Bluetooth Low Energy (BLE) is a 2.4 GHz wireless technology which has been widely adopted in consumer devices. Available in most modern smartphones, it is a popular choice for connecting battery powered devices like sensors or beacons to mobile handsets. A low energy consumption, while being beneficial for all BLE applications, becomes a key requirement for applications containing larger numbers of BLE devices like smart home sensors or beacons in indoor localization due to the high cost of battery replacement. In this paper, we investigate the energy consumption of different BLE System on Chips (SoCs) which provide the foundation to most BLE devices. Based on extensive measurements, we present a vendor-independent model for BLE power consumption which allows energy consumption prediction from key design parameters. This helps in the tradeoff between engergy consumption and BLE performance. We concentrate on advertising mode which governs the energy consumption in popular applications like beacons.