Efficiency is the number solar marketing leans on hardest, and it is the one most often misread. A 22% panel is not 22% better than an 18% panel. Efficiency tells you how much power a panel produces per square foot — and unless square feet are what you are short of, it is close to irrelevant.
What the percentage actually measures
Efficiency is the share of the sunlight landing on the panel that comes out as electricity. Standard test conditions deliver 1,000 watts of light per square metre, so a 20% efficient panel produces 200W per square metre of panel area.
That is the whole definition. It says nothing about reliability, nothing about how the panel behaves when hot, and nothing about price. Two panels rated 400W produce the same 400W under the same conditions regardless of their efficiency — the more efficient one is simply physically smaller.
The comparison that matters instead
| Panel A | Panel B | |
|---|---|---|
| Efficiency | 22% | 19% |
| Rated output | 400W | 400W |
| Physical size | ~19 sq ft | ~22 sq ft |
| Price | $260 | $200 |
| Dollars per watt | $0.65 | $0.50 |
| Energy produced per year | Identical | Identical |
Panel B produces exactly as much electricity for 23% less money, and costs you three extra square feet of roof. If you have the roof, B is the better buy and the efficiency number told you nothing useful. If you do not have the roof, A is the only one that fits, and efficiency is suddenly the most important spec on the sheet.
When efficiency genuinely matters
- A small or complicated roof where you cannot fit enough panels to cover your usage
- Van, RV and boat roofs, where the mounting area is fixed and small
- Tiny houses, for the same reason — this is where paying for efficiency clearly pays
- Shaded roofs where only part of the plane is usable
- Where a permit caps your array by area rather than by output
Outside those cases, efficiency is a way to charge more for the same kilowatt-hours.
The specs that deserve the attention instead
Temperature coefficient is the underrated one. Panels lose output as they heat, typically 0.3-0.5% per degree above 25°C, and a roof in summer reaches 55-65°C. A panel with a -0.29%/°C coefficient loses roughly 10% at 60°C; one at -0.45%/°C loses closer to 16%. Across a hot climate that gap is worth more real energy than a few points of efficiency.
Degradation rate determines what you still have in year 25. At 0.4% a year a panel retains about 90% of its output after 25 years; at 0.7% it is closer to 84%. Over a system's life that compounds into a meaningful difference.
Warranty length and the company behind it matter more than either. A 25-year warranty from a manufacturer that folds in year six is a 25-year warranty on paper only.
One efficiency figure that is genuinely misleading
Be careful with cell efficiency versus module efficiency. Cell efficiency measures the silicon alone; module efficiency measures the finished panel including the frame, gaps between cells and the glass. Cell efficiency is always the higher, prettier number, and it is not what you are buying. Compare module efficiency or nothing.
Once you know your available area, the panel-count guide tells you whether efficiency is your constraint at all — for most roofs it is not.