base load service; the corresponding time for the addition of a 1 GW unit is 1 - 1 1/2 y years. If the SPS is added to a 50 GW power pool, 3-4 years are required to return the base load units to base load service. Increased maintenance costs for these units will result from this displacement but the resources available for this program were insufficient to assess the size of this increase. Purchase of Bulk Power Power is often purchased from nearby utilities either directly or by automatic purchases directed by regional power pools encompassing several different utilities (e.g. New England Power Exchange). Utilities purchase this power because they cannot generate it themselves or it would cost them more to do so. Base load power is usually purchased only when the utility has not built the appropriate base load generators (e.g., non-generating municipal utilities and slippage of the construction schedules for nuclear power plants). However, utilities often purchase power to meet their intermediate and peak load requirements. As previously discussed, each generator is scheduled for use according to its incremental cost of generation. Since the incremental or operating costs of the SPS should be low it should be used as a base load plant. This would be true even if the total cost of energy from the SPS is higher than from conventional plants. However, if the SPS is owned by an independent organization and the energy is priced at its total cost, the SPS may be used only to meet intermediate or peak loads. For this reason, two other ownership/pricing concepts have been investigated. If the SPS were ''leased" to the utilities, the rental costs would be fixed and payment would be required even if the power were not used. The incremental cost to the utilities would be zero. On the other hand, if the incremental cost of SPS energy were artifically
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