**A.2 Variable cost**

The variable cost (*VC*) represents the cost of energy supplied by the system. It is affected by the load variation, the type and size of generating units, and the number of hours of operation. Also, these costs are related to the cost of operation and maintenance (fuel, scheduled maintenance, interim spare parts, staffing, wages, and miscellaneous expenses) and can be evaluated as

$$\text{VC}\_{T} = \sum\_{t} \sum\_{k} \left( \varepsilon ES\_{k} \cdot ESC\_{k} \cdot NUU\_{k} \right)^{t} \tag{A.2}$$

where : expected energy supplied by unit of type *k*; : energy supplied cost of unit of type *k* (SR/kWh).

The total system costs (*SCT*) for the entire expansion plan can be estimated by summing all the above individual costs at every stage of the planning period as being expressed in the following equation:

$$\mathbf{SC}\_{T} = \mathbf{FC}\_{T} + \mathbf{VC}\_{T} \tag{A.3}$$
