In truth, it's my purpose to discover the best way to use things people normally have around the house to create enough power to operate a nightlight. So using tap water is rather ideal for my plans. Does anyone posses knowledge of different metals and electrolytes and what their combinations can yield as far as voltage and amperage?
Glad to bring back a fun childhood memory Coca Cola.
Voltage potential varies by two factors, dissociative capability and concentration.
Amperage varies by amount of ions present (amount of electrolyte multiplied by concentration).
Dissociative capability - ability for a acid or base to put ions into solution, also applies to salts (a salt by definition is an acid anion coupled to a metal cation, so table salt is the sodium salt of hydrochloric acid).
Basically, there are strong and weak acids, based on dissociation. Strong acids will always form stronger electrolytic solutions. Pour vinegar into a battery and then sulfuric acid at the same concentration and you tell me which one is stronger.
Concentration - this one's kinda obvious. More ions in solution creates a larger potential. How do you render an acid safe(r) in the lab? Dilute it. Same principle with batteries. Higher concentration, more ions, increased potential. That's the reason 12V high-load car batteries are potent enough to etch metal.
Car batteries are the reaction of sulfuric acid stripping electrons off of lead plates.
Amperage is a trickier concept, although not by much. It varies by total amount, although this can be misleading. A water tower-full of brine (salt water) turned into a gigantic battery will sustain a higher amperage than a car battery mainly because it has a higher ionization (amount of creating ions), even though it would have a MINUSCULE voltage. More electron flow is higher current.
Therefore to increase amperage you can play around with two notions, total volume and concentration. If you have sensitive areas that won't tolerate high concentration but you have sufficient space, you can build a large battery.
Hopefully this is of some aid to you, I am a chemical engineering major at my university. Let me know if you need any more help.
solidus
Edit: as for specific examples, I can end up digging up tables and calculating values for you if you're able to get a water assay or description of your electrolyte source.