The Seattle City Council passed an ordinance on Monday protecting the personal data of Seattle City Light customers from being sold. The ordinance passed as new “smart meters” are installed in homes around the city.

Smart meters, which are part of City Light’s Advanced Metering Infrastructure program, frequently collect data on a customer’s energy use and can show exactly how much is being used. The meters track total energy consumption in 15-minute intervals and upload the data to Seattle City Light six times per day.

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“This new ordinance will provide basic protections for every person in Seattle who buys power from the City of Seattle,” said Shankar Narayan, Technology and Liberty Program Director for the ACLU of Washington.

Narayan has claimed that the data can then be analyzed to show what people are doing in their homes at any given time, like cooking, sleeping, or what devices are being used.

The ordinance says the collected data cannot be sold and can only be used for legitimate purposes such as utility services, grid management, or energy efficiency programs.

Seattle City Light began installing smart meters in 2016 and expects to have the entire system installed by 2019.

Customers who do not want the advanced smart meters can opt out.