It depends on a lot of factors. It’s less, but definitely not negligible. Especially if you have a fence running north/south with bifacial panels, you will generate most of your power in the morning and the evening when demand is highest. If you have some kind of net metering, the extra power in the evening could outpace panels in a normal configuration from a monetary perspective.

Many typical configurations also suffer from decreased efficiency due to heat, which is the worst around noon on a clear day. Vertical panels get the least direct light around then.

It’s also better for dirt and snow like others have mentioned.

I’ll hunt for an article with some actual numbers.