Would it be correct to say that electricity supply has been portioned to people in some areas of Tokyo?
The verb connected to 'portion' is 'apportion'. But this means to regulate how much of something you put here, how much you put there. It doesn't describe a supply that is simply intermittent, or random.
So when I do my business accounts at the end of the year, I have to apportion some business expenses if there has been any private use. For example, if I use the business car for some social driving, then I might apportion the expense of that car 80/20 percent business/private.
Another possible verb here is 'ration', but that also implies regulation of the supply. We ration a supply to people if we have very little of it, giving it out in small doses. This might suit your purpose here.
If you simply want to say that the electricity is going on and off all the time, try 'fluctuate' as a verb, or 'fluctuating', 'unreliable' or 'haphazard' as adjectives. Also there are two more in my first paragraph!
Rolling blackout - Wikipedia, the free encyclopedia
If done intentionally it is called a "rolling blackout." There is not enough electricity for everyone to have 24 hours/day service, so the utility lights up certain sectors at a time, with blackouts in others. This way everyone has some electricity part of the day.
The article states that Japan started doing this on March 14, due to a loss of capacity from the offline plants.