There are 45, 60, and 85 watt power adapters for the MacBook line of portable devices (the Air, the Pro, etc.). Does the 45 watt charger which comes with the MacBook Air work with the 15" MacBook Pro for example?
You can use it, but how much actual charging goes on may be minimal. It depends how much power your MBP is drawing (i.e. if you're doing CPU or graphics intensive tasks, you'll draw more power, thus less charging), but I suspect the 45W won't do much more than slow the rate of battery drain. If your MBP is asleep or turned off, it should charge the battery, just slower than the standard charger.