Bind textbox to float value. Unable to input dot / comma

Try adding a StringFormat definition to the binding. Like so:

<TextBox Name="txtPower" Height="23" 
    TextWrapping="Wrap" Text="{Binding Path=Power, Mode=TwoWay, 
    UpdateSourceTrigger=PropertyChanged,StringFormat=N2}"></TextBox>

If you have .NET 4.5 or newer, you may enforce the pre 4.5 behaviour

System.Windows.FrameworkCompatibilityPreferences.KeepTextBoxDisplaySynchronizedWithTextProperty = false;

See Sebastian Lux's blog: With .NET 4.5 it is no longer possible to enter a separator character (comma or dot) with UpdateSourceTrigger = PropertyChanged by default. Microsoft says, this intended.