The first raid for Destiny 2, titled Leviathan, is set to go live on Wednesday, September 13. While it was previously rumored that you would need a power level above 280 to participate, Destiny 2 director Luke Smith confirmed on Twitter that a Guardian's Power Level can range from 260-280.
Destiny 2's first Raid has a Power range of 260-280.— Luke Smith (@thislukesmith) September 11, 2017
Bungie was coy about revealing the exact Power Level range leading up to the Raid, encouraging players to just keep grinding as hard as they can. Most players just assumed that the Raid would be leveled at somewhere between 280 and 300, so the 260 Power Level entry point is a bit lower than the figure thrown around. Still, it's suggested that you try and shoot for 280 if you want to have the smoothest time during the Raid.
In the first Destiny, Guardians would have a Light Level instead of a Power Level, but both were determined by the strength of your gear. For each point of Light lower than the recommended Light Level, your Guardian's damage was reduced by 3 percent, which quickly adds up. Presumably, Power Levels will work in a similar fashion, though no concrete numbers currently exist.
For Guardians looking to hit Raid-ready Power Level before Wednesday, we have a comprehensive guide for leveling and all things Destiny 2 so you should definitely check it ou if you want a shot at the Raid loot that go live this week.