Is it ok to have a power inverter draw more watts than the battry has?

I have a 12v 18 ah battery and I'm expecting to run it for about 3 hrs so then that is 72w coming from the battery, correct? Ok then I have a 200w continues power ac to dc inverter so is it ok to hook them up and can i charge something like an iPhone. Thanks.

Added (1). Sorry i put this in car audio i just have no die where it really belongs

You could run 18ahr/3hr= 6amps for 3 hours. 6 amps x 12 volts =72 watts for the 3 hours.

The battery has 18ahr x 12 volts = 216 wattHr. Your iPhone needs 5 volts at 1 amp, or 5 watts. If the inverter and iPhone charger were 100% efficient, you could charge the iPhone for 216 watthr/5 watts = 43.2 hours.

The inverter is not that efficient, only maybe 80%, and the charger is also maybe 80% efficient. Figure 0.80 x 0.80 = 0.64. And that times 43.2 hours is 27.6 hours.

Instead of an inverter, get a 12v DC to 5v DC converter for $5.00, they are more efficient than the inverter converting up to 120 vac then the charger converting back down to 5 vdc. They are the cheap Chinese cigarette lighter charger adapters.