In the solar industry, there are a lot of measures of quality, but the best measure is the degradation of a solar panel over time. As a solar panel sits in the elements and generates electricity, it degrades and produces less energy over time. The higher quality the panel, the less it will degrade.
Installers should be able to provide you data on quality, and a data sheet with information about the panel will help, but in general, a mono-crystalline panel is better than a multi-crystalline panel in terms of degradation. Mono-crystalline is generally higher efficiency, too, so they're an option consumers should consider.
Based on third-party testing by PV Evolution Labs, Fraunhofer, and others, SunPower is often named one of the highest-quality panels in the industry because it's a mono-crystalline n-type construction, which increases efficiency even further than competitors and improves reliability.
A 2012 study by Fraunhoffer tested SunPower and four other manufacturers' solar panels under extreme environmental conditions like heat, cold, mechanical loads, and ultra-violet light. The study found SunPower's panels degraded 1.3% versus 5.4% for conventional panels made by anonymous manufacturers -- even though Fraunhofer said it selected among top crystalline silicon module manufacturers like Yingli Green Energy, Trina Solar, Canadian Solar, Sharp, and Kyocera. In a durability test last year, the same lab found SunPower once again crushed these competitors.
SunPower put its advantage in the chart below, which shows the initial advantage it has in rooftop solar -- because of efficiency -- and how its advantage over competitors grows over time because of lower degradation and greater durability.
This is an illustration from SunPower, but it shows how higher quality panels will perform better than conventional panels over the full life of a solar system. Remember, these are assets that will last longer than most cars, so quality matters.