"The mathematical equation that describes this relationship is:

    V = IR

where V is the potential difference between two points of interest in volts, I is the current in amperes, and R is a circuit parameter, measured in ohms (which is equivalent to volts per ampere), and is called the resistance."

Since we are lazy we go to an Ohm's law calculator site:

http://www.onlineconversion.com/ohms_law.htm

We plug in our 12V and 1000W to get amperage needed at 12V and find we need 83.3A of power.

HTH,