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The easiest place to pick up the numbers is via the serial monitor of the Arduino IDE when you have the programming lead plugged in to your emonTx. If you look at the sketch you're running on it, you should see two sections that you are interested in. The first is a data structure that tells you the variables sent to you RPi and the order in which they are sent. It should be something like:

typedef struct { int power1, power2, power3, Vrms; } PayloadTX;         // neat way of packaging data for RF comms
PayloadTX emontx;

The second is a series of "Serial.print(.....)" statements that send the data to the serial monitor.

Coming to the numbers varying, 200 W is not a heavy load. Your c.t is rated at 100 A which is roughly 24 kW, so 200 W is only 0.8% of full scale. The ADC is only 10 bits (and one of those is effectively a sign bit), and we expect  a few tens of "watts" of noise from the digital circuits in the processor. Here is some reading:

http://openenergymonitor.org/emon/buildingblocks/measurement-implication...
http://openenergymonitor.org/emon/emontx/accuracy
http://openenergymonitor.org/emon/node/1385
http://openenergymonitor.org/emon/node/2311
Trystan did some wonderful animated pages demonstrating how the errors vary here:  http://vis.openenergymonitor.org/dev/adc/noiseless, here: http://vis.openenergymonitor.org/dev/adc/noisy and here: http://vis.openenergymonitor.org/dev/adc/noise.mp4.

Thanks a lot. Pretty explanatory. (only "wonderful animated pages" produce 404)

Then, if we take that we have 512 (2^9) steps of 24kW, then 200W will take at least 4 of these steps...

And  how can I diagnose why there is no change in readings when I connect clamp to some ports?

And these are W, which are sent from EmonTX or some "Steps" of this 512/1024 scale?

"Then, if we take that we have 512 (2^9) steps of 24kW, then 200W will take at least 4 of these steps..."

I think you are saying you can draw out the top half of the current wave as a set of 2-bit numbers. You should certainly see a 200 W load, but you cannot expect to measure it with any great accuracy. If you can't see a current, it is usually one of two things: you haven't pushed the plug on the c.t. fully home, or you've clipped the c.t. around both line and neutral.

Provided you have calibrated the emonTx, the numbers should be Watts for real power and 1/100 V for the voltage. You can always work through the sketch to verify that. If you have not calibrated it, the numbers will only be close.

(I get 404 for those diagrams too now - I'll ask Trystan what he's done with them.)

you've clipped the c.t. around both line and neutral

Sure, I did it! It is so logical, taking into account diameter of the clamp :) I think that it should be written in large red letters for those, like me, who think that electricity is sold in barrels on farmers' markets :)

I fixed this and now my readings are much closer to the reality.

Thanks a lot, you made my day! :)

"It is so logical,..."

Not when you consider that the c.t. usually needs to go around a meter tail! Our USA constructors can't even use our standard 100 A c.t. because at half our voltage, they need twice the current for the same power, hence their cables are even thicker.

"I think that it should be written in large red letters for those, like me, who think that electricity is sold in barrels on farmers' markets :)"

I shall send the Marcolins to get you for heresy like that!  Actually, it is spelled out quite clearly in a few places - like the installation notes...