The log-normal shadowing radio model has frequently been used to model radio propagation conditions. There exist accurate calculation methods for estimation of interference power sum statistics in fixed-topology wireless networks based on this radio model. Here we publish essential additions to these estimation methods to expand their use to sensor networks and ad-hoc networks with changing topology. To our best knowledge this has not been done before. Taking into account radio propagation conditions, density of nodes, size of the network, traffic load per node and MAC protocol characteristics we present a calculation method for the estimation of interference power sum statistics in wireless ad-hoc and sensor networks. The accuracy of the calculation method is verified by simulations. We highlight the influence of MAC protocols on interference and show that an increase in network size or in node density does not necessarily lead into higher interference values. Our results can be ...