How to Read a Solar Panel Spec Sheet (Without an Engineering Degree)
A spec sheet looks like a wall of numbers. Only a handful actually change your results. Here are the ones that matter.
You don’t need to understand every line — you need the six that affect production, longevity and your roof’s look.
The numbers that matter
- Wattage (W): rated output per panel. Higher means fewer panels for the same total — today’s premium residential panels are ~400–440W.
- Efficiency (%): how much sunlight becomes electricity (good modern panels are ~21–22%). It matters most when roof space is tight.
- Temperature coefficient: how much output drops as the panel heats up. A smaller (closer to zero) number is better — and a reason solar loves Minnesota’s cold.
- Degradation / year-25 output: good panels still produce ~88–92% of original output after 25 years.
- Cell type: monocrystalline (and half-cut/bifacial designs) outperform older poly panels.
- Warranty: separate product (defects) and performance (output) warranties — both should be 25 years or more.
What we default to
iSolar’s default is a premium all-black ~430W bifacial panel (half-cut cells, 30-year product and performance warranty) — high output, low temperature loss, and a clean look on the roof. We’ll always show you the exact model on your proposal, not a vague “Tier-1.”
Frequently asked.
Is higher wattage always better?
It means fewer panels for the same output, which helps tight roofs. But efficiency, temperature behavior and warranty matter just as much.
What's a good panel warranty?
25–30 years on both product and performance. Our default panels carry 30-year coverage, backed further by our Solar Insure warranty.
Have a question we haven't covered?
Talk to a local iSolar expert — free, no pressure. Call (651) 565-1140.