i hope some resident engineers can help out...

i've seen a few DIY pages explaining how to do this -- i.e. run car, plug in DC to AC inverter, plug appliances. Just curious:

-- Is it safe to do so, for extended periods of time(e.g. >24 hours)?

-- how can you tell how many watts the car can produce at idle? Would revving the engine pump up the wattage? This is different from the rated hp(which refers to the mechanical power) of an engine right?

-- most examples i see mention pretty lightweight devices: laptops, cell chargers, etc. Can you safely plug in appliances with motors(fans)? i think it's wishful thinking that you can plug in even a small A/C or ref?