Try:

on 500:Text:.access power*:#: {
  Msg $chan $nick Has accessed my central power control.
  timer 1 5 Notice $Nick Hello, you can use .power on ; .power off .power standby
}
On 500:Text:.power on*:#: {
  If (%power == on)  Notice $nick Denied: Power is on.
  elseIf (%power == off) { 
    Notice $nick Approved: Power is now on. 
    Msg $Chan $Nick has turned the power on. .enter to start.
    set %power on
  }
}
On 500:Text:.power standby*:#: {
  If (%power == on) {
    Msg $chan $nick has turned central power on standby, this is temporary to reduce lag. 
    set %power standby
    Ignore $chan
    .Timer 1 30 Ignore -rs $chan
    Msg $chan Standby is complete, enjoy.
    set %power on
  }
}
On 1:Text:.enter:#: {
  If (Power == on) {
    If ($nick,entered == yes)  Notice $nick you have entered!
    elseIf ($nick,entered == no)  Notice $nick Welcome to 
  }
}

Edit: I don\'t see why you check if (%power == on) and then set it as on