which power adapter to use?

I must have 10-15 different power adapters. Some previously shipped with Apple products, I've purchased some equipped with various ports, and some have been freebies. I'm noticing that some of the adapters won't charge my iPad Pro. You plug it in and nothing happens, so I'm thinking it must be related to the power limitations of the adapter.

Some adapters give their output in watts and others specify voltage output, while some will list both. Which is more important to consider in using an adapter--volts or watts? How do you know the required voltage/wattage required to charge a particular device so you don't leave it plugged in all night only to discover the next morning that it didn't charge?