Using energy generated with fossil fuel causes global warming due to the greenhouse effect, which threatens our environment. One of the challenges for New Generation Networks (NGN) is then the reduction of energy consumption, in particular at the BSs (Base Stations) which use about 85% of the total network energy. We contribute to the research with a mathematical model that calculates the total power consumption of a BS and enlightens the way to minimize it. First, we analyze the power consumed at every different component of the BS. Second, based on the cost incurred in turning off the BS's power amplifiers, we show how to decide whether it is convenient to keep the BS idle during those intervals in which no traffic has to be sent, or to turn off the amplifiers. Our model is evaluated by means of numerical examples, and shows that interesting power gain can be obtained under a large spectrum of load conditions.