The question kind of intrigued me, so I figured out a way to get a sort of answer. There's a road I drive quite often, that drops about 2300 feet in 9 miles, at a fairly constant gradient. When the battery has fully charged, the Insight coasts at about 70 mph. This is in gear, so with fuel cutoff it ought to be getting all the engine friction, but no power.
Figure that with me and the dog in it, the Insight weighs about 2100 lbs. Multiply that by elevation, and you get total potential energy. Figure how long it takes to cover 9 miles at 70 mph, and you have power dissipation. I left the calculator at home, but if you crunch the numbers, you find that it takes about 17 HP to move the Insight at a steady 70 MPH on level ground.