User blog comment:B214/Volume 23 discussion blog/@comment-31764731-20170825182223/@comment-29963364-20170826174034

Issei hasn't done any training to increase his Demonic Power. So far, he's only focused on increasing how much he can handle his Boosted Gear.

So while the amount of Demonic Power he has is only low-class, his Boosted Gear makes it worth Ultimate/Satan-Class (God-Class in DxD Mode).