The most fundamental formula for Joule heating is the generalized power equation:
where
*  is the power (energy per unit time) converted from electrical energy to thermal energy,
*  is the current travelling through the resistor or other element,
*  is the voltage drop across the element.
The explanation of this formula () is:
Assuming the element behaves as a perfect resistor and that the power is completely converted into heat, the formula can be re-written by substituting Ohm's law, , into the generalized power equation:
where R is the resistance.
Voltage can be increased in DC circuits by connecting batteries or solar panels in series.