Quote:
Originally Posted by mrblah
I'm fairly new to electricity for RVs and from what I gathered, DC current isn't = as AC current. In fact: DC current * 10 = AC current.

^ That's not a proper relationship. For these calculations you can ignore the fact that one is DC or AC. What you want is the voltage. 12 volts and 120 volts in this case.
AC and DC describes the flow of the electricity. You can have any combination of volts and amps in AC or DC. There are high voltage DC power lines in excess of 100,000 volts. It's also not uncommon to see a power adapter around the house that is 12 volts AC.
Quote:
Originally Posted by mrblah
So if his microwave is 1040W (on a 120V plug) it would pull 1040/120 = roughly 9 AC amps, am I wrong?
So he would need 90 DC amps to make it work, make it 100 DC amps to make it safe.
Is that too close and that's why you say it's not ok (@jazty)? What am I missing?
I'm learning all this so I figured I'd use Zephod_beeblebrox2's situation to understand better. Sorry to thread jack!

You've got the right idea here for the most part. You just need to represent the data correctly.
"9 AC amps" should be "9 amps @ 120 volts AC"
"90 DC amps" should be "90 amps @ 12volts DC".
Amps = Watts / Volts
Therefore
Watts = Amps * Volts
and
Volts = Watts / Amps
1040w / 120v = 8.66a @ 120v
1040w / 12v = 86.6a @ 12v
So 86.6amps divided by two batteries = 43.3amps per battery (in a perfect world).
Your calculations are right, I just used more precise decimal places.
Also, here's a simple calculator for doing any of the above calculations:
http://www.supercircuits.com/resourc...ampsconverter
This is also a handy calculator for sizing cables for your DC install:
http://www.solarwind.co.uk/cablesizingDCcables.html