Inverter DC Current Calculator

Work out the DC current a marine inverter draws from the battery bank, and check it against the BMS continuous and peak ratings before you buy.

By GabrielReviewed September 2026

Inverter DC current calculator

An inverter's AC rating tells you what it can run. The number that sizes your BMS, cable, busbar and fuse is the DC current it pulls from the bank — and that is the figure most often missing from a quote.

Inverter AC output

The largest load you expect to run, not the inverter's badge.

System voltage

12.8 V nominal

Inverter efficiency 88%

Typical inverters sit in the high 80s to low 90s at useful load.

Surge factor 2x

Motor and compressor starts draw well above running current.

178

A continuous from the bank at 12.8 V

Calculation: 2,000 W ÷ 0.88 ÷ 12.8 V = 178 A
Surge current to allow: 355 A at 2x
Same load at other voltages: 89 A at 24V · 44 A at 48V
Cable for a 3 m run, 3% drop: 50 mm² or larger

Check this current against the BMS continuous rating and the surge figure against its peak rating — they are different specifications, and marketing material often quotes only the larger one. The cable suggestion is a voltage-drop calculation only; current rating, bundling, ambient temperature, insulation type and AS/NZS 3004.2 all still apply.

An inverter's AC rating tells you what appliances it can run. It tells you almost nothing about whether your battery system can support it.

The number that matters is the DC current drawn from the bank, and it is the figure most often absent from an installation quote.

Why the DC current surprises people

Converting DC to AC is not free, and low system voltage multiplies the current. A 3,000W inverter on a 12V system can require well over 250A from the battery once conversion losses are counted. That single number determines:

the BMS continuous current rating you need,
the cable size between battery and inverter,
the busbar and switch ratings,
the fuse selection,
and whether 12V is the right system voltage at all.

Choose the inverter from simultaneous loads

A 3,000W inverter is not automatically better than a 2,000W one. Larger inverters can have higher idle consumption, and they demand heavier DC infrastructure throughout.

Size from the appliances that will genuinely run at the same time. On many boats, staggering the kettle, the microwave and the induction hob — a habit, not a purchase — avoids an inverter upgrade entirely, along with the cabling and BMS capacity that would have come with it.

Continuous is not peak

Motors and compressors draw far more current starting than running. Air-conditioning compressors, washing-machine motors and some pumps all do this.

Check two things separately:

The continuous DC current against the BMS continuous rating.
The surge current against the BMS peak rating, and how long the BMS will tolerate it.

These are different specifications, and marketing material frequently quotes only the larger, more impressive one. A battery advertised with a high peak figure may have a much lower continuous rating.

Idle draw matters on a boat

An inverter left on continuously consumes energy doing nothing. Over a 24-hour period that idle draw can rival a fridge. If the inverter only serves occasional loads, switching it off between uses may save more energy than any efficiency difference between models.