Buying
Two panels on the market today:
- Panel A: 400W, 19.8% efficiency, -0.36%/°C temperature coefficient, $280
- Panel B: 400W, 22.6% efficiency, -0.25%/°C temperature coefficient, $370
Both panels are rated at 400W under STC. Panel B is physically smaller and will outperform Panel A on hot days. Panel A produces the same annual electricity on an unlimited roof at lower cost.
The $90 per panel difference across 22 panels is $1,980 in system cost. For a homeowner with plenty of roof space in a moderate climate, that $1,980 buys real money and meaningfully extends payback time. For a homeowner with a tight roof in Phoenix, Panel B’s smaller footprint and lower temperature coefficient might be worth it.
Efficiency is a tool for matching a panel to a situation. It’s not a universal quality ranking.
— Allen
