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Ohm's law and power

The four-way relationship between voltage, current, resistance and power, and what it implies for part ratings and heat.

Ohm's Law CalculatorEnter any two of voltage, current, resistance and power, and get the other two with every step shown. The trap is power: it goes up with the SQUARE of voltage or current, so doubling either one quadruples the heat.
Resistor Power Rating and Derating CalculatorA power rating is a promise about cool air, not about the part. Work out what a resistor may actually dissipate at the temperature it will really see, and what rating to order so it is not running at its limit.
Electricity Energy and Running Cost CalculatorWatts are a rate; a bill is charged on energy. Turn a power draw and a duty cycle into kilowatt hours and money, and see why the hours matter more than the wattage.
Wire Resistance Calculator by Gauge, Length and MaterialEvery conductor is a resistor. Get the resistance of a run of wire from its AWG gauge, its length and what it is made of, with the cross-section and the temperature correction shown.
Voltage Drop Calculator for a Cable RunA long cable is a resistor in series with your load, and it takes its share of the voltage. See what reaches the far end, what fraction is lost, and the smallest gauge that stays inside the usual 3% limit.