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1) Show below is a partial solution to the problem, but this will only provide you with amp measurements. In order to monitor the wattage, you would need to monitor the current on each leg connected to an amp meter......ie.......L1 and L2 and then multiply each current leg by 120 volts to convert to watts. Not sure if this is an acceptable solution for your application or not?
A) At the 30 amp double pole Interlok generator breaker, install a 30 amp rated solid core current transformer (CT) toroid on each of the breaker hot legs. Then extend the current transformer wires and connect them to a panel amp meter. Solid core CT's take up less panel space than split core CT's.
The solid core current transformer does not require any physical wire termination to a circuit breaker. When the toroid is placed around the hot conductor, it senses the amount of current via inductance. This will solve your problem since it sounds like your panel is maxed out and you cannot use any tandem or quad breakers to free up breaker space. If a solid core current transformer with a panel amp meter is not acceptable as a partial solution, then a sub-panel would need to be added in order to free up circuit breaker slots.
B) Refer to the 2 links shown below for an example of a CT application:
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