Adding a switch to a water boiler
Using 1kW at 120v means your boiler is drawing 8.33A of current:
1000W / 120V = 8.33A
so this is what the switch much be capable of handling continuously.   

A 10A rating should be safe &amp; 15A or more would be best.  

This simplistic formula only works for 'simple' things which turn electricity into heat, not things like pumps &amp; compressors.