[jpk]

Without getting too technical, battery capacity is rated in amp-hours, which gives a general idea of how many amps can be supplied over a period of time. If you multiply the amp draw times the length of time you need to supply that, you get amp-hours.

In your example, the fan draws 2.5 amps and you want to run it for 8 hours, that's 20 amp-hours.

Between your stereo and the XM, you're probably looking at between 1 and 2 amps draw; more if you turn up the volume to jam to Zepplin; so 12 hours would be 12-20 amp-hours or so.

Phone chargers are typically 1/2 amp or so, and a laptop charger might pull up to 10 amps.

All together, you're looking at roughly 50-70 amp-hours over the course of a day, depending on how much the laptop gets used.

A good 12 volt deep-cycle battery will provide up to about 120 amp-hours, so one fully charged battery should be able to cover a day and a half. You could hook up two batteries in parallel (+ to + and - to -) and get 240 amp-hours.

Keep in mind that putting power back into the battery works the same way; if you have a 10 amp charger, it needs roughly 5 hours to put 50 amp-hours back into the battery, so two hours of running your generator and charging will more or less cover the power to run the fan.

I'm simplifying things a bit here and ignoring things like efficiency and heat loss and that kind of stuff, but this should give you an idea of how this works.