Honestly you can run the BK-9 on batteries, at least for a few hours. Total current draw is well within the range of an external power bank for a laptop computer. Add a battery-powered amp (Roland makes several) and your busking career is off to a good start! If you don't have an amp, a bluetooth stereo speaker like the JBL Charge Extreme might even be an option.

I can't personally endorse the EK-50 nor the EW-410. There are a lot of Middle-of-the-Line advanced arrangers which have low draw and could be run from batteries: BK-5, BK-7m + controller, PSR-SX600, etc. If you are determined to stay in the low price range, the Casio CT-X3000 or even the new CT-S400 would be my pick over the EW-410. The ability to control the bassline (Bass Inversion) is HUGE! And the chord recognition system on the Casios is better than an entry-level Korg or Yamaha. My $.02.