12.5 amps should be plenty for most household appliances, even a microwave or a small refrigerator. I'd imagine, though, that you want to pull this much amperage only with the alternator turning/engine running.
Okay. Here's the equation:
P=VI (Power (watts) = Voltage (volts) x Current (amps))
With the numbers provided above, the available current (I, in amps)
should be...
I = 150/12 = 12.5 A
Yes, I am a physics nerd.