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[Reply #15 on:]

The USB connection provides up to 500mA. Any battery capable of providing that amount of current should suffice. You need not worry about providing too much current, since current is pulled, not pushed.

[Reply #16 on:]

Thanks, Paul.... That's good to know about current!

Gerald

[Reply #17 on:]

9V batteries are one of the most efficient and most expensive ways to spoil energy.

The best of them contain 500mAh, their internal resistance is around 50 Ohms, which means they will hardly deliver 150mA, as the terminal voltage drops down to around 7.5V which is just enough to satisfy a non-low-drop 5V regulator.

The ethernet shield will suck more than 100mA alone....

[Reply #18 on:]

Thanks!  It's great to know all this useful information!