One therm is 29.3071 kilowatt-hours. The therm is a gas billing unit defined as exactly 100,000 BTU, and it exists so that a gas bill and an electricity bill can be compared without a calculator — although in practice they are still printed in different units.
About the units
A therm is 100,000 British thermal units, which is 105.5 megajoules. It was adopted for gas billing because the energy content of natural gas varies with composition, so utilities meter the volume delivered and then multiply by a measured calorific value to arrive at an energy figure. The customer is billed for energy, not for gas. British bills state therms or kilowatt-hours depending on the supplier and the era; American bills state therms or hundreds of cubic feet. The unit has no scientific use whatsoever — it exists purely for commerce.
The exact factor
The derivation is 100,000 BTU × 1,055.05585262 J ÷ 3,600,000 J per kWh = 29.30710702 kWh. A useful set of equivalents: one therm is 105.5 MJ, one CCF of natural gas is very roughly one therm depending on gas quality, and a typical British household using gas for heating consumes 500 to 1,200 therms a year, which is 15,000 to 35,000 kWh.
Where you meet this conversion
Gas billing in the United Kingdom and the United States. Comparing the cost of gas heating to electric heating requires putting both on the same energy basis, and this is the conversion that does it — after which the price per kilowatt-hour can be compared directly, though efficiency differences between a boiler and a heat pump then have to be applied on top. Energy performance certificates, carbon accounting and fuel switching calculations all pass through the same step.