65W will definitely work.  45 W will work if the system has only integrated (no AMD or nVidia) GPU.  
My mother can't find her charger and I'm trying to order her a new one.  There is a choice between a 45W and 65W charger.  Which one is the correct one and how would I know?  I took a picture of her laptop and I don't see wattage in the power specs, just voltage and amps. Thanks.
Replacement charger for Inspiron 15 5000 Series