I thought I'd add a little to this in case anybody's searching for it in future. Just a reminder that the Time Capsule I bought was a first generation "flat" model, and not one of the vertical column-style models being sold right now (mid-2015).
My Time Capsule gets hot. This is a known problem and as mentioned above is the chief cause of PSU failure.
Spin down of the hard disk would help but it doesn't appear to be happening in the traditional sense. The disk I used is a Western Digital RE4-GP model, which apparently is an server-grade version of the "Green" WD disk.
The drive apparently contains "green" electronics that amongst other things spins down slightly when the drive isn't in use. I've heard the drive make a spin down-like noise (like a tiny drill slowing down!), yet I'm still able to feel the drive vibrating as if spinning. Weird. I'm hoping to dig out my watt meter to see if the power usage changes across time. (Note that early versions of the firmware of this disk spun down TOO MUCH, although I guessed that's been fixed in more recent releases.)
The Time Capsule offers no control over spin down via the AirPort Utility on a Mac/iOS, and WD don't offer any firmware control either (and in any case I don't think the spin down setting would stick across power cycles anyway).
So, the whole thing's hot 24 hours a day. I live in the UK where ambient temperatures are (cough cough) never that high but in places like Australia or Florida this can be a huge problem.
My solution has been to
(a) remove the silicone mat on the bottom of the Time Capsule. Apparently this is designed to dissipate heat but it's main job is to protect users from the metal plate beneath which, because the PSU is contained within the device, could be electrically live (it almost certainly won't be, ever, but safety regulations are there for a reason I guess). On the bottom I've stuck some rubber feet, as found on many other electrical deices. There are lots of small holes in the metal plate but some people have augmented this by drilling their own larger holes. Argh, too much trouble for me. On the bottom I've stuck some rubber feet, as found on many other electrical devices.
(b) plug it into to a basic timer switch so that the Time Capsule is powered down between the hours of 12am-7am, when the family is sleeping. I'm a little worried that the Time Capsule is being abruptly powered down at this time, but then again there doesn't appear to be any way to shut it down cleanly via software (again, the AirPort Utility is pretty cr*p). Time Capsule drives are formatted with Mac OS Extended Journalling FS, just like most Macs, and I know that this means recovery from unexpected power downs is pretty good.
In fact, I've put the timer on the socket for the entire desktop's computing equipment so that everything is powered down during that time. I'm a little concerned about the MacBook Pro cycling the battery during this period but Apple claim this is a very healthy thing, and in fact keeping the magsafe attached at all times is bad.
