On a bright sunny day with the strobe just off camera and pretty close to the subject takes about 1000 watt seconds if you diffuse the light a bit with a softbox and your max sync speed is 1/250th or so. Without the box you could probably get by with about 600 or 700 watt seconds. If your flash sync is 1/500th the the flash power you need is about half that.
I shot this with an Elinchrom Ranger DC pack at close to full power which is 1100 watt seconds using a small photoflex softbox to diffuse the light and a 40 degree fabric grid to keep the light tight and where I put it. I shot a whole series of photos like this, just follow this link
http://www.ernierice.com/stories/sportraits/index.html if you'd like to see them.