How to measure power Arduino' s consumption

Hello everybody,
i'm new to this forum and this is my very first post.
I'm confortable with SW.
I'm much less confortable with electronic. So far i have setup a circuit able to read temperature, display it on a LCD display, activate a motor over a given threshold and send data over USB to a application. From a SW point of view everything has been ok.
For the hardware i had to go through Arduino's book (which is indeed very well done).
Now i would like to know how much power is Arduino consuming while running.
Does anyone have a clue on how to do this ?
I tried to google but i found nothing interesting (suitable to my competences).
Thanks in advance for your cooperation.

Do you have a multimeter?
Put the meter in current mode (mA mode) and put the probes in series with the +voltage feeding into the barrel jack and measure the current going into the board.
Power = voltage x current.
Power actually used by the board is 5V x current read by the meter.
What is your source voltage? Anything above 5V is power wasted in the 5V regulator: (Vs - 5V) x current = wasted power.

Best case is having a 5V wallwart with switching regulator feeding 5V into the Power Header on the card, so the onboard 5V regulator is not used.

If you are interested in how to save power (as in battery powered applications), this is a great reference.

You can also measure the power without a multimeter, if you would like to do that either because you don't have one or you would like the arduino to have this information (to show a graph of the power consumption and display in on a LCD-screen etc.)

As CrossRoads have explain, you need to know the current and the voltage in to calculate the power.

The arduino can quite easily measure the voltage between the current in and ground. The site explains how to measure the voltage accurately: Measuring accurate voltage with an Arduino – AutoTT.

To get the current place a small resistor (or multiple resistors in parallel to avoid exceeding the power rating of the resistors. (This can be a good idea as the smaller the resistors are the better.)) in series with the power supply and measure the voltage over the resistor with the arduino. The current into the arduino can then be calculated by Ohm's law (current = voltage / resistance). If you got a multimeter able to measure the resistance of resistors, measure it to get a more accurate result.

And finally power = voltage x current.