Private Sub Worksheet_Change(ByVal Target As Range)
    Application.EnableEvents = 0
    If Range("C27") = "" Then Range("C27") = "Укажите присоединяемую мощность в кВт"
    Application.EnableEvents = 1
End Sub

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
    If Not Intersect(Target, Range("C27")) Is Nothing Then
        Application.EnableEvents = 0
        If Range("C27") = "Укажите присоединяемую мощность в кВт" Then Range("C27").ClearContents
        Application.EnableEvents = 1
    Else
        If Range("C27") = "" Then Range("C27") = "Укажите присоединяемую мощность в кВт"
    End If
    If Target.Cells.Count > 1 Then Exit Sub
    If Not Intersect(Target, Range("C24")) Is Nothing Then
        slancalendar.Show
        Target = slancalendar.Value
    End If
End Sub