yes they produce 1.5 volts
Onions don't have volts.
Volts
Zero volts.
This will still only produce 12 volts. It will produce 1200 watts. watts is the result of Volts times Amps.
13.5-16 volts DC
none!
plant onion seed and wait a year, onions are the biennial bulbs
I'm not sure I understand your question. An alternator will have to produce 13 volts to charge a battery at 13 volts. Most alternators are able to produce 14+ volts but the range is 13.5 to about 15.5 volts.
lots
2 volts
A good AA battery has about 1.5 volts across it.
3 volts