If I have my cell phone charging via a transformer plugged into the wall, is it still consuming energy when I unplug my phone? For that matter, any transformer, since it is more or less electrically isolated from the unit it is powering, does it continue to use electricity when the unit is unplugged from it (or turned off)?
Yes. But not much. I’ll see if I can find or calculate some figures.
Touch your transformer and you’ll feel that it’s a little warm. That’s electricity being dissipated as heat.
ski’s answer is right. Something to think about is that it can actually draw a fair amount of current, even an ideal transformer that uses no power. The way the current alternates at 60 times per second is out of phase by 90 degrees with the voltages’ alternating, so the average power is zero (energy is sloshed into and out of the transformer, but the average transfer is zero).
I thought they used energon cubes.
When a transformer is under no-load conditions, the primary winding will act as a high value impedance, being make up of inductive reactance and resistance, the resistance will disspate heat.
In addition the transformer core will still be magnetized alternately north and south at mains frequency so there will be some hysteresis loss which is also disspated as heat.
However in the case of a charger unit such as your cell phone charger, there will likely be some components on the secondary that are required to rectify and smooth the reduced voltage, and these will consume a small amount of power.
What everyone else has said so far is true for a simple transformer. Your phone charger is, however, more than just a transformer.
First of all it is (most likely) a switched power supply, which means that the electricity is rectified, and chopped to several kHz, before entering the transformer. The chopping is controlled by electronics, and is probably stopped when there is no load.
Therefore I believe that the actual transformer part of the charger does not dissipate energy when not in use. There will still be dissipation from the rectifier and control circuit though.
Please note that this post is written in my best belifef of how such a device works, although I have neither designed nor dissassembled one myself. Anyone with first hand knowledge, please refute or confirm my guess.
Whatever the cause, if your transformer is warm, it is using power. (Unless it is sitting on the heater.) Pull it out of the power socket.
I checked this thread because I knew someone was going to make this joke. 
(And remember, too much energon causes your systems to lock up, so stay in beast mode to protect yourselves 
)
Popup- In every power supply design I’ve seen, the transformer comes first, and the switching power regulator runs off the low-voltage secondary side of the transformer. So even if the switching regulator shuts off, you’re still going to have the primary side of the transformer connected to the grid, wasting power.
Huh?
For switching power supplies that do not have power factor correction (PFC), the first thing the AC “sees” is a full wave rectifier circuit, followed by high voltage electrolytic capacitors, followed by a solid state switch (usually a FET), and then a transformer optimized for high-frequency operation. The FET is controlled by a high frequency PWM circuit located on the secondary (low voltage) side of the transformer.
With no load on the output, most switching power supplies still draw a little bit of current, called the “idle” or “stand-by” current. This is because they want to keep the closed-loop controller “awake.” How much current depends on the particular supply and manufacturer.
Linear supplies are more predictable, as others have already pointed out. Even with nothing connected to the supply’s output, the transformer’s primary winding offers a load to the 120 VAC, albeit a light one. As casdave pointed out, self-heating is due to the resistive component of the transformer’s input impedance, plus some hysteresis loss in the magnetic circuit.