Your current adapter should have the wattage on it - buy the same. If you no longer have it, you can search yoir laptop by the service tag on the Dell Support page (it should detect it, if not the service tag is on a label on the bottom cover) and check the configuration there - it should have the original adapter listed. Buy the same.
If in doubt, just buy the 240W as that should work even if it used 180W.