Tons of TNT to Therm (EC)

One ton of TNT releases 4.184 GJ — roughly the energy of 120 liters of petrol or the electricity an average US home uses in 1.2 days. In blast terms, one ton of TNT in open air produces lethal overpressure within about 15–20 meters and can shatter windows at 100+ meters. The MOAB bomb (11 tons TNT) flattened structures across a 150-meter radius.

They differ by factors of 1,000: 1 kiloton = 1,000 tons, 1 megaton = 1,000,000 tons. Conventional bombs are rated in tons (the MOAB is 11 tons). Tactical nuclear weapons are rated in kilotons (Hiroshima was ~15 kt). Strategic thermonuclear warheads are rated in megatons (modern US warheads are 0.3–0.475 Mt). The scale spans nine orders of magnitude.

The 2020 Beirut explosion was caused by 2,750 tonnes of ammonium nitrate and was estimated at roughly 500–1,100 tons of TNT equivalent. For comparison, the Oklahoma City bombing (1995) was about 2 tons TNT equivalent, and the Halifax explosion (1917) was roughly 2,900 tons. Beirut ranked among the largest non-nuclear explosions in history.

Small meteor airbursts are rated in tons or kilotons of TNT. The 2013 Chelyabinsk meteor released about 440,000 tons (440 kt) — roughly 30 times the Hiroshima bomb. The Tunguska event (1908) is estimated at 3–15 megatons. The Chicxulub asteroid that ended the dinosaurs released roughly 100 trillion tons (100 million megatons) of TNT equivalent energy.

The largest planned non-nuclear explosion was the British demolition of Heligoland fortifications in 1947, using 6,700 tons of TNT equivalent. The largest accidental explosion was the Halifax harbour disaster (1917) at roughly 2,900 tons. The largest conventional bomb, the US GBU-43/B MOAB, yields about 11 tons — tiny compared to accidental industrial blasts.

The EC therm is defined as exactly 105,505,600 joules; the US therm is 105,480,400 joules — a difference of 25,200 J (about 0.024%). The discrepancy arose from slightly different historical BTU definitions. For residential gas billing the difference is negligible, but in large-scale energy trading involving millions of therms, the distinction can affect settlement amounts.

The UK Gas Act 1995 mandated a switch from therms to kWh as part of broader metrication. One therm (EC) equals 29.3071 kWh. The change aligned gas billing with electricity billing, making it easier for consumers to compare energy costs. Older UK customers and industry veterans still refer to therms colloquially, and wholesale gas markets continued using therms for years after the retail switch.

A typical UK home uses 500–800 therms (EC) per year for heating and hot water, equivalent to roughly 14,700–23,400 kWh. Well-insulated newer homes may use under 400 therms, while large Victorian houses with poor insulation can exceed 1,200 therms. Ofgem's energy price cap is set in pence per kWh, but converting back to therms gives about £2.50–£3.50 per therm at recent rates.

One cubic meter of UK pipeline-quality natural gas contains roughly 38.5–39.5 MJ, which is about 0.365–0.374 therms (EC). Gas meters measure volume in cubic meters, and the utility applies a calorific value correction to convert to kWh (or therms). The correction factor varies by region and season because gas composition changes depending on the source field.

The therm (EC) was once the standard trading unit on the UK's NBP (National Balancing Point) gas market. In 2020, the ICE exchange switched NBP contracts from pence per therm to pence per kWh. Continental European hubs like TTF have always traded in euros per MWh. The therm is fading from professional use but remains in legacy contracts and older billing systems.