You can easily compare the energy consumption of two sites from the Analytics page.
To do this, select the first site and period you want to compare from the Graph Controls panel and press the Update button to generate that chart.
To overlay the other site's data, press the + icon above the chart in the Graph Tools section, choose Meter as the overlay type, and then select the site that you'd like to overlay.
Upon clicking the Apply button, you will see the energy consumption for both sites on the chart. You can also overlay additional data sets if you wish to compare more sites.