Megawatt Hour to Kilocalorie (th)

MWh is the natural unit for grid-scale transactions because power plants and large industrial loads operate in the megawatt range. Quoting in kWh would produce unwieldy numbers — a 1 GW nuclear plant generates 24,000 MWh/day, not 24,000,000 kWh. Spot markets like the US PJM or European EPEX quote prices in $/MWh or €/MWh, typically $20–$80/MWh in normal conditions.

One MWh powers the average US home for about 1.1 months (since the average is 886 kWh/month). In Europe, where consumption is lower (~300 kWh/month), one MWh can cover about 3.3 months. A single MWh is also enough energy to drive an electric car about 5,000–6,000 km, or to run an industrial air compressor for roughly 4 hours.

US wholesale prices typically range from $20 to $80/MWh depending on region, time of day, and fuel costs. European prices are generally higher at €50–€150/MWh. During extreme events — heat waves, supply shortages — prices can spike above $1,000/MWh for brief periods. Negative prices (below $0/MWh) also occur when wind or solar oversupply the grid.

A modern onshore 3 MW turbine at 35% capacity factor produces about 9,200 MWh/year. A large offshore 15 MW turbine at 50% capacity factor generates roughly 65,700 MWh/year. Capacity factor — the percentage of theoretical maximum output actually achieved — varies with wind resource, turbine technology, and maintenance downtime.

Current lithium-ion battery costs (~$150–250/kWh) make 4-hour systems economical for peak shaving and solar time-shifting, but 24-hour storage would cost 6× more with diminishing returns. Grids instead layer solutions: batteries handle the evening peak (4 h), gas turbines cover overnight baseload, and pumped hydro or compressed air provide longer-duration backup. Iron-air and flow batteries are emerging for 100+ hour storage at lower cost per kWh, potentially closing the gap by the 2030s.

Most foundational biochemical data — ATP hydrolysis (~7.3 kcal/mol), glucose oxidation (~686 kcal/mol), amino acid combustion values — were measured and published in kcal th before SI adoption. Rewriting decades of literature, lecture notes, and exam banks to kJ would introduce conversion errors and confusion. The field maintains kcal th by convention while acknowledging SI equivalents.

The standard free energy change (ΔG°) for ATP → ADP + Pi is approximately −7.3 kcal th/mol (−30.5 kJ/mol). Under actual cellular conditions, the value is closer to −12 to −14 kcal/mol because reactant and product concentrations differ from standard state. This energy drives muscle contraction, nerve impulses, protein synthesis, and virtually every energy-requiring process in living cells.

The classic Atwater factors (4 kcal/g carb, 4 kcal/g protein, 9 kcal/g fat) are averages from 19th-century bomb calorimetry, adjusted for digestibility. They can be off by 5–25% for specific foods. Almonds deliver ~20% fewer usable calories than labels claim because cell walls trap some fat from digestion. High-fiber foods also overcount. The FDA allows ±20% tolerance on label accuracy, so a "200 kcal" bar could legally contain 160–240 kcal.

Complete aerobic oxidation of one mole of glucose (C₆H₁₂O₆) releases approximately 686 kcal th (2,870 kJ). The human body captures about 38–40% of this in ATP; the rest dissipates as body heat. This is why exercise makes you warm — over half the food energy your muscles consume is released as thermal energy rather than mechanical work.

Fat molecules are highly reduced — their carbon atoms are bonded mostly to hydrogen, with very little oxygen. Oxidising them releases maximum energy because every C-H bond is converted to C=O and O-H bonds. Carbohydrates are already partially oxidised (they contain oxygen in their structure), so less additional oxidation is possible. Gram for gram, fat stores 2.25× more energy, which is why evolution favored fat as the body's long-term energy reserve — it packs the most kcal per gram of tissue weight.