An electrical element is a resistive load. If one knows a element's rated power, one can determine it's resistance. An element's resistance does not change.
element_resistance = voltage * voltage / wattage
A 2000W, 120V element has a resistance in the range of:
element_resistance = 120 * 120 / 2000 = 7.2 ohms
element_wattage = 120 * 120 / 7.2 = 2000W
While wattage is amperage multiplied by voltage, the equations above take advantage of Ohms law, which states that E (voltage) = I (current, a.k.a amps) * R (resistance). If we want to determine the I (amps) for a given voltage and resistance we divide voltage by resistance. The part of the equation equation above that divides 120 by 7.2 gives us the current draw for this element.
How much current (amps) does the element above draw?
I (amps) = E (voltage) / R (resistance)
I = 120 / 7.2 = 16.67 amps, which means that one cannot plug this element into a 15A circuit because it will blow the breaker.
The National Electrical Code (NEC) states that no more than 80% of a circuit's rated current carrying capacity can be drawn continuously. Eighty percent of 15A is 12A. A 1650W element draws 13.75A (1650 / 120 = 13.75). However, that part of the code is for continuous current draws lasting more than 3 hours. As long as the current draw is not for more than 3 hours continuous operation, an element of up to 1800W (120 * 15 = 1800) can be operated on a 15A circuit. A directly immersed 1800W element is capable of bringing 6 to 7 gallons of wort to a boil; however, it will not be fast.
One last thing, the reason why using a 240V-rated element on a 120V-rated circuit requires one to divide the rated is wattage by 4 can be seen if we use the equation above. A voltage or 240 is 2 * 120; thus, the square of 240 is 4 times the square of 120.
element_resistance = (2 * 120) * (2 *120) / 5500
We can factor out the 2s, resulting in an equation that demonstrates this physical property:
element_resistance = 2 * 2 * 120 * 120 / 5500 = 4 * 120 * 120 / 5500 = ~10.47 Ohms
The element's resistance does not change when operated on a 120V circuit.
element_wattage = 120 * 120 / 10.47 = 1375W
We can verify this calculation by dividing 5500 by 4, which equals 1375