perhaps you are confusing a few different terms...
filterguy said the most important thing to think about at the start:
"The key thing to remember is that Watts out of the inverter is roughly equivelent to Watts into the inverter."
watts are the instanteous demand of power.
The "roughly" part is because everything that gets converted from one thing to another, loses something in that conversion.
So here is a real simple way to look at it:
the watts I want OUT of my system is equal to the watts going INTO my system PLUS some additional watts to handle the conversion
This helps you not get caught up in all the potential units of measure, just convert everything into watts for the first step.
The maximum wattage of your inverter has NOTHING to do with the wattage of your batteries, as long as you have enough.
How much power(this is watts) can your inverter deliver at its maximum, this is the most it will use (you can ask the inverter for anything UP TO this maximum amount), your inverter has no idea what is providing it the power, it just watts enough to meet whatever you are asking for on the output side.
How much energy (usually talked about in watt-hours) you have stored in your batteries is how LONG it can feed the inverter. You can have a 10000watt inverter run off a battery pack you hold in your hand...of course it will only run for a fraction of a second hehe.
Notice that storage (batteries) involves how much energy is stored(watt hours = battery voltage * battery amp hours), inverters are described in terms of the most watts they can handle.
This is how you figure out how long yor inverter can provide power.
As to your question of how huge that amperage number looks for a 12v battery system, the bottom line is, yes, as the voltage goes down on the input the system will need to draw more current to make up the needed POWER.
That is why increasing your input voltage to inverters and solar panels really helps deliver more power without the penalty of more amps.
I am sure if you have been looking into this you know the power loss in a system via wires is almost entirely due to wire heat losses...which is from the flow of current.
If you have an electric stove or resistive space heater...those are just wires with too many amps going thru them!!
batteries in series means you ADD the voltages which is how you get more energy. current stays the same, voltage increase.
batteries in parallel means you ADD the amps which is also how you get more available energy, the
if you have two 12v 100Ah batteries:
in series you would have 24v * 100Ah = 2400Wh (Wh ==watt hours)
in parallel you have 12v * 200Ah = 2400Wh
but the current(amps) in your series setup would only need to feed the inverter HALF as much current for the same amount of energy.
well this turned into a rather long winded write up...
Ok, here is the simple simple simple answer:
The inverter will only draw in enough power to supply whatever is needed on the output side.
If there is a 1 million AMP hour battery on the input and you plug a 60watt bulb on the output, the invert will just draw in 60watts of power (plus the little bit of conversion penalty extra that was mentioned).
