Energy is an abstraction representing approximately the maximum amount of calories can be used in a day. The lower it gets, the less effective you are, because your body is resorting to more difficult-to-extract energy sources (fat or protein instead of blood sugar, depleted glycogen reserves in muscles, etc). Additionally, your metabolism can only actually absorb so much nutrients in a given day, it takes time to digest food and extract nutrients from it.

The exact biological functions aren't really relevant to gameplay, but they are physical processes to base our equations on.

Points of reference

• A small, sedentary person uses ~1500 kcal/day
• An average male soldier marching all day uses ~6000 kcal/day
• An Olympic athlete in an endurance event can use as high as ~12000 kcal/day
• Calories metabolized by humans per-gram
  • Fat: 9
  • Protein: 4
  • Uncooked starch: 2
  • Cooked starch: 4
  • Sugars: 4
  • Cellulose: 0.2
  • Alcohol: 7

Equations

Current food and water needs

Food and water advance only with a character's authoritative strategic clock. Settlement life currently assumes that ordinary meals and drinking water are provided, including lazy catch-up and explicit rest. Entering a settlement restores one day of short-term food and hydration reserve and refills every owned waterskin.

Travel uses 6,000 kcal and 4 litres of water per full day, applied proportionally for partial days. A travel ration supplies 6,000 kcal. A waterskin carries 4 litres. Characters automatically eat personal rations and drink carried water whenever their short-term reserve would otherwise become negative.

Unsupported hunger reaches full strategic incapacitation after three marching days beyond the food reserve. Unsupported thirst reaches it after one marching day beyond the hydration reserve. Both curves are quadratic, begin only below zero reserve, and combine with pain, blood loss, fear, and fatigue. They do not currently kill a character.

The character panel presents each signed physiological balance as a centered meter. Positive reserve fills right, normalized to the ordinary one-day reserve. Once the balance crosses zero, hunger or thirst fills left according to its quadratic incapacitation contribution. Carried rations and water are not included; they affect the meter only when automatically consumed.