What are the chances of Energy Siphon appearing for a reward

If you're here from the future, the damage calculator pointed out by Oblivious Sage is outdated as of Damage 2.0. Warframe's wikia site has an up-to-date, extensive paragraph on damage calculation, along with formulas.

The system is quite complex, but the gist is:

• Enemies (and NPCs, and warframes) are made of "health materials" - ie. different types of health, shields and armor.
• Every "health material" applies different multiplier against different damage types.
• When you attack, the final damage within your attack is calculated separately for each damage type.
  • Health is a plain, old health bar. If it's depleted, the enemy dies.
  • Shields is a separate health bar that serves as a protection for health bar. If it's depleted, the damage goes to health.
  • Armor is not a health bar. It adds its own multipliers on top of health's multiplier (cumulatively) and reduces damage the health takes. However,
    • the more armor the enemy has, the more weight armor's multiplier has over health's multiplier,
    • damage types strong against armor reduce that protection (dealing more damage), while damage types weak against armor increase that protection (dealing less damage) - on top of the damage multiplier.

When you use it you get a small amount of XP, the amount of xp is larger if it is a stealth scan and will count as 2 scans.

Since the Codex Scanner shows boxes and can see them through walls it can be used to spot hidden rooms with boxes.

It is also very handy for spotting enemies through walls if you are trying to do stealth missions.

Before the release of the Codex scanner the developers said on a live stream that maxing data on a target would give a bonus towards the target like extra crit, but at this time there is no indication that this feature has been added.