Cheap (eBay) strobes:how many watts is "enough"?

Situation:tight budget,already aquired two 59" (150cm) parabolic silver/black umbrellas and used them w/Speedlites 580 for occasional (not pro) portraits of 1-2-3 persons.
Feel like it's tIme to upgrade to something more powerful and controlable....as the title says,what's the minimum accepted strobes' power (except the answers like "...you can never have enough" or "...the more,the better")?
TIA for any decent opinion!
Regards,

There is no single minimum. It all depends on what you are going to shoot and what you want the lights to do.

That said B&H has a 150ws light kit with stand and soft box for $139. And the good news is it is quite happy to run on a modified sine wave if you want to use it on location.