Destiny 2’s Director, Luke Smith, revealed on twitter earlier today what the raid power level requirement is going to be.
According to the tweet, the first Destiny Raid called Leviathan is for Power Levels 260-280.
Destiny 2’s first Raid has a Power range of 260-280.
— Luke Smith (@thislukesmith) September 11, 2017
If it’s like Destiny 1, the first enemy encounters in the raid will likely be 260 and then the enemy levels increase as you progress in the raid. The endboss is probably level 280 or higher.
To be safe, try to hit 285-290 in preparation for the raid. To learn how to do that, check out the Destiny 2 Power Level Guide: How to Reach 300 Power.