You can use a clamp meter to measure current through a single conductor.
Current is the number of electrons flowing through the wire per unit time, and is measured in amperes. If you know the voltage, you can use that to calculate power (watts):
Power [watts] = Current [amps] X Voltage [volts]
If this is a standard residential power system you can assume 110V or 220V for the voltage, or you can measure it with a voltmeter/multimeter. If you want kW instead of watts, you divide by 1000. (The formula on your website includes a "Power Factor" which is based on the appliance being used. In most cases it's close to 1, so you can just leave it out unless you need to be super-precise or happen to know that you have a low power factor.)
The major limitation of a clamp meter in measuring household power is that it can only be clamped around a single conducting wire, not the entire romex cable. If you clamp around the entire cable you will get a reading of 0, since the two conductors have opposite currents and they cancel each other out. This limitation means the meter is probably only practical inside the circuit breaker box or an electrical box, where space and safety concerns may make it a bad idea.
Ok you wrote a book. Proposing all manner of third rate hackery. And what does it boil down to? You want to get 5000W out of your 5000W generator. Quick question.
What is 240 x 21 ?
By my math, it's 5040. There's your 5000W. You do get it out of the big NEMA L14-20 connector.
I have no idea where you got 41A. I'm pretty sure you made that up, probably by dividing 5000 by 120. I seriously doubt it was on the generator spec. There's a way if you really really want that, but as you get educated, you will realize you do not.
What is it you're missing? The odd idiom of North American 2-pole service. I don't blame you for not getting it... It's weird.
Your house is served by +120V, neutral (0V), and -120V. I just described an instant in time, they're AC so they will reverse position 120 times a second. The poles are called L1 and L2 and the middle is Neutral.
240V loads grab L1 and L2. 120V loads grab either pole and neutral. Which pole they grab is nearly random and that's the idea, to make them average out so loads are balanced.
For you, with 21A on each pole, balancing is a big deal. You'll have a problem if you put 30A of load on one pole. So you'll need to get into the gory details of what is on which pole, and manage accordingly.
Step 1: Control MWBCs so they don't kill you
I don't recommend rearranging things on a panel because you can break a type of wiring called a multi-wire branch circuit. Find an electrician and tell him to do exactly this:
find every multi-wire branch circuit in my home, and make sure both its hot wires are served from the same 2-pole breaker.
Step 2: get rid of double-stuff breakers
If your panel is stuffed, and has lots of breakers that have 2 breakers in 1 space, those will drive you absolutely bat crazy. ack... You know what, to heck with all that.
Let's just get you a new subpanel with the appropriate interlocks, and move the loads you want the generator to power into this new subpanel. Make this subpanel quite large (at least 20 space) realizing you'll use 4 spaces just for the interlock.
In a perfect world, your new panel will have ammeters which will tell you how close to 21A each pole is getting. Even better get one of those new fangled whole house monitoring systems. Ask a new question on how to get one to work in a generator interlocked panel.
Step 3: rearrange your loads in the panel
Now finally, it's time to learn the gory details of how poles are assigned in a panel. Read my posting here. Your panel may differ, but probably not by much.
Move your loads into the new panel, and consciously and carefully balance the loads. For instance if your table saw is on L1, put your dust collector on L2. Stuff like that.
Best Answer
Take a look at a system called TED (The Energy Detective), it can be used to monitor the whole house or individual circuits, or both and gives you a simple interface either stand-alone or to your smart phone or tablet.