Input Power Calculator solves input power, output power, efficiency, or steady DC, single-phase AC and balanced three-phase AC real power. It also estimates output and loss when equipment efficiency is supplied; the heat equivalent assumes losses dissipate locally.

Passive equipment model. AC uses RMS values and true power factor; three-phase mode assumes a balanced system and line-to-line voltage.

Required for input/output solves. In electrical mode, blank omits output and loss estimates.

Input Power Formula

The following formula applies to steady DC. For single-phase AC it gives real power only at unity true power factor; balanced three-phase line quantities require the √3 factor.

IP = IV × II
  • Where IP is the Input Power (watts)
  • IV is the input voltage (volts) 
  • II is the input current (amps) 

To calculate the input power for DC, multiply the input voltage by the input current. For AC real power, power factor matters: single-phase real power is P = VRMS · IRMS · PF, and balanced three-phase real power is P = √3 · VLL,RMS · IL,RMS · true PF. Use true PF = P/S, including distortion; displacement cosφ alone may not suffice. The AC magnitude sqrt(S² − P²) is non-active power, and equals reactive magnitude only under sinusoidal conditions. Efficiency is useful output divided by real input; it is separate from power factor. See Fluke’s power-factor explanation.

How to Calculate Input Power?

The following two example problems outline how to calculate the Input Power.

Example Problem #1:

  1. First, determine the input voltage (volts). In this example, the input voltage (volts) is measured to be 86.
  2. Next, determine the input current (amps). For this problem, the input current (amps) is measured to be 54.
  3. Finally, calculate the Input Power using the formula above: 

IP = IV * II

Inserting the values from above and solving the equation with the input values gives: 

IP = 86 * 54 = 4644 (watts)

Example Problem #2: 

Using the same process as example problem 1, we first define the variables outlined by the formula. In this case, the values are:

input voltage (volts) = 4

input current (amps) = 56

Entering these values into the formula or calculator above gives us 

IP = 4 * 56 = 224 (watts)