British Thermal Units (IT) to Therm (EC)

BTU/hour expresses cooling or heating power, not energy alone. A 12,000 BTU(IT)/h rating corresponds to approximately 3,517 W of thermal power. Watts can describe thermal power as well as electrical input; the two quantities must not be confused. An air conditioner's electrical input depends on its efficiency and operating conditions, rather than being the same as its cooling capacity.

A rough rule of thumb is 20 BTU per square foot of living space in a temperate climate. A 300 sq ft bedroom needs about 6,000 BTU/h; a 1,500 sq ft open-plan living area needs roughly 30,000 BTU/h. Actual requirements vary with insulation, ceiling height, climate zone, and window area. Poorly insulated older homes may need 30–40 BTU per square foot.

BTU is a unit of energy (heat); BTU/h is a unit of power (rate of heat flow). When an air conditioner is labelled "12,000 BTU," the industry shorthand actually means 12,000 BTU per hour. Technically one BTU equals about 1,055 joules of energy, while 1 BTU/h equals about 0.293 watts. The distinction matters for energy calculations but is routinely blurred in product marketing.

Natural gas energy quantities may be billed in millions of BTU (MMBtu) or therms. A therm nominally represents 100,000 BTU, but the legal U.S. and EC therm variants have their own fixed joule definitions. This converter keeps those therm variants separate; neither should be assumed to equal exactly 100,000 BTU(IT). Always check the unit convention used on the bill.

The UK metricated energy units in the 1970s–1990s, switching gas billing from therms (100,000 BTU) to kilowatt-hours and scientific work to joules. The "British" in BTU reflects 19th-century British steam engineering origins, not current usage. Today the BTU is almost exclusively an American unit, used for HVAC, gas pricing, and appliance ratings across the US.

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.