Portfolio Manager views annual data in 12 month blocks starting on the first day of the first month, and ending on the last day of the last month. If your energy bills are from mid-month to mid-month (Jan 15 - Feb 15), then you will need 13 bills to equal "12 full months" of data (because neither Jan nor Feb count as full months).
To view your property's "Current" and "Baseline" annual period's, select the "Goals" tab and view the table under the "Metrics Comparison for Your Property & Your Target" header.
More examples on how the annual period is determined:
|Start Date||End Date||Annual Period|
|1/1/2015||12/31/2015||Dec 31, 2015|
|1/15/2015||2/15/2016||Jan 31, 2016|
|4/1/2014||3/31/2015||Mar 31, 2015|