Megawatt Hour to Erg

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.

Astrophysics literature built decades of reference data in CGS units before SI became dominant. Key constants like solar luminosity (3.828 × 10³³ erg/s) and supernova energy (10⁵¹ erg, called a "foe") are baked into textbooks and databases. Switching to SI would require rewriting thousands of reference values, so the field maintains CGS by convention.

A core-collapse supernova releases roughly 3 × 10⁵³ ergs total, of which about 99% escapes as neutrinos. The visible light and kinetic energy of the ejected shell account for about 10⁵¹ ergs — a unit so common in astrophysics it has its own name: one "foe" (ten to the Fifty-One Ergs). In joules, that is 10⁴⁴ J, or the Sun's total output over 10 billion years.

An erg per second is the CGS unit of power, equivalent to 10⁻⁷ watts. Astronomers quote stellar luminosities in ergs per second because the numbers align well with astrophysical scales: the Sun emits 3.846 × 10³³ erg/s, and supernovae peak at ~10⁴³ erg/s. Using watts would give the same exponents minus seven — less tidy for a field that already juggles 40-digit numbers daily.

CGS (centimeter-gram-second) is a metric system that predates SI, formalised in the 1870s. It derives mechanical units from cm, g, and s: force in dynes (g·cm/s²) and energy in ergs (dyne·cm). CGS was standard in physics until the mid-20th century, and its Gaussian variant remains preferred in electromagnetism and astrophysics because Maxwell's equations take a simpler form.

One erg is 10⁻⁷ joules — roughly the kinetic energy of a mosquito in flight or the energy of a single grain of sand falling one centimeter. You would need about 10 million ergs to equal one joule, or 42 billion ergs to match the energy in a single dietary Calorie. The erg is useful precisely because atomic and astronomical quantities span so many orders of magnitude.