When connecting directly to the system, you can use any Dell AC adapter that provides equal or higher wattage to what the system requires.  I've had a 130W AC adapter in my living room for years now and I've used it with systems that have required 45W, 65W, 90W, and 130W power supplies.  All of them work just fine, and some of them will even charge the battery a bit faster with a higher-capacity adapter.  Technically you can also connect an adapter that provides lower wattage, but you can end up with performance throttling and sometimes even battery consumption as the system tries to adapt to the power deficit, so that's obviously not recommended.

When connecting to a dock, you typically (always?) need to have at least the minimum AC adapter wattage required by the dock, otherwise the dock itself may not work and/or the system won't charge.  You can certainly connect a higher wattage AC adapter, although you may or may not see a benefit from doing so.