Measure electrical consumption from electric panel

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I want to hack my electrical distribution board a little, to measure the energy used by each circuit.

Thus, I want to use a little arduino or maybe a raspberry pi and plug it to my circuits to take measures (I assume I should use things like ampmeter or voltmeter). Then I could make charts about what use energy the most in my home (and eventualy point energy leaks).

Unfortunately, I have less-than-basic knowledge about electricity, so I don't have a clue about how to measure energy.

So here are my questions :

• where and what should I plug to my circuits ?
• how many measures would be enough to be valuable ? (one every millisecond ? second ? hour ?)
• would this system consume a non-negligible amount of energy ?

EDIT : as Will Dean stated, I should start with something small. Please consider this question to apply to a simple plug, and not an entire electric panel :-)

FINAL EDIT : I have to admit this seems way too dangerous for me since there is too much information I don't know about the electricity way across my home. So the final answer is : I give up for now. Any advice to how to teach myself is really welcomed