How this calculator works
A watt is one joule of energy transferred per second. For a resistor, voltage and current also obey Ohm's law, allowing the power equation to be rearranged around any pair of positive known values. The calculated resistance is an operating-point value and may change with temperature. Reactive AC loads require power factor rather than the purely resistive model used here.
Formula
P = VI = I²R = V²/R; V = IR. For known P and R, I = sqrt(P/R) and V = sqrt(PR).
Worked example
A 12 V supply delivering 2 A to a resistor transfers 24 W and sees 6 ohms. A 4 ohm resistor dissipating 16 W carries 2 A with 8 V across it.
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Frequently asked questions
Can I use an AC appliance rating?
Only if the load is effectively resistive. Use watts to amps with power factor for other AC loads.
Does a 100 W power supply always use 100 W?
No. Its rating describes capacity; the actual load and supply losses determine consumption.
How do I get watt-hours?
Multiply average watts by hours. Use the related watt-hour calculator for energy and battery charge modes.