In grid applications the heterogeneity and potential failures of the computing infrastructure poses significant challenges to efficient scheduling. Performance models have been shown to be useful in providing predictions on which schedules can be based (N. Furmento et al., 2002) and most such techniques can also take account of failures and degraded service. However, when several alternative schedules are to be compared it is vital that the analysis of the models does not become so costly as to outweigh the potential gain of choosing the best schedule. Moreover, it is vital that the modelling approach can scale to match the size and complexity of realistic applications. In this paper, we present a novel method of modelling job execution on grid compute clusters. As previously we use performance evaluation process algebra (PEPA) (J. Hillston, 1996) as the system description formalism, capturing both workload and computing fabric. The novel feature is that we make a continuous approximation of the state space underlying the PEPA model and represent it as a set of ordinary differential equations (ODEs) for solution, rather than a continuous time, but discrete state space, Markov chain.