A gas furnace with 80% efficiency means that 80 out of 100 units of gas it uses will be converted into heat to warm your home, while the remaining 20 units will be lost as exhaust. A 95% efficient gas furnace will convert 95 units of gas into heat and only lose 5 units as exhaust. This means that the 95% furnace will use less gas and be more cost-effective in the long-term to heat your home. Additionally, a 95% furnace has additional features to capture more heat from the exhaust gases, making it even more efficient.