Ah, I see. I figured they'd keep the input energy constant because otherwise you're not saying much at all (you could figure that the efficiency plummets and you'd be better off with a space heater).
They could choose the lowest temperature, find it's power input, then fix it at that amount. But at low temperatures the concern is usually less "efficiency" and more "am I going to freeze to death", because in most of the cold areas temperatures aren't usually that cold for that long (though in others it definitely is, and if you're running at 1.6 COP for a significant amount of time, you're better off with more insulation than a heat pump).