I didn't know that! So really they're partially implemented.
If I needed to power a facility and didn't have a PG or couldn't get a build permission to build one I assume one day I'd be able to use one of these Portable versions at either +3 or +5 power.
What I'd really like to know is if this is likely to be possible any time soon?
Even when/if that is implemented it's unlikely those items will truly be able to replace power generators on a permanent basis or they'd end up being used instead of power generators to optimise city designs for things such as tax even further than is already possible. It would also eliminate the ability for ground forces to cut a cities power thus making ground combat less useful when we will probably be trying to achieve the opposite effect.