Gigajoule to Erg

In cold-climate countries, 30–60 GJ per year is common for heating and hot water. A well-insulated modern home in Germany might use 20 GJ; a drafty older home in Canada might use 100+ GJ. Australians use about 40 GJ/year on average. Each gigajoule costs roughly $8–$15 depending on local gas prices.

One tonne of coal holds roughly 24–30 GJ depending on grade. One tonne of crude oil contains about 42–44 GJ. One tonne of LNG holds roughly 54 GJ. One tonne of dry firewood stores about 16 GJ. These figures explain why oil and gas are preferred for transport — they pack more gigajoules per kilogram than solid fuels.

One gigajoule equals 277.78 kWh. At an average electricity price of $0.15/kWh, one gigajoule of electrical energy costs about $42. The same gigajoule from natural gas costs $8–15. This price gap is the main reason gas boilers remain popular for heating in countries with cheap pipeline gas.

A single-aisle jet like the Boeing 737-800 burns about 10–12 GJ per flight hour. A six-hour transatlantic flight on a wide-body aircraft can consume 300–400 GJ of jet fuel. The entire global aviation industry uses roughly 12 billion gigajoules of fuel per year — about 3% of total world energy consumption.

At 2,000 kcal/day (8.4 MJ/day), a person consumes about 3.07 GJ of food energy per year. Over 80 years, that is roughly 245 GJ — equivalent to about 6,000 liters of petrol. Your entire lifetime food energy would fit in a medium-sized fuel tanker, which is a humbling thought.

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.