Your first question should be "did Congress specifically authorize me to use Procurement funds to purchase energy/electricity with my procurement budget line item"? If not, then you fall back to the general policy for the use of appropriations. In this case, it seems to be pretty clear that the purchase of energy/electricity should be made with Operations and Maintenance (O&M) funding. The most pertinent reference to support this determination is the Department of Defense’s Financial Management Regulations (DoD FMR); specifically, the Expense/Investment Criteria section, as follows: 
 DoD FMR: https://comptroller.defense.gov/Portals/45/documents/fmr/Combined_Volume1-16.pdf
B. Basic Distinctions Between Expense and Investment Costs.
The criteria for cost definitions consider the intrinsic or innate qualities of the item such as durability in the case of an investment cost or consumability in the case of an operating cost and the conditional circumstances under which an item is used or the way it is managed. The following guidance is provided to determine whether a cost is either an expense or an investment. All costs are classified as either an expense or an investment.
 1. Expenses are the costs incurred to operate and maintain the organization, such as personal services, supplies, and utilities.
2. Investments are the costs that result in the acquisition of, or an addition to, end items. These costs benefit future periods and generally are of a long-term character such as real property and personal property.

C. Policy for Expense and Investment Costs
3. Costs budgeted in the Operation and Maintenance (O&M) and Military Personnel appropriations are considered expenses. Costs budgeted in the Procurement and Military Construction appropriations are considered investments. Costs budgeted in the Research, Development, Test and Evaluation (RDT&E), Base Realignment and Closure (BRAC), and Family Housing appropriations include both expenses and investments.
 In summary, unless you had received specific authority granted to you via that FY’s Appropriation or Authorization Acts for your procurement budget line item to purchase energy/electricity, or unless the energy/electricity being purchased was specifically in support of a procurement (i.e., the energy/electricity that a contractor uses in the production of a procurement-funded item), the most appropriate appropriation to use would clearly seem to be O&M.
 Also,  and as a best practice in general with any issue pertaining to fiscal law, you should always consult your local comptroller and legal offices to ensure that the specific details of your particular situation are being legally addressed.