Is there any tool that tells me the min and max battery consumption rate of an app?
Apps don't consume battery. The device consumes the battery as it does things like run apps. So the only way to measure battery use is physically.
Fortunately, devices record battery use themselves. There is most often an area in Settings where you can view the approximate percentage of battery used by apps, cell standby, etc.
You could also kill/remove everything except your app and see how long the battery lasts on a full charge, and compare it to how long the battery lasts without your app running.
The best way to ensure low battery use is good design, though. Your app should not be busy waiting, otherwise it will consume much more battery than needed. You also mention (I think) that your app won't work if the phone is rebooted. Plenty of other alarm apps work after the phone is rebooted, so I think you need to look into that.