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A new study on water usage inside U.S. homes found toilets led the way for the highest water use, followed closely by showers, while dishwashers used the least.

The new research also comes with some surprises, including the strong association of humidifiers to high water usage, while other findings may be less surprising, such as that heavily regulated cities in California having the lowest water usage in the study. The research is in Earth's Future.

With data from more than 26,000 single-family homes across 39 cities, the study had a larger data pool than previous research. It is one of the first and largest snapshots of how households in the United States use water inside their home, with previous research not distinguishing between indoor and outdoor water use.

Landon Marston, an author on the study, said research into indoor water use was difficult before this study.

"We didn't have a great understanding of how much water was being used or how it was being used inside homes," said Marston, a researcher at Virginia Tech. "So, we kind of relied on these crude estimates to pinpoint that, and it's not that precise."

The data was provided by Flume, a company that sells sensors that can help detect water leaks in homes. The sensor monitors water usage and uses machine learning to identify which appliance is using the water. The result is then confirmed by the people in the household.

Marston and his team focused on the cold winter months of December and January. "In the summer, you might have some misclassification of water use. For example, outdoor water use mistakenly classified as indoor or vice versa," Marston said.

Las Vegas, Phoenix and Tampa ranked the highest cities for water usage while the lowest cities all fell in California. Marston and his team speculated in the research that California may rank that low due to state water conservation regulations. On average, cities with low water usage tended to be warmer and wetter compared to cities with high water usage that were cold and dry. In drier cities, indoor features like humidifiers were more common.

"What was really a driving factor was toilet events and showers," said Marston. "We now look at this across the nation and find a similar pattern. That's really a driving force."

The researchers hope the information could help communities adapt water usage plans to better fit. "Conservation efforts focused on reducing indoor residential water use can be most effective if they target these two end uses," said Yunus Naseri, lead author of the study.

Toilets used around 40% of all indoor water in all household types. Showers followed toilets, using about 30% of water. Faucets and other miscellaneous objects made up the rest.

Homes built before 1985 were found to be associated with lower water usage compared to homes built later. Cities with low water use often had a higher number of these older homes. Homes built after 1985 were more likely to use humidifiers, which were more common in high use cities and were associated with an increase in water usage in the homes they were found in.

Marston noted there are limitations to the study. The sensors were self-bought by consumers, some with rebate incentives. The data was collected by Flume for internal use but provided to researchers for the purpose of the study. This process could have influenced the outcome, while still providing a large dataset with which to build off. However, this research serves as a good pilot program to open up more questions on how Americans use water, Marston said.