**3. Modeling of Cost**

Calculating the *life-cycle cost* of the EES is an accurate and sound way to estimate the overall cost spent on an asset over the course of its useful life, thereby including the initial capital costs, the projected operating costs, and the maintenance costs, plus possible disposal costs or final residual values of the asset. This Section maps the characteristics of the various models of the life-cycle cost with respect to the layered structure outlined in Section 2.2.

#### *3.1. Main Characteristics of the Cost Layer*

The cost of the whole EES is a combination of *component-specific* cost items—that is, that can entirely be determined locally for each component (e.g., initial investment cost, operation, and maintenance cost); and of *global* costs—that is, costs that require an aggregation to be computed; this is essentially the case for electricity cost, whose computation implies the calculation of the balance of the power flow. Therefore, the adoption of a bus-based architecture for the cost layer is relatively natural and straightforward:


The characteristic information managed in this layer is cost, interpreted as a numeric value in some currency. Cost is thus the only *layer-specific signal* that connects all components to the layer-specific bus, and all components are directly connected to the cost bus. Notice that this architecture may not reflect the actual *physical* organization among the components in the actual EES. For instance, in the real-world EES, a certain component may be connected to the physical power bus through a DC-DC converter; nonetheless, in this *virtual* cost layer, both the component and the DC-DC converter are independently connected to the cost bus. Figure 2a shows a pictorial representation of the general structure of the cost layer.

**Figure 2.** Organization of the cost layer: property-specific signals (**a**), inter-layer signals shared by models of the same component (**b**), and with the power bus (**c**).
