Formulas and sources
Each formula this calculator uses, where it comes from, and what it leaves out. "How it works" shows the same formula with your own numbers.
Power, DC
DC mode
- P
- power, in watts
- V
- voltage, in volts
- I
- current, in amperes
- R
- resistance, in ohms
From current and resistance, V = I × R is worked out first; from voltage and resistance, I = V ÷ R.
Source: Uses the SI's definition of the volt as one watt per ampere (BIPM) [1].
Power, single-phase AC
AC 1-Phase mode
- P
- real power, in watts
- V
- RMS voltage
- I
- RMS current
- pf
- power factor: real power ÷ apparent power, 0 to 1
- Z
- impedance magnitude, so V = I × Z
Source: Uses the IEC definitions of apparent power (RMS voltage × RMS current) and of power factor (real power ÷ apparent power) in IEC 60050-131 [2].
Power, three-phase AC
AC 3-Phase mode
- P
- real power, in watts
- V
- line-to-line RMS voltage
- I
- line current
- pf
- power factor, 0 to 1
- Z
- impedance per phase
A balanced load. An impedance is per phase, so V = √3 × I × Z.
Source: Standard formula; no single publisher.
Apparent and reactive power
AC 1-Phase and AC 3-Phase modes
- S
- apparent power, in volt-amperes (VA)
- Q
- reactive power, in var
- P
- real power, from the line above
On AC 3-Phase, S = √3 × V × I. Both are exact for a sinusoidal supply.
Source: Uses IEC 60050-131: apparent power is RMS voltage × RMS current, and real and reactive power are the two parts of it, so S² = P² + Q² [2]. The var is the SI Brochure’s name for the unit of reactive power [3].
Energy and cost
With Calculate Energy Consumption & Cost on
- E
- energy a day, in kWh
- P
- real power, in watts
- h
- hours the load runs a day
Cost is energy × your price per kWh. A month is 365 ÷ 12 days and a year is 365.
Source: Standard formula; no single publisher. The month of 365 ÷ 12 days is CalcTrail's own; no publisher.
Sources
- Bureau International des Poids et Mesures (BIPM). The International System of Units (SI), section 2.3.4, Table 4, p. 133. 9th edition (2019), version 4.01, June 2026.
- International Electrotechnical Commission (IEC). IEC 60050-131, International Electrotechnical Vocabulary, Circuit theory, IEV 131-11-41 to 131-11-46, apparent, active and reactive power, the var, power factor. Electropedia entry, read October 5, 2026.
- Bureau International des Poids et Mesures (BIPM). The International System of Units (SI), Table 8 and its note (n), pp. 140-141. 9th edition (2019), version 4.01, June 2026.
What this calculator does not cover
- This multiplies the RMS voltage, current and power factor you enter. It does not model harmonics, an unbalanced three-phase load, a delta-connected impedance, motor starting current or efficiency.
- The energy cost assumes the load draws the same power every hour it runs.
Sources checked on .