I've just been talking about this subject on the Stony Stratford thread.

You don't need much amplification for busking, but you do need sound reinforcement. Unless you can find a pitch with plenty of walls and other surfaces to reflect sound (ie the underground) there is a tendency to thrash your voice in the open air. I'm willing to bet that my voice is much more powerful than yours or alanbits (ask Breezy), but I still find it's a good idea to use a small PA if I'm going to sing outdoors for any length of time (and if you're trying to raise cash you'll need to).

There is also a difference in the tone of your voice when you are having to project hard - a lullaby won't sound right sung FF (in a concert, club, or on an operatic stage the audience will listen to PP with bated breath - but not in the street!) When you use just a small PA you can achieve a much better range of dynamics and tones.

In most places you need a clear sound radius of at least 30 metres to busk effectively, otherwise your public will have walked past you before they've even heard enough of you to decide that you're worth listening or contributing to. Don't forget that you will most likely be competing with traffic and pedestrian noise, shop window distractions, and possibly other buskers. After all, the bible discourages "hiding your light under a bushel"!

Even if you have the vocal power, a guitar being thrashed like hell (to the detriment of both instrument and sound quality) will not carry very far in the open, so you can do with the system to provide you with a balanced mix.

I, myself use a small soundcraft mixer feeding a car sound sound system amp, powered by a small car battery (I use an inverter to get 240v for the mixer). The speakers are small Yamaha monitor ones.

I think that you'll want something much simpler and the NOBELLS or PEAVY (if you can find one) amps should both be fine. I'm not sure what batteries they use - it'll probably be either a load of "D" cells or there'll be a socket for 12 volts. If you do need a 12v supply, a rechargeable dry lead-acid battery is probably your best bet. Remember, a 15 watt amp will normally only be drawing a small current - it's only for the odd fractions of a second that you tend to hit peak power.

Finally, what are you planning to use for microphone and guitar pickup? Ideally, I would recommend a small headband boom mic for the vocal side and a transducer inside the guitar. That way you don't have to cart a stand around with you, the whole setup is less obvious to the public, you take up less space, and you can move around a lot more.