Why? Wouldn't a large enough capacitor last longer than batteries? UPS batteries I've seen are only rated for 3 years, and half of my recent laptop batteries have physically swelled after 3 years of use.

It would, but the size would be impractical and it's too costly. Quick estimate, let's assume we need 10W for 10 seconds, that's 100 Joule of energy. The energy stored in a capacitor is 0.5CV^2. Say we use a 10V capacitor then C = 2 Farad. They exist, but they are very large (look them up on amazon for instance). You'll probably need more like twice the capacity though because it's impossible to extract all energy from the capacitor, and it's lossy to convert it to a constant +5V / +12V.

Instead of 10 V and 2 F, how about going for 2.5 V and 50 F? A capacitor with those specs is only 40 mm long and 18 mm diameter [1]. That shouldn't be too hard to fit in a typical server or desktop. That's under $4 in quantity.

[1] http://www.mouser.com/ds/2/257/Maxwell_HCSeries_DS_1013793-9...

Theoretically it's possible, but in practice the lower input voltage makes it harder to convert it to +5/+12V. It becomes increasingly lossy and expensive - to convert 0.5V to 12V at 10W is not trivial to begin with. Voltage across a capacitor drops continuously while discharging (unlike a battery). So with a 2.5V capacitor, being able to use it between 1.25V and 2.5V is already pushing it. On average, discharge current at 10W is around 8A. The internal resistance of the capacitor (ESR) better be very low (it probably isn't) at this low voltage - even if it's 0.1 Ohm, at 8A we already lost 0.8V from out meager 2.5V, and now the useful energy is just 1.7-1.25 = 0.5C0.45V^2 = 5J, just enough for 500ms at 10W.

Interesting. 500 ms would would probably not be enough time to save everything (although maybe on an SSD based system it would be...), but it would probably be enough time to save select information that would allow ensuring that the disk is in a consistent state.

The 10 F capacitor has an ESR of 0.075 ohm, but that's at 1 A. They have a 100 F model that is 0.015 ohm at 10 A, and a 150 F that is 0.015 at 15 A. Based on your calculations, these look like they would have a good chance of giving enough time to save everything (especially on an SSD system).

Those are physically bigger but should still fit in a normal desktop or server.

(There is another manufacturer that has up to 630 F!)

The idea is rather to have it built in. A retail consumer shouldn't have to worry about these things. And the idea would not be to sustain a long power interruption like a UPS, just to give time to flush caches and avoid corruption. Which should be much cheaper than a UPS.