Lenny, The input power, volts times amps, will reflecct the required output load times some efficienty factor. Unless the manual says how efficient it is, I'd use 60% as a guess. You'd have to know the power requirements of each load, add them together and and call that 60% of what'd be required from the 12 volt system.
If you know the ampere hour rating of your battery, multiply it times 12 to get a very rough approximation of power hours availab.e don't even come close to expecting that kind of total power, you might presume you'd get half the estimated total power from the battery before it gets so low you won't start the engine again. On the other hand modern electronics don't take all that much power unless they are making a lot of noise or heat. Make sure your inverters are designed for electronics, some are only intended for lights or motors and don't produce a good enough sign wave form or good enough frequency and voltage regulation to keep electronics happy. Hope that helps some, but probably not <GRIN> Tom Fowle
