A cell phone is a transmitter just like a radio tower, and higher power is required to send the signal farther. Every modern cell phone automatically adjusts its transmitter power based on reception. Fewer bars means faster battery drain when you are using your phone. Switching to 2G will not necessarily save battery. If you have a 3G tower nearby, then your battery will last longer. And if you have a spotty 4G connection, you'd best plug in, or don't use it. However, given equal signal strength, a 4G signal is MORE efficient per byte than 3G.