Elevator installation in a building in which there is an electricity distribution network that is connected to the electricity supply (11, 12) of the building, and which elevator installation comprises: a plurality of elevator cars; a control (6A, 6B, 14), which is configured to form a run plan for driving elevator cars on the basis of service requests; a plurality of hoisting machines; a plurality of power supply devices for a hoisting machine that are connected to the electricity distribution network of the building, each of which power supply devices is configured to drive an elevator car according to the run plan with a hoisting machine, by supplying electric power via the electricity distribution network to a hoisting machine driving an elevator car, and also by supplying electric power from a hoisting machine braking an elevator car back to the electricity distribution network; characterized in that the aforementioned control (6A, 6B, 14) is configured - to form alternatives for a run plan for driving elevator cars on the basis of service requests, - to estimate the electric power which the hoisting machines need for implementing the aforementioned alternatives, and also - to select for use from the plurality of different alternatives a run plan, which when implemented causes the electric powers of the hoisting machines, when summed together (P ∑), to smooth the power variation occurring in the electricity supply (11, 12) of the building. a plurality of elevator cars; a control (6A, 6B, 14), which is configured to form a run plan for driving elevator cars on the basis of service requests; a plurality of hoisting machines; a plurality of power supply devices for a hoisting machine that are connected to the electricity distribution network of the building, each of which power supply devices is configured to drive an elevator car according to the run plan with a hoisting machine, by supplying electric power via the electricity distribution network to a hoisting machine driving an elevator car, and also by supplying electric power from a hoisting machine braking an elevator car back to the electricity distribution network; characterized in that the aforementioned control (6A, 6B, 14) is configured - to form alternatives for a run plan for driving elevator cars on the basis of service requests, - to estimate the electric power which the hoisting machines need for implementing the aforementioned alternatives, and also - to select for use from the plurality of different alternatives a run plan, which when implemented causes the electric powers of the hoisting machines, when summed together (P ∑), to smooth the power variation occurring in the electricity supply (11, 12) of the building. - to form alternatives for a run plan for driving elevator cars on the basis of service requests, - to estimate the electric power which the hoisting machines need for implementing the aforementioned alternatives, and also - to select for use from the plurality of different alternatives a run plan, which when implemented causes the electric powers of the hoisting machines, when summed together (P ∑), to smooth the power variation occurring in the electricity supply (11, 12) of the building.