Web applications are run on a wide variety of devices, many of which are battery powered. Using modern web standards, it is possible to gain information about the battery level and charging state. This information may be used to adjust the operation of web applications. For example, applications can access the battery level of a device to calculate if there is enough remaining energy to complete the current task. If not, the application can react by offering several options such as charging the device for a specific amount of time or changing the energy source. The user may be notified once the battery is charged sufficiently. Data may be lost if power is lost before data is saved. Accordingly, monitoring the battery level may enable an application to save data or prompt a user to save data when the battery level reaches a predetermined threshold.