[Irish stargazer (John)]

After almost 18 months without long exposure imaging I am very keen to get started again. When I was imaging at The Mornington Pensinula Astronomical Society site they had electricity conveniently at each observing platform so there was no need for batteries to run the netbook for guiding.
I have seen devices at Officeworks from Targus that can be used to power a laptop from a car lighter socket (so I presume a portable powerpack might work also). They look like big USB type adapters but are quire small and neat. The spec looks like they draw 10amps so the typical powerpack wont last long. Any idea what a typical 240V invertor would pull?
Any recommendatons on a portable power set up that would give 6-8 hours of use?