PowerCalcLab

Inverter calculator

Inverter DC Current Calculator

Convert an inverter's AC load into the DC watts and amps the battery must actually supply. Use voltage under load—not only the nominal label—to avoid understating current.

Advanced assumptions

Results update as you type. The calculator runs entirely in your browser — nothing you enter is sent anywhere.

Estimated DC operating current111.6 A139.4 A with the entered design margin

Calculation breakdown

Total DC input power1,316.3 W
Design current139.4 A
Entered battery/BMS limit150 A
Battery/BMS current checkPass
DC current = (AC load ÷ inverter efficiency + idle W) ÷ DC voltage
= (1200 ÷ 0.92 + 12) ÷ 11.8
= 111.6 A; 139.4 A with margin
  • Use the lowest credible DC voltage under load. Current rises as battery voltage falls.
  • The design-current figure is a planning check, not a conductor, fuse or breaker prescription.

Before you rely on this

High DC current can overheat conductors and connections. Have cable size, voltage drop, fusing, disconnects, BMS limits and installation rules verified for the actual system.

Method

How the number is reached

The inverter must draw more DC power than it delivers as AC because conversion is not lossless. Its own consumption is added before dividing by the battery voltage measured or expected under load.

A separate planning margin is shown because a nominal operating estimate should not be mistaken for a protection or conductor rating.

DC W = AC load W ÷ efficiency + inverter idle W
Operating A = DC W ÷ DC voltage under load
Design A = operating A × (1 + margin)

Symbols

DC
Direct-current battery side
AC
Alternating-current load side

Worked examples

The same maths, applied

Example

1,200 W AC load on a 12 V-class battery

Battery voltage is 11.8 V under load, inverter efficiency is 92%, idle draw is 12 W and design margin is 25%.

DC power = 1,200 ÷ 0.92 + 12 = 1,316.3 W
Operating current = 1,316.3 ÷ 11.8 = 111.6 A
Design current = 111.6 × 1.25 = 139.4 A

Result: 111.6 A operating and 139.4 A with margin.

Limits of the model

What it assumes, and where it stops

Assumptions

The AC load and efficiency represent the same steady operating point.

Not covered

Does not calculate conductor ampacity, voltage drop, fuse class, short-circuit current or code compliance.

Sources

Where these figures come from

  • Inverter Smart — Technical specificationsVictron EnergyManufacturer ratings that distinguish continuous watts, continuous VA, temperature derating, peak power, maximum efficiency and zero-load consumption.
  • SI UnitsNIST Office of Weights and MeasuresDefinitions of the volt, ampere, watt and joule used for the unit conventions on this site.
  • Wiring Unlimited — Battery bank wiringVictron EnergyManufacturer guidance for series, parallel and series/parallel battery banks, balanced current paths and connection safety.

Next steps

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Last meaningful update: 2026-08-19. This date changes only when the model, the sources or the guidance change.