How many joules in a kilowatt?

One watt is defined as one joule per second. So one kilowatt is defined as 1000 joules per second.

Don't confuse Energy (joules) with Power (watts). Power is the rate of production of energy, so it must have dimensions which include per second or per hour, or per some other unit of time.

Actually, the definition of joule is a unit of work just like the definition of watt -- it's the amount of work done by one newton (0.101971621 kilograms) done over the distance of 1 meter. here's how merriam-webster defines it -

Definition of JOULE

: a unit of work or energy equal to the work done by a force of one newton acting through a distance of one meter

here's how they define a watt -

Definition of WATT

: the absolute meter-kilogram-second unit of power equal to the work done at the rate of one joule per second or to the power produced by a current of one ampere across a potential difference of one volt : 1⁄746 horsepower

so you can see, joules are also expressed in seconds, hours, etc. a watt can be used to express a total power without using it as a time based quantity, just like joules, when viewing it as to how much power, energy, what have you, at any given time slice. a circuit whose load is 1 amp, and has 1 volt across that load, is using 1 watt. in 1 minute, the total energy used would be 60 joules. the thing is, not all equations need joules expressed in time -- they just use the total amount of joules required (E=MC2 for example says you can create a given mass (in kilograms) if you have enough energy (in joules) divided by the speed of light squared (in kilometers per second -- i.e. 300,000 squared). thus in theory, you can create 1kg of mass with 90000000000 joules of energy. so, to make a long explanation just slightly longer, you can make 1kg of mass with 90 GW (gigawatts) of energy applied for 1 second. pretty nifty, but seems to require an awful lot of power to make a small amount of mass.