MJ means megajoule, and mega is a prefix that means "million".
1cal=4,184J
It may be better answered by asking it backwards: "How many Joules are in a kWh?" 1 Joule is 1 Watt-Second. Knowing this, you can derive that 3,600 Joules = 1 watt-hour. 3,600,000 Joules = 1...
1 BTU = ~1,055.056 joules
1/1000