The app uses Bluetooth Low Energy (BLE) and the Google / Apple Exposure Notification system which is designed to have minimal impact on your mobile phone battery, especially if you normally have Bluetooth on.
If your phone battery is draining much faster than usual, you can check how much battery the app is using compared to other apps. You can find this on your phone by following these instructions (the steps may be slightly different, depending on your phone):
Apple iOS: Settings -> Battery
Android: Settings -> Device Care -> Battery
Under day-to-day phone use, battery usage by Exposure Notifications should be under 5%. However, if your phone is idle (for example if you are not using it much), Exposure Notifications may contribute to higher percentages of battery consumption. This does not necessarily mean the app or Exposure notifications are using more battery overall.
When determining how much battery is being used by the app, you should check over the course of a week to get a better understanding of battery activity.
You can find a breakdown of battery use in your phone’s settings. The breakdown of battery usage will depend on what combination of apps are running on your phone at that time. If a lot of apps are being used, each app will show a smaller percentage of battery usage. However, if a single app is running, this may appear as 100% battery usage at that specified point. It is therefore important to consider which other apps may also be in use.