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.
FORTUNE'S FUN FACTS TO KNOW AND TELL: #44 Zebras are colored with dark stripes on a light background.