Dumb question - what does power supply do?

I've been reading about these Alien Bees units and they sound like decently priced units for home use. I can't figure out why you need the power supply "on location", but evidently, not at home. Don't you just plug them into a wall outlet directly??

Are these power supplies batteries, so you don't have to plug them in? I know this is probably the mother of all stupid questions, but I've never worked with a portrait setup except plain old photofloods in a college class more years ago than I care to remember.

Thanks.

John

At home they plug into the wall. The portable unit is a big battery and a box to change the power to the right type (tired, brain no think of right words). ie 12V DC to 110V AC.

John, the AlienBees Vagabond system is - as they say on their website - portable power. The Vagabond units would let you take the flash units out into the woods, for example, where there are no AC receptacles to plug into.

The Vagabond just looks like an inverter to me. Converts DC to AC power.

It is, but it provides a smooth sine wave and was engineered to specifically be able to work with the Paul Buff flash units. Many flash units will croak on power from conventional inverters that put out square wave or "modified sine wave" power.