I have a 850GB iPhoto library - what happens in new Photos app?

[xraydoc]

So I've got a pretty large iPhoto library - years and years of pictures.

If I turn on iCloud photos and switch to the new Photos app, what happens to my library? Does all 850GB get uploaded to iCloud?

Do I need the 1TB data plan for $19.95 per month?

What happens when I inevitably need more than that? Between my photo library and the iCloud backups for my iOS devices, I suspect I'll need more than 1TB almost instantly...

 

[donlab]

you have the option to store local when photos first launches and the option can be found in preferences

 

[xraydoc]

you have the option to store local when photos first launches and the option can be found in preferences

Interesting. So if I store locally, what goes up to the cloud? Do I have browse access from my devices (does it serve off my local machine -- which would be fine, actually)? Or does it just behave like iPhoto used to?