If I just press on/off switch once it goes to sleep mode and wakes up instantly when I press it again.
Does it consume battery during this time? I understand to run system clock etc. some power will be consumed but how does it compare with fully switching the unit off?
In addition, some tasks and hardware can be running when the iPad is in Sleep mode - for example the WiFi and 3G are switched on and the iPad can be programmed to check for email every so often, even when it's in sleep.
Have a look here for the official line on extending your iPad's battery life
Yes, the device still consumes battery, mostly for apps connecting to the Internet, such as checking for mails. But the most battery is used for powering the display, so as long as that is off, will the device rarely use any battery.