Solar panel efficiency, explained
Efficiency is the percentage of sunlight a panel converts to electricity — typically 19–23% for residential panels in 2026. It only matters when your roof space is limited. If you have room, a cheaper lower-efficiency panel produces the same power for less money.
What efficiency does and does not tell you
A 22% efficient panel produces more watts per square metre than an 19% one. That is the whole claim. It says nothing about durability, degradation rate, warranty or how it performs in heat — and it does not mean a kWh from it is worth more.
Worked comparison
| Standard panel | Premium panel | |
|---|---|---|
| Efficiency | 19.5% | 22.5% |
| Output per panel | 400 W | 450 W |
| Panels for 8 kW | 20 | 18 |
| Roof area used | ~39 m² | ~35 m² |
| Typical price premium | — | +15–30% |
Both make 8 kW. The premium panel saves about 4 m² of roof. If you have 4 m² spare, that saving is worth nothing — and you paid 15–30% more for it.
The spec that matters more: temperature coefficient
Panels are rated at 25°C and lose roughly 0.3–0.4% of output per degree above that. A roof in Phoenix easily hits 65°C, which is a 12–16% real-world loss. A panel with a better temperature coefficient (closer to −0.29%/°C than −0.40%/°C) can outproduce a nominally more efficient one in a hot climate. Almost nobody markets this, and it is the more useful number in the sunbelt.
The three panel types, and where efficiency actually differs
| Type | Typical efficiency | Where it makes sense |
|---|---|---|
| Monocrystalline | 20–23% | The default for residential roofs in 2026. Best output per square metre, so it wins wherever space is limited. |
| Polycrystalline | 15–17% | Cheaper per panel, largely displaced on homes. Fine where roof space is genuinely plentiful. |
| Thin film | 10–13% | Light and flexible, so it suits structures that cannot take the weight. Needs far more area for the same output. |
For most homes the choice is which monocrystalline panel, not which technology. And at that point the efficiency difference between a good panel and a very good one is a percentage point or two, which matters far less than where you put them.
Efficiency falls, slowly and predictably
Panels degrade at roughly 0.5% a year. After 25 years a panel rated at 22% is producing around 88% of what it did new, which is why performance warranties are typically written as a floor at year 25 rather than a promise of no decline at all.
This is worth knowing when comparing quotes. A panel with a slightly lower starting efficiency but a better degradation rate can out-produce a higher-rated one over the system's life. The starting number is the one that gets advertised; the degradation rate is the one that decides what you actually get.
What actually drives your output
- Shading — dwarfs everything else. A tree can cost a third of your output.
- Orientation and pitch — 10–25% swing.
- Local sunlight hours — the difference between Tucson and Seattle is roughly 60%.
- System losses — inverter, wiring, heat, dust: ~15%.
- Panel efficiency — a few percent, and only if space is tight.
It is last on the list, and it is the thing salespeople talk about most.
What this looks like in real cities
Measured rooftop data, spanning the range from fastest to slowest payback we have found:
| City | Sun hrs/yr | Rate | Payback |
|---|---|---|---|
| Kahului, HI | 1,627 | 42.1¢ | 3.5 yrs |
| Daly City, CA | 1,783 | 31.2¢ | 8.8 yrs |
| Enid, OK | 1,825 | 12.1¢ | 10.9 yrs |
| Northampton, East Midlands | 1,004 | 26.1¢ | 12.9 yrs |
| Trois-Rivières, QC | 1,347 | 7.9¢ | 31.9 yrs |
Frequently asked
What is a good solar panel efficiency in 2026?
Most residential panels now sit between 20% and 23%. Efficiency matters mainly when roof space is tight, because a higher-efficiency panel fits more output into the same area. If you have ample roof, cheaper panels at slightly lower efficiency often produce more total power per dollar spent.
Do more efficient panels produce more electricity?
Per square metre, yes. Per panel, not necessarily — a 400W panel produces 400W at rated conditions whatever its efficiency figure. Efficiency describes how much of the sunlight hitting the panel becomes electricity, so it determines how much power you can fit onto a limited roof area.
Why do panels underperform their rated wattage?
Ratings are measured under standard test conditions that real roofs rarely match. Heat, dust, wiring losses, inverter conversion and imperfect angles each take a share. A derate of roughly 15% from rated output to delivered energy is normal, and every estimate on this site already includes it.
Related guides
General information, updated 2026-08-22. Not financial, tax or engineering advice.