Please can you tell me how to compare the energy imbalance of the broker agents using the available data?
All the data you need for this is in the state log generated by the server. Specifically, imbalance information is in the BalancingTransaction instances. Here are a few from game 9 in the September 2012 series:
In this case, the BalancingTransaction classname is followed by fields objectId::new::brokerId::time::kwh::charge.
We have a tool that parses these logs and gives you the ability to extract the data you are interested in. Just pull down the powertac-tools repo and import the logtool into STS. One of the examples in that package extracts imbalance data and formats it as a simple matrix. Note that you need to have the 'logtool' branch of the common repo imported into STS for this to work. If you already have STS set up with the common module, just switch to the logtool branch before you try to build/run the logtool. It has a README file that should help you get started.