In case you don't have enough space in your home screen, a clear battery level indicator is available on status bar so that you can see it even when you are browsing the web or sending text messages. It can be configured to show in 3 different colors to reflect high/medium/low battery levels. You can also define your own alerts, so that Battery Widget will automatically notify you about battery status changes.
Battery Widget will not drain your battery, as it does only what it needs to do. When the device is idle, it is not listening to any battery information updates from the system if there are no active alerts