Be careful calculating the wattage and working from that. 240V even at a tiny current will fry things...

You need to consider voltage and current separately.
1) Voltage. Virtually all USB chargers put out 5V - 5.5V, and this is what your Kindle (and phone, tablet etc) wants. As long as the voltage is about 5-6V you will not have a problem.
DON'T use anything with a higher voltage. It won't be faster, it will just go bang!
The Asus charger which is putting out 16V is very much non-standard, and I'm certainly not impressed with them putting that voltage out through a USB port. Get a marker pen and write "NOT FOR ANYTHING ELSE" on it!
2) Current. Remember that current is sucked (drawn) not pushed. The Kindle sucks as much as it wants, and as long as the charger is capable of supplying that much, everything will be fine.
USB from your PC will allow up to about 500mA (0.5A) to be drawn. This is the "norm", if you like, and everything will work with it, but it is quite low, and that's why charging your Kindle from your USB port is slow.
I believe Kindles can draw up to about 800mA (e-Ink - Fires can probably draw quite a lot more).
Avoid a charger which gives
less than 500mA. You sometimes find these for old phones. Using one of these can damage the charger (not the device).
As long as the charger is rated higher than 500mA, it doesn't matter how high the rating is. Remember this is the maximum the charger can let out, so even if it's higher than the Kindle rating it won't matter.
Most chargers are 1A, which is ideal and will be faster than your USB port. Some chargers are 2A or more. You can use any of these, but once the rating is higher than the Kindle's maximum draw (about 800mA I believe) it won't be any faster.
The cheap ones for sale on Amazon tend to be 1A, anything higher than that tends to be more expensive.
Think of it like sucking water up a straw: the limiting factor is how hard you can suck. Using a thin straw you won't get much water, and if you suck too hard you could make the straw collapse. Sucking water up a hosepipe will still only let you get as much water as you can suck, but you won't do any harm.
(The water will probably taste horrible out of a hosepipe, but that's a different matter!

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TL
R summary: use a charger rated at 5 - 5.5V, 1A.