How to use sodium percarbonate

Note that sodium percarbonate is not all that stable and breaks down into sodium carbonate (a.k.a. washing soda) and free oxygen which escapes into the atmosphere. There is no way of knowing if the sanitizing agent still has its peroxide component. That means that pre-used sodium percarbonate may or may not be still effective. The only way to tell is by whether or not your batch of beer has gone off. Is that worth the risk? In my personal opinion it isn't.

1. Does sodium percarbonate really leave behind only water? No!

Sodium percarbonate rapidly dissolves in water and dissociates into sodium, carbonate and hydrogen peroxide. 1

So no- not just water. But it does break down into "safe" compounds.

2. When using sodium percarbonate, is it necessary to completely fill all vessels and leave them to soak?

It depends on what you're trying to do. Sodium percarbonate is marketed pretty widely as everything from a cleaner to a one-step sanitizer. I've typically only ever used it as a cleaner. If you want to clean a really gunky dirty fermenter- you might need a soak. If you just want to clean the dust out of a carboy then a quick rinse probably suffices. When cleaning a 5 gallon carboy I usually half fill it- and roll it around on the floor for a while until it's had good contact.

If you're using it as a sanitizer, it probably needs more contact time than something like iodophor or starsan, the packaging should tell you that.

As most of us here at homebrewing are likely only casual chemists at best, you might want to ask chemistry if you have any serious chemical questions/concerns!