I don't know what the wattage is but I use the wall charger from amazon in my bedroom and the c h larger that came with my Windows phone in the living room with no trouble. And I'm sure my wall sockets are 110. Hope that helps.
Kindle Fire HD charges with a standard USB voltage (around 5 Volts). It's the charger (transformer) that needs to handle 100 and/or 220 Volts. Most these days do. This information should be written or stamped onto the charger.