The highest grade is Grade L.

 Sunpower A grade Solar Cell

As you can see in the table above in the SunPower Prime pricelist, the only three grades available for sale are; Ultra Premium Performance(L)(23.7%), Ultra High Performance(K)(23.1%), Ultra Performance(J)(22.6%), with their efficiency evidently decreasing from L to J.

For example, one common practice, unfortunately, is for certain resellers to buy cheap and low-quality Grade A SunPower cells for 1USD/Watt on the Chinese market, and sell them on Amazon for 150-170USD/100W.

They are deceitfully marketed as top-quality cells, and non-surprisingly problems start to arise around one year after purchase.

Sunpower A grade Solar Cell

When it comes to buying stuff from China, “you get what you paid for” is generally good advice to follow.

There’s always a drawback to buying something for cheap, although it might not be too apparent at the beginning.