Kilocalorie (nutritional) to Watt Hour

Most weight-loss guidelines recommend a deficit of 500 kcal/day below your maintenance level, which typically means 1,200–1,800 kcal/day for most adults. A 500 kcal/day deficit yields roughly 0.45 kg (1 lb) of fat loss per week, since one kilogram of body fat stores about 7,700 kcal. Going below 1,200 kcal/day is generally not recommended without medical supervision.

Almond cell walls are rigid and resist digestion — about 20% of the fat in whole almonds passes through the gut unabsorbed. A USDA study found that almonds provide ~129 kcal per 28 g serving, not the 170 kcal on the label. Walnuts and pistachios show similar discrepancies of 5–20%. Food labels use standard Atwater factors that assume full digestibility, which overestimates usable energy for structurally intact whole foods like nuts, seeds, and legumes.

The Atwater system assigns 9 kcal per gram of fat, 4 kcal per gram of protein, and 4 kcal per gram of carbohydrate. Alcohol provides 7 kcal/g. These rounded values have been the basis of food labeling since the 1890s. Actual digestibility varies — fiber-rich carbohydrates yield fewer usable kcal because the body cannot fully break them down.

A marathon (42.195 km) burns approximately 2,200–3,200 kcal depending on body weight, pace, and efficiency. A 70 kg runner typically burns about 2,600 kcal; an 85 kg runner about 3,100 kcal. That is roughly equivalent to 35 bananas or 13 slices of pizza. Elite runners complete the distance in about 2 hours, so their metabolic rate during the race exceeds 1,300 kcal/hour.

It uses the FAO food-energy relationship of 1 kcal = 4.184 kJ = 4,184 J, matching the thermochemical kilocalorie factor. The International Table kilocalorie is a different variant equal to 4,186.8 J; it is not used by either nutritional selection here. The nutritional Calorie (Cal) and nutritional kilocalorie (kcal) selections are aliases for the same energy scale.

Watt-hours account for both current and voltage, giving the true energy stored. A 10,000 mAh power bank at 3.7 V holds 37 Wh, but at 5 V output it delivers only about 7,400 mAh due to voltage conversion losses. Airlines use the Wh rating (max 100 Wh carry-on) because it reflects actual energy — and therefore actual fire risk — regardless of battery voltage.

Most smartphones have batteries rated at 10–18 Wh. An iPhone 15 Pro holds about 12.7 Wh; a Samsung Galaxy S24 Ultra about 18.4 Wh. For context, fully charging an 18 Wh phone from a wall outlet costs less than 0.01 kWh — roughly one-tenth of a cent on a typical electricity bill.

Most airlines allow lithium-ion batteries up to 100 Wh in carry-on luggage without approval. Batteries between 100 and 160 Wh (e.g., large camera or drone batteries) require airline permission, and batteries above 160 Wh are banned from passenger flights. A standard laptop battery is 50–100 Wh; a large power tool battery can exceed 160 Wh.

Watt-hours map directly to how consumers think about devices: a 50 Wh battery powering a 10 W laptop lasts about 5 hours — simple division. Expressing the same battery as 180,000 joules gives no intuitive sense of runtime. Airlines also adopted Wh for lithium battery safety limits (100 Wh carry-on threshold) because it communicates energy density risk in a unit engineers and passengers can both grasp.

A typical laptop battery holds 50–100 Wh, so a full charge from empty uses 50–100 Wh of energy (plus about 10–15% lost as heat in the charger). At average US electricity rates, that is roughly 1–2 cents per charge. Over a year of daily charging, a laptop costs about $4–$7 in electricity — far less than most people assume.