V = I × R, and P = V × I. Fill in any two values and the other two are worked out. The power cards below use power factor, so a motor or discharge load is not treated as a pure resistor.

Voltage, current, resistance and watts

Leave the unknowns blank. If you fill in more than two, the first consistent pair is used and anything that disagrees is flagged.

Single-phase kilowatts

kW = V × I × power factor ÷ 1,000. Use 1 for a resistive load such as a heater.

Three-phase kilowatts

kW = √3 × line voltage × line current × power factor ÷ 1,000. 400 V line is about 230 V phase.

Guide arithmetic only. It does not allow for cable temperature, motor efficiency, or the difference between displacement power factor and true power factor on electronic loads.

These tools are quick guides for a ballpark figure. They are not certified design or test software, and a result on this page is not a design, a certificate, or a substitute for the current edition of BS 7671. Check the working and cross-reference it before you apply it to a real installation. Thanet Electrical Ltd does not accept liability for loss or cost arising from reliance on these figures, except where the law does not allow that liability to be excluded, including death or personal injury caused by negligence.

  All tools