An energy cost optimization system for a building, the system comprising: - HVAC equipment configured to satisfy a building energy load; and - a controller configured to: - generate a cost function defining a cost of operating the HVAC equipment over an optimization period as a function of one or more energy load setpoints for the HVAC equipment, wherein the energy load setpoints are decision variables in the cost characterized in that the controller is further configured to: - modify the cost function to account for both an initial purchase cost of a new asset to be added to the HVAC equipment and an effect of the new asset on the cost of operating the HVAC equipment, wherein both the initial purchase cost of the new asset and the effect of the new asset on the cost of operating the HVAC equipment are functions of one or more asset size variables included as new decision variables in the modified cost function; - perform an optimization using the modified cost function to determine optimal values for the decision variables including the energy load setpoints and the asset size variables; and - operate the HVAC equipment using the optimal values of the energy load setpoints. - HVAC equipment configured to satisfy a building energy load; and - a controller configured to: - generate a cost function defining a cost of operating the HVAC equipment over an optimization period as a function of one or more energy load setpoints for the HVAC equipment, wherein the energy load setpoints are decision variables in the cost characterized in that the controller is further configured to: - modify the cost function to account for both an initial purchase cost of a new asset to be added to the HVAC equipment and an effect of the new asset on the cost of operating the HVAC equipment, wherein both the initial purchase cost of the new asset and the effect of the new asset on the cost of operating the HVAC equipment are functions of one or more asset size variables included as new decision variables in the modified cost function; - perform an optimization using the modified cost function to determine optimal values for the decision variables including the energy load setpoints and the asset size variables; and - operate the HVAC equipment using the optimal values of the energy load setpoints. - generate a cost function defining a cost of operating the HVAC equipment over an optimization period as a function of one or more energy load setpoints for the HVAC equipment, wherein the energy load setpoints are decision variables in the cost - modify the cost function to account for both an initial purchase cost of a new asset to be added to the HVAC equipment and an effect of the new asset on the cost of operating the HVAC equipment, wherein both the initial purchase cost of the new asset and the effect of the new asset on the cost of operating the HVAC equipment are functions of one or more asset size variables included as new decision variables in the modified cost function; - perform an optimization using the modified cost function to determine optimal values for the decision variables including the energy load setpoints and the asset size variables; and - operate the HVAC equipment using the optimal values of the energy load setpoints.