Toyota Repair: 12 volt ignition source, avalon xls, starter solenoid

Question
I have a 1999 Avalon XLS and have installed a hydrogen generator on this car and I would like to have this unit be activated only by the ignition. I now have to turn it on and off as I am hooked directly to the battery.  
 It needs to be a good source as this unit draws 15-20 amps.  
 Can I go to the starter solenoid?  Any advice would be appreciated.
Thanks.

Answer
The small wire on the starter solenoid only has 12 volts when the key is in the start position. I recommend going directly to the back of the ignition switch, find the solid black wire and splice into it, this will provide 12volts from the battery going through a 30A fuse before reaching the switch, anytime the switch is in the ON position the device will have power to it.