Does this ups include the battery? Or do I need to purchase it separately?

Yes, battery is included unless something has changed since I bought mine. My guess is that if the battery is NOT included there would be a statement to the effect in the product description. Hope that helps. It's a good UPS.

How much noise while battery not in use?

The EATON 5S1500LCD UPS is noted for its silent operation.

Questions & Answers: Eaton 5S1500LCD 5S UPS 1500VA 900 Watt 120V LCD Line-Interactive Battery Backup ECO USB - Tower - 2 Minute Stand-by - 110 V AC Input - 115 V AC Output - 10 x NEMA 5-15R

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Eaton 5S1500LCD 5S UPS 1500VA 900 Watt 120V LCD Line-Interactive Battery Backup ECO USB - Tower - 2 Minute Stand-by - 110 V AC Input - 115 V AC Output - 10 x NEMA 5-15R

Load Capacity (VA) 1500 VA
Load Capacity (Watt) 900 W
Plug/Connector Type NEMA 5-15P
Receptacle Type NEMA 5-15R
Receptacle Detail 10 x NEMA 5-15R
Number of Batteries 1
Ideal for protecting sensitive electronics from power interruptions: Backup/Run Time (Half Load) 12 Minute & Backup/Run Time (Full Load) 2 Minute

$276^.99 –