Megawatt Hour to Therm (EC)

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.

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.