Energy consumption is arguably the key factor in the design and operation of Wireless Sensor Networks (WSNs). This holds both for normal operation and maintenance operations – such as software updates. Whereas software updates will probably be infrequent, they must still not consume a significant fraction of a WSN’s energy reserve; also, the required consumption must be known before triggering an update, in order to ensure that it can complete. Software updates are an expensive operation: there can be a significant volume of data, guaranteed delivery is required, and normal data fusion algorithms cannot be used to reduce the communication load. In this paper we present a node-level energy consumption model for the evaluating the efficiency of software updates on wireless sensor nodes. This model is then used to derive a novel minimum energy benchmark equation. This paper also presents some new power measurements, and uses these, along with published data, to interpret the benchmark in quantitative terms for some specific hardware platforms. This benchmark provides a standardized and quantitative figure to use in comparing software update algorithms. The methodology is also applicable to establishing energy benchmarks for other tasks in the WSN domain.