Pack Light: The more maneuverable you are, the better the experience will be. Along with getting to and from your location, once there you may have to search around a bit to find a good spot that is not already taken by a performer. The less you lug around as you go from street corner to street corner, the happier you will be. If possible, get a dolly or hand truck to wheel your gear around on and make your life easier.
Power on the Sidewalk: You have 3 standard options: Extension Cord, car battery, or battery-powered equipment. The extension cord works if you can find an outlet (like in a subway). However, this method limits your abilities to move to different locations if your favorite spot is taken. You also risk people tripping over it. Car batteries are effective, but they are heavy and setting them up can be a pain. If you know a lot about how to do this, go for it – otherwise, I recommend the third option: Battery-powered equipment.
This is the most convenient method. It allows you to be mobile. If your equipment takes standard batteries, invest in some rechargeable ones. Duracell makes a great “Duracell Value Charger” that has served me well. Rechargeable batteries are a bit more expensive, but after you switch them a few times, the investment pays off.
Making Noise: Because I am a musician, I will offer what I know about battery powered amplifiers. I have experience with the Roland Micro Cube Guitar Amplifier and the Crate TX15 Battery Powered Guitar Amp . The latter of the two is my favorite, and the one I use. The TX15 has two inputs (ideal for guitar/microphone input), and one of them is designed to either take a microphone or a 1/4 inch instrument cable. It has a rechargeable lead battery which lasts many hours on end, and it does not lose its charge too poorly as the years go by. This amplifier can also get quite loud, which is nice. It also lasts a long time (6-8 hours).
I owned the microcube for a few weeks. It, too, is a great option. It takes AA batteries (which you can power with the rechargable Duracell batteries I’ve had success with). It has some effects, which is nice too. The biggest advantage the microcube has over the TX15 is that it is lighter and smaller. However, it is not as loud.
NOTE: The Roland CUBE Street Battery Powered Stereo Amplifier is a third option that looks great, but I have never tried it out, so I can’t speak for it.