Ultracapacitors use nanomaterials to store a static charge on either side of a vast surface area of insulating material. The voltages are low, but the charge density is high. Ultracapacitors exceed lead-acid energy densities, and lab units far exceed the capacity of conventional batteries. The technology continues to evolve, as energy storage capacity increases and costs decrease. For the data center, we expect ultracapacitors to become an option to replace batteries as the backup power source for an uninterruptible power supply (UPS).