A performance model, previously proposed to characterize the performance of grid infrastructures, is extended to evaluate federations of grids by aggregating their performance parameters. These parameters can then be used to develop scheduling policies based on them. The new model can be used to take scheduling decisions based on them and hence to aid in the development of scheduling policies. The model has been validated using the performance results obtained in the execution of a high throughput computing application on an enterprise grid composed of Globus Toolkit Web Service resources and a GridGateWay giving access to gLite resources from the EGEE infrastructure.