In the US, Rokus come with a power supply included in the box. Depending on the Roku model it will either be a USB power adapter to be used with an included USB cable or a dedicated "wall wart" power supply with an attached cord. (Some have reported a USB adapter may not be included in some geographical regions.)
The USB ports on some televisions, if they even have one, only supply 0.5 amp or less, which is not enough to power a Roku reliably. The preferred method is to plug the power supply directly to your house power.
If yours is a model powered by USB and you don't have a Roku adapter handy, any other you have from another device should work as long as the fine print on it says it outputs at least 1 amp.
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