Food labels
A food Calorie, written with a capital C or as kcal, is 1,000 small calories, which is 4.184 kJ. Labels in Europe and Australia give kilojoules first: a 2,000 kcal day is 8,368 kJ. To read kJ as Calories, divide by 4.184: 500 kJ is about 120 kcal.
Electricity and heating
A kilowatt-hour is 3.6 million joules: a 1,000-watt heater running for an hour. Gas and heating are often billed in kWh or BTU, and 1 kWh is 3,412.14 BTU. Air conditioners are rated in BTU per hour, which is power rather than energy: a 12,000 BTU/h unit moves 3.52 kW of heat.
Questions
Which calorie is used here?
The thermochemical calorie, exactly 4.184 J, which is what food labels use. The BTU is the international table BTU, 1,055.06 J. Other definitions differ from these in the fourth significant figure and are not listed.
Can it turn watts into kWh?
Not by itself, because watts are power and kWh energy. Multiply the watts by the hours and divide by 1,000: a 60 W bulb on for 10 hours uses 0.6 kWh.