As an end user power off or power on option is not available. you can only Add Host or Remove host from SDDC . however this should be done after doing a proper analysis, because at any point of time these hosts are part of vSAN cluster and a minimum requirement for the vSAN should always be full filled. in case of failure of any host in SDDC there is an automated Mechanism which restarts the faulting hosts and if even after reboot the host is not fixed It will automatically move towards the replacement of the host.
In addition to what was said, you may also be interested in the Elastic DRS feature (nice introduction from Frank D http://frankdenneman.nl/2018/07/17/introduction-elastic-drs/ ) which allows VMC to automatically scale up or down host capacity based on policies.