The main objectives of the design of wireless networks is to answer the increasing demand of users in terms of capacity and coverage. However, a new challenge for future wireless networks consists in limiting the energy consumption with a limited impact on the network performance. Therefore, reducing the base stations (BS) transmit power should be an objective while designing green cellular networks. However, decreasing the BSs transmit powers may degrade network performance. This paper proposes an evaluation of the degradation of the capacity and coverage/quality of service (in terms of outage probability) due to a reduction of the BS transmit power. In this aim, we first establish closed form formulas of the capacity and coverage/QoS (quality of service), taking into account a dynamic traffic. This allows to quantify in a simple way the fundamental importance of the BS transmit power. We particularly show that the transmit power can be decreased, without impacting the quality of service, as long as a small capacity decrease is accepted according to networks characteristics.