Engineering Guide

AC Power Factor, Supply Current, Voltage Drop, and Runtime Budgeting

A useful power budget follows energy from the source through conversion and distribution to the load. Real power, apparent power, current, voltage drop, usable battery energy, peak demand, and time-weighted runtime must remain distinct.

PowerIntermediate19 min readUpdated October 7, 2026

Source-to-Load Workflow

Step	Action	Engineering Check
Define load states	List operating, peak, startup, standby, sleep, and fault power with duration or duty cycle.
Separate AC quantities	Keep real power, apparent power, reactive power, displacement, distortion, RMS voltage, and RMS current distinct.
Refer load to source	Include converter efficiency, power factor, quiescent demand, and simultaneous-load assumptions.
Size current paths	Calculate continuous, peak, and transient current at each voltage rail rather than transferring current unchanged between rails.
Budget distribution	Calculate source, cable, connector, switch, fuse, and PCB voltage drops at the relevant current and temperature.
Budget energy	Convert capacity to usable watt-hours at the actual voltage, discharge rate, temperature, and SOC window.
Estimate runtime	Divide usable energy by time-weighted average input power, then check peak-current and cutoff constraints.
Validate	Measure RMS input, real power, rail current, endpoint voltage, temperature, capacity, and shutdown behavior over corners.

Power and Energy Quantities

Quantity	Meaning	Unit or Boundary
Real power P	Energy converted per unit time	W
Apparent power S	Vrms x Irms	VA
Reactive power Q	Quadrature power component for sinusoidal conditions	var
Power factor	P/S	Dimensionless; includes waveform distortion in total PF
DC load power	Vload x Iload	W
Source input power	Pload/efficiency plus other losses	W
Voltage drop	Current x path resistance	V
Battery energy	Voltage x ampere-hours	Wh; voltage is operating-point dependent
Runtime	Usable energy/average input power	h; only a first-order energy estimate

AC Power Factor and Source Current

Formula reference

Single-phase source model

S = Vrms x IrmsPF = P/SIrms = P/(Vrms x PF)Q = sqrt(S squared - P squared) for the applicable sinusoidal model

Variable definitions

Total PF includes displacement and distortion

Use true RMS measurements for nonsinusoidal current

Efficiency is not power factor

Use the Power Factor Calculator for AC relationships.

Supply Current and Distribution

Formula reference

Conversion and path model

Pinput = Pload/efficiencyIdc,input = Pinput/VinputVdrop = I x RpathPloss,path = I squared x RpathVload = Vsource - Vdrop

Variable definitions

Use source-side current for source wiring

Rpath includes the complete loop

Check continuous and peak states separately

Use the Power Supply Current Calculator and verify delivery with the Voltage Drop Calculator.

Battery Energy and Runtime

Formula reference

First-order energy budget

Enominal(Wh) approximately Vnominal x Capacity(Ah)Eusable = Enominal x usable fraction x conversion efficiencyPaverage = sum(Pstate x duty state)Runtime = Eusable/Paverage

Variable definitions

Actual battery voltage changes during discharge

Rate, temperature, aging, cutoff, and peaks reduce usable energy

Do not use rounded display values in chained calculations

Use the Battery Capacity Runtime Calculator for an energy estimate and validate against the battery discharge specification.

Worked Examples

AC current

A 500 W load at 230 V and PF 0.8 draws about 2.72 A RMS, not 2.17 A.

Apparent power

The same 500 W, PF 0.8 load requires 625 VA from the source.

DC source current

A 60 W load supplied from 24 V through a 90% efficient converter needs about 2.78 A at the 24 V source.

Rail conversion

A 5 V, 4 A load is 20 W; from 12 V at 85% efficiency it draws about 1.96 A, not 4 A.

Path drop

A 2.78 A source current through 0.18 ohm round-trip resistance drops 0.50 V and loses about 1.39 W.

Load voltage

A 24.0 V source minus 0.50 V distribution drop leaves 23.5 V at the converter input.

Nominal energy

A nominal 12 V, 10 Ah battery is a 120 Wh label-level estimate.

Usable energy

At 80% usable capacity and 90% conversion efficiency, 120 Wh yields 86.4 Wh at the load.

Runtime

An 86.4 Wh usable budget at 18 W average load gives a first-order 4.8 h runtime.

Duty cycle

A 30 W active load at 20% duty plus 2 W idle at 80% duty averages 7.6 W before conversion losses.

Common Mistakes

Using watts and volt-amperes interchangeably.

Calling cos(phi) the total power factor for every nonlinear load.

Sizing AC current with P/V while ignoring power factor.

Transferring load current unchanged across a converter voltage ratio.

Applying efficiency in the wrong direction.

Adding independent peak currents that cannot occur together, or averaging peaks that do occur together.

Ignoring round-trip resistance and connector loss.

Calculating voltage drop at room temperature only.

Using nominal battery Ah without voltage, SOC window, rate, temperature, or aging effects.

Using peak power instead of time-weighted average power for runtime.

Using average power alone without checking peak-current cutoff.

Treating a first-order runtime result as a guaranteed service time.

Related Calculators

Power FactorOpen Calculator Power Supply CurrentOpen Calculator Voltage DropOpen Calculator Battery Capacity RuntimeOpen Calculator

Support reference

FAQ

What is the difference between watts and volt-amperes?

Watts measure real power; volt-amperes measure apparent power from RMS voltage times RMS current. Their ratio is power factor.

How do I calculate AC current from power?

For single-phase AC, use current equal to real power divided by RMS voltage and total power factor, subject to waveform and topology assumptions.

Is power factor the same as efficiency?

No. Power factor relates real to apparent input power. Efficiency relates useful output power to real input power.

How do I calculate power-supply input current?

Divide required output power by efficiency to get real input power, then divide by source voltage for DC or by RMS voltage and power factor for single-phase AC.

Why is input current different from output current?

A converter transfers power between different voltages and also has losses. Current changes approximately with the inverse voltage ratio after efficiency is included.

How do I calculate voltage drop?

Multiply current by total path resistance, including supply and return conductors, connectors, protection devices, switches, and PCB paths.

How much voltage-drop margin should I allow?

Set the limit from the load minimum input, source tolerance, transient behavior, regulation, temperature, and reliability requirements rather than one universal percentage.

How do I convert battery capacity to energy?

Multiply ampere-hours by an appropriate battery voltage for a first estimate. Accurate energy requires integrating voltage over discharge.

How do I estimate battery runtime?

Divide usable battery energy by time-weighted average source power, then apply conversion, temperature, rate, aging, and SOC-window derating.

Why does actual runtime differ from the calculation?

Battery voltage curve, rate capability, cutoff, temperature, aging, converter efficiency, standby demand, and load peaks all change usable energy.

Should I size a supply from average or peak current?

Check both. Thermal and energy budgets use suitable averages, while regulation, current limit, wiring, connectors, and startup must tolerate relevant peaks.

How should I validate a power budget?

Measure real input power, RMS current, rail currents, endpoint voltages, peak events, temperatures, battery discharge, and shutdown threshold under real operating modes.

Summary

Separate real and apparent power, refer load power through conversion efficiency, calculate current at each rail, close the voltage-drop loop, and estimate runtime from usable energy and time-weighted source power. Verify both average budgets and peak constraints on hardware.