Nexus 5 adapter output is 5V and 1.2A.
something interesting that i found on internet that might be important to everyone to be aware of including myself as i am also doing the wrong thing using lower amp adapter to charge my Nexus 5.
Here is a check list I copied from
www.topmic.com..
It apply to notebook adapter but i believe the concept is same.
The following three condition should be met for you to use this charger safely.
1) Output Voltage is within +/- 5% of your old charger's output.
For example, if your old charger provides 19.5V, then you can use a charger that provides 20.5V or 18.5V.
2) Output Current is the same or higher than that of your old charger.
For example, if your original adapter provides 3.42 Amps, then you can use a charger that provides 3.42 Amps, 3.9 Amps or higher.
3) Connector tip must fit securely into your laptop.
Therefore : you need a match or higher current, otherwise it can cause overheating, might as well not function properly, even, shorten your battery life.
Meaning, I can't use my blackberry adapter with 0.8A.

Thanks man. I also did some research and came across the same findings as above.
It also appears that even if you use a 2A charger, if your device is rated for 1.2A, it will only draw 1.2A and not the full capacity afforded by the charger. Can anyone confirm this?