This question marked as off topic because it's "seeking for hardware shopping recommendations".
The question title itself makes clear this is an incorrect reading ("Calculating an appropriate wattage for my server's new PSU?" - specifically asks about "calculating an appropriate wattage", not what model to buy).
It asks how one can appropriately calculate a suitable wattage that's appropriate for a file server build with a large number of HDDs. That's as much on topic as asking how many cores, how much RAM, what speed processor, or any other "how much is appropriate for these needs" question.
It even says up front, "I don't need a brand recommendation" and "It's the size that needs to be sensible. What PSU size would be "safe" for this build?"
What size PSU do I need?
- How much power is enough?
- How to determine what size backup power equipment is needed?
- How much power supply does a typical PC build need?
- How much voltage (and ampere) does the PWM input on 4 pin fan need?
- How can I evaluate how much power (from power supply) is needed based on PC configuration?
- What size power supply would I need for a computer with these specifications?
What size RAM do I need?
- RAM and Windows 7, how much do I need?
- How much RAM can I put in an ASUS Q550LF?
- How do I determine how much RAM I actually need?
- How much RAM do you need to run Windows 7 usably in a VMWare virtual machine?
What size storage do I need?
- ReadyBoost - How much space needed?
- How much storage I need for this CCTV system and what capacity it has?
- How much disk space do I need for running an Ubuntu Live CD?
Can the question be reopened, please?