Adders are one of the most important digital components used in any arithmetic applications. Many improvements in past have been made to improve its architecture. In this paper, we present two new symmetric designs for Energy efficient full adder cells featuring GDI (Gate-Diffusion Input) logic. The main design objectives for these adder modules are to operate at Low-Power with reduced area but also provide full-voltage swing. In the first (AEG-FA) design, a new approach of Inverted and Non-Inverted Carry-ins were taken to give complementary Carry-out and Sum with desired performance. These were then applied in different combinations to form higher bit width Adder architecture. This provides a higher degree of design freedom to target a wide range of applications, hence reducing design efforts. The second (PEG-FA) design is based on conventional approach which tries to reduce the critical path delay and lower switching activity in GDI circuit, providing Low-Power and high speed digital component at full voltage swing circuit. Many of the previously reported adders in literature suffered from the problems of low-swing and high noise when operated at low supply voltages. These two new designs successfully operate at low voltage with high signal integrity and driving capability. In order to evaluate the performance of proposed full adders, we incorporated 8-bit ripple carry adders. The studied circuits are optimized for energy efficiency using 45 nm CMOS process technology. The comparison between these novel circuits with standard full adder cells shows improvement in terms of Area, Delay, Power and Power-Delay-Product (PDP), Area-Delay Product (ADP), Area-Power Product (APP). At architecture level proposed adder shows 12.8% over CMOS, 14.8% over hybrid and 11.4% over other GDI logic power savings, by having almost 55% reduction in area.