dbus: org.freedesktop.UPower: EnergyRate is a positive value

The EnergyRate is a positive value since most of the laptops are ACPI based system and report the power status through power_now sysfs attribute. Moreover, the ACPI specification mention that the battery power status is a positive value, measured in watt.

https://learn.microsoft.com/en-us/windows-hardware/design/component-guidelines/acpi-battery-and-power-subsystem-firmware-implementation

Fix: #252 (closed)

Signed-off-by: Kate Hsuan hpa@redhat.com