You can swap the firmware and use pretty much whatever wattage you like.
I've got one that is submerged in mineral oil with a pump loop & silent fans.. It's literally silent.
I really should plug it back in...
You can only use such an asic only when you have free electricity.
You can even without firmware update remove 1 board with chips and reduce power consumption.
IF you intend to sell immediately. Everyone seems to only consider current exchange rates. If you look at where things are going and consider the future value, it's a different story...
I've had mine running well as low as 600W at about the same efficiency as running at full power. That said, yes, the S9 is out-dated and highly inefficient by comparison to other ASICs now.