Jim - I can't shed much light for you, but 1.5c worth: a lot of units can not be used underway, as the strain on the bearings from the precession is high. Some can.
Nigel -
I just finished rebuilding my electrical system over the summer. I recommend Nigel Calder's book, which has a lot of heavy reading explaining everything in detail, available a lot of boating shops.
Pretty much everything electrical you buy tells you somewhere how much power it draws. Nigel Calder has some tables in his book that answer your exact question, but I recommend looking at every circuit you have on your boat, and making a list of its current draw, and how much you will run it. For example, a typical marine tungsten bulb uses about 0.7A, so your three running lights, on for 10 hours, will use 21 Ah. Stereo depends a lot on the volume, if you have an ammeter in the system you can turn on such items and see how much they are using.
My book, and all my calculations are in storage right now, otherwise I would pass them on, but I did figure that by going to LED lighting (running lights, anchor light, cabin lights, a tiny fraction of tungsten filament power draw), I could run everything except refridgeration off a solar cell mounted on the cockpit stanchions. Bear in mind that is planned for latitudes<33N. I had one 4D, and then I kept one small house battery for engine start only, so running the alternator was my backup.
Take a look at my post on refridgeration a few minutes ago for my thinking on battery capacity. This winter I was planning to install a fridge, and towed water generator to power it, but bad weather has stopped play.
Gareth
Freyja E35 #241 1972