What power adapter do I need to use when traveling to the US?

You are better off buying a plug adapter in India for the USA, rather than the other way around. A lot of what is sold in the USA would be for adapting a US plug to a foreign socket. You can find adapters for foreign plugs, they just aren't common.

Power wise, it depends on your device needs. Most modern electronics (phones, tablets, laptops, etc) have power supplies that can handle voltages from 100 to 220 VAC both 50 & 60 hz. Look at your power source, it is usually stamped into the plastic or written on the label what voltage range it accommodates. If you need a step up transformer for something else, you again may find it easier at home.

I can't recall the last time I bought an electronic device that didn't have a universal power supply; and I am including here laptops, desktops, mobile phones, e-readers, SLR cameras, mirror-less cameras, gaming consoles, hair drier, microwave, refrigerator, washing machine ... and that's all I could think of in terms of recent purchases.

The oldest device I own, a laptop bought 7 years ago - came with a universal power supply.

Therefore, what you most likely need is simply a plug converter; which you can purchase at most airport duty free shops.

These are devices that take the power cord plug, and adapt it to the socket that is in use in the destination country.

I would personally avoid the universal adapters, like this:

These are useful for a short trip but their bulky size means that you need to be careful where you plug it in, as it may take up too much room and block another available socket.

If you are planning to buy one, get one from a reputable brand - there are lots of companies making these and if you get one that's really inexpensive, the locking mechanism (the slider that ejects the right plug) gets loose and doesn't lock very well. Not only is this an annoyance, but it is also a potential safety hazard.

If you are frequently traveling to one country; carry a many-to-one adapter for each device. These are cheaper to buy, and easier to use as you simply plug it in and go; here is an example of a many-to-US adapter:

Unfortunately these are difficult to find at airports - you are likely to find these at a hardware or electronics store. Of the airports I frequent, only ONE had a store that offered the many-to-one adapter.

The many-to-many adapter can be found at most airport shops; they can also be found on most airline's on board shopping catalogs.

CAUTION When using "Voltage Converters" or "Step up Transformers" you need to make absolutely sure that you do not exceed the rated power of the device. If you want to bring devices that consume significant power (iron, hair dryer, etc) you will need a big and heavy transformer. Lightweight devices typically do not have the required power and connecting an iron to a small $20 voltage converter will at best not work or just break and at worst start a fire.

You can either buy plug converters or universal travel adaptor for $2 to $5 in most supermarkets or phone shops. This is just a converter not step up adaptor, if you are looking to charge your mobile phones or laptops which comes with adaptor they will work fine with 120 Volts but if you have electronic items like iron box etc, which doesn't have adaptor plug will require a "Voltage Converter" which you may find in auto electrical or hardware shops and it will cost around $15 to $20.

It's much better to buy your converters at home. I would say that 99% of the converters for sale in this city are to allow US travelers to plug their devices into foreign plugs. I'm aware of only one store that has a few that are for plugging foreign devices into US plugs.

An exception to this general pattern is airports that get a lot of international arrivals sometimes sell converters that international travelers would want--but as with everything in the airport they're way overpriced.

Here's my collected knowledge about power adapters and travel power strips:

1. If you need a three pole travel power strip try the Voltage Valet PS2 Travel Power Strip.
2. The Skross World Adapter PRO is an absolutely unique three prongs (earthed) adapter (family). Yes, Tumi also sells an adapter like that but if you look closely you'll find the Skross logo on it :) OK, that's about three prongs, let's move on to two prongs.
3. The Kirkland power adapter is a knockoff of this Japanese product not bad but there's better now. It's available from China for even cheaper.
4. The Wonpro Nano is win. Yes it's not flat but it's small enough to be pardoned for it. It's quite probably a Nissyo NP-10 knockoff but since the Nissyo is unavailable outside of Japan I can't be bothered. I had more success with this in the UK than the one above. The one above sometimes failed me with UK sockets but this one didn't. The way the plug unscrews for the EU plugs is just ingenious.
5. If you need to plug in more than two devices, then add a Road Warrior 47.

Either ship a Wonpro Nano to your hotel in the USA or if you still have the time (could be two months) then order the flat one from China. You will be covered wherever you go in the future.