Having the Revolar app running in background will not dramatically decrease your smartphone's battery nor will it dramatically use up your data.
Keep in mind your location is only shared with your loved ones when an alert is active. By using GPS only when needed (ie. during an alert), this not only helps protect your privacy, it also helps with not wasting data.
The Revolar wearable is able to conserve energy by using Bluetooth® Low-Energy technology (also known as Bluetooth Smart), and only connects when it needs to. This means a longer battery life for both your wearable and your Smartphone.