This thread should provide some advice for how to calculate the aggregate data. The short version is you will need to create Items to hold the sums and rules to update the summary Items when your power meters report new data. NOTE: it requires persistence.
Presented in a table is not easy using the built in UI. Each Item gets its own row on the UI. You could cludge some something together using String Items and Rules though. However, charts are easier. See the Charts wiki page.
One thing to note is if you are using rrd4j as your persistence you need to store the Items you want to chart every minute regardless of how infrequently they are updated.