Data aggregation is one of the most important communication pattern in wireless sensor networks (WSN), as it reduces communication volume significantly in typical monitoring scenarios. However, data aggregation is also assumed to reduce the energy consumption. We present a real-world evaluation using real sensor nodes in a wireless testbed with energy measurement capabilities. Our results show, that the impact of data aggregation regarding energy-efficiency is far lower than expected. We suggest, that simple packet aggregation is sufficient for most concast scenarios as it provides equivalent energy-efficiency without typical disadvantages of data aggregation.