Many data centers rely on evaporative cooling, or “swamp cooling,” where warm air is drawn through wet pads. Data centers typically evaporate about 80% of the water they draw, discharging 20% back to a wastewater treatment facility, according to Shaolei Ren, an associate professor of electrical and computer engineering at the University of California, Riverside. Residential water usage, by comparison, loses just 10% to evaporation, discharging the other 90%, Ren said. (A spokesperson for Google said the company doesn’t have a standard percentage because any data center would see some variation based on factors like location, temperature and humidity.)