A 12v battery. I never knew DC was more efficient than AC! WOW GOOGLE IS SO COOL!
I'm not sure if you are being sarcastic. A 12v battery in the power supply is more efficient than taking DC -> AC -> DC. That is what a UPS does, each conversion introduces loss. Having the battery in the power supply means there is no conversion so less power loss.
The goal of science is to build better mousetraps. The goal of nature is to build better mice.