Obviously the reason they have that amount of power on station is to use it to run the station. So the first obvious math hour is that you cannot have all the power for 15 seconds.
Conversely the notion of using extra power that is unconsumed is valid. The Ad Astra guys with VASIMR plan to scavenge extra power to charge the batteries for their engine so they can run reasonable tests of 15 minutes of their 200kW engine. So not out of the question in principle.
Of course, once VASIMR is on station, if it happens, then that spare power is spoken for.
Better question, is a 1 Megawatt laser enough to matter to debris? Two approaches:
- Small stuff vaporize it.
- Larger stuff, let the emitted burst of gases act to slow it down, drop orbit, hoping it is enough for it to reenter.