Evaporative coolers (ECs) provide significant gains in energy efficiency compared to vapor compression air conditioners, but simultaneously have significant onsite water demand. This can be a major barrier to deployment in areas of the world with hot and arid climates. To address this concern, this study estimated where in the world evaporative cooling is suitable, the water consumption of ECs in these cities, and the potential that greywater can be used to reduce the consumption of potable water in ECs. ECs covered 69% of the cities where room air conditioners (ACs) are likely to be deployed, based on comfort conditions. Varying with climate, water consumption due to ECs ranged from 200 to 650 L/household/day, with the potential for greywater to provide 100% to 40% of this amount, respectively, based on conservative cooling load calculations. In the Mediterranean, the Middle East, Northern India, and the Midwestern and Southwestern United States alkalinity levels are high and water used for bleeding will likely contribute significantly to EC water consumption. Upfront costs for household GW systems are variable, however, in many cases the combined cost of an EC and GW system can be lower than comparable vapor compression ACs. Moreover, regions of the world that face problems of water scarcity find that the benefits substantially outweigh the costs.