JanJ
Newer Roku devices have the additional "Power" setting. (Settings/System/Power)
Older models do not have an additional "Power" setting. (Settings/System)
Model numbers can be found under Settings/System/About. Roku Ultra and Ultra LT models prior to the 4800/48xx series will not have the Power setting.
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Make sure to press Home on your Roku remote when you are finished streaming so that Roku returns to Home Screen. This will cease streaming data usage. (Some data will still be seen when Roku checks for updates and talks to Roku servers intermittently).
On older models, can also unplug them so they have no power. (no power = no data use). Or you can use with programmable timers/on-off switches/smart plugs at outlet to deprive them of power.
When doing the above, may have to manually check for Software Updates since device likely will miss its daily automatic Software update check. (Settings/System/Software Update [Check Now]