Enter the voltage and current into the calculator to determine the running watts (real power). For AC loads, you can also enter the power factor (PF); if PF is left blank, the calculator assumes PF = 1.0.

Running Watts Calculator

Enter any 2 of Voltage, Current, or Running Watts to calculate the missing value (Power Factor is optional; if omitted, PF = 1.0).

PF

Running Watts Formula

The following formula is used to calculate the running watts (real power) for a single-phase AC load (using RMS volts and amps) or for DC/resistive loads (where PF = 1):

 RW = V * I * PF

Variables:

  • RW is the running watts (W), also called real power
  • V is the voltage (V)
  • I is the current (A)
  • PF is the power factor (unitless, typically between 0 and 1 for most loads)

To calculate the running watts, multiply the voltage by the current and then multiply by the power factor (PF). If PF = 1 (common for DC and purely resistive loads), the formula reduces to RW = V × I.

What are Running Watts?

Running watts, also known as continuous watts, refer to the amount of real power a device or appliance needs to operate under normal conditions. This value is commonly used when sizing generators, though generators may also be rated in volt-amps (VA) or kVA (apparent power). For DC or purely resistive loads, running watts can be found using W = V × I. For AC loads where the power factor is not 1, real power is W = V × I × PF, and V × I by itself represents apparent power (VA). For example, if a device operates at 120 volts and draws 10 amperes at PF = 1, it uses 1200 W; at PF = 0.9 it uses 1080 W. It’s also important to note that some devices may require more power to start up (known as starting watts or peak watts) than they do to continue running.

How to Calculate Running Watts?

The following steps outline how to calculate the Running Watts.

  1. First, determine the voltage (V) of the appliance.
  2. Next, determine the current (I) drawn by the appliance.
  3. Next, determine the power factor (PF) (for AC loads). If unknown and you only have volts and amps, you are calculating apparent power (VA) rather than real power (W).
  4. Finally, calculate the Running Watts (real power) using RW = V × I × PF (for DC or resistive loads, PF = 1).
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

voltage (V) = 120

current (I) = 5

power factor (PF) = 0.9

running watts (RW) = 120 × 5 × 0.9 = 540 W