How much does Android use Wi-Fi for network discovery when not connected? When I have Wi-Fi turned on, but not connected to any network, how much does Android use the Wi-Fi function? I'm asking this because apparently Wi-Fi activity uses considerable* amounts of battery.

Is the device constantly scanning for Wi-Fi signals to find a network to connect to? Or does it only do it at some intervals to save battery, so that all significant battery use happens only when it's connected? Are there any settings related to this?

(I have Android 2.2.1 but I'm interested in information about other recent versions as well if there are significant differences.)

* - "considerable" should not be interpreted as a synonym of "large" or "too much". I'm not suggesting Wi-Fi drains my battery immediately, just that the energy consumption is worthy of considering.
I'm pretty certain the wifi drains more when scanning than when actively connected to a network. So when you are not going to use wifi, it is better to turn it off and use cell network (which you'll be using anyways while not actively connected to a wifi).