New Yorkers are about to get zapped on their electric bills. Starting May 28, households in the five boroughs will be socked with rate increases of up to 12 percent to run appli ances -- after the feds de cided that power companies needed an extra $500 million a year, The Post has learned. And that's on top of another 4 percent state-OK'd Con Ed rate hike that kicks in today. A typical apartment resident's electric bill of $74 -- based on 250 kilowatt hours of power each month -- will go up to $86, or $12 more than last...