Batteries store DC, but most household appliances expect AC at mains voltage. An inverter is the component that bridges the two: it takes the low-voltage direct current from your battery and turns it into the alternating current a kettle, laptop, or fridge is built for. Knowing how it does that — and where it loses energy and hits its limits — is what lets you read an inverter's specifications instead of just its headline wattage.

This guide assumes you already know the difference between AC and DC and between power and energy.

How the conversion works

DC flows steadily in one direction; AC swings back and forth many times a second. To manufacture that swing, an inverter uses fast electronic switches arranged as a bridge that reverses the direction of current through a transformer, and the transformer steps the voltage up to mains level. Victron describes its Phoenix units, for example, as using a "full bridge plus toroidal transformer topology," producing an output in the region of 120 V or 230 V at 50 Hz or 60 Hz depending on the model and region.

Conversion is active electronic work, so it is never perfectly efficient, and the quality of the AC "swing" it produces varies between inverters. Those two facts drive most of what follows.

Waveform: pure sine vs modified sine

Not all AC out of an inverter looks the same. There are two broad kinds:

  • Pure (true) sine wave — a near-perfect replica of grid power, with low harmonic distortion. It runs anything the grid would.
  • Modified sine wave — a cheaper, stepped approximation. As one inverter maker puts it, it "converts the DC input in multiple steps to mimic a true sine wave curve," which leaves it with high harmonic distortion.

The difference is not cosmetic. Simple resistive loads — a kettle, a heater, an incandescent bulb — generally tolerate modified sine, though some devices "may be running hotter than normal." But sensitive electronics and many motors need pure sine: laptops and modern electronics, medical equipment such as CPAP machines, microwaves, and variable-speed or AC motors can misbehave, run inefficiently, or be damaged on a modified-sine supply. Unless you are certain every load is simple and resistive, choose a pure sine inverter — it is the safe default and what the rest of this guide assumes.

Continuous power vs peak power

An inverter carries two power ratings, and confusing them is a classic sizing error:

  • Continuous power — what it can supply indefinitely.
  • Peak (surge/start-up) power — a much larger figure it can supply for a short moment.

Peak power exists because some loads demand a brief inrush far above their running draw. Victron's datasheet notes the high start-up figure is "needed to start loads such as power converters for LED lamps, halogen lamps or electric tools," and motors are the classic case — a fridge compressor or pump can pull several times its running power for a fraction of a second at startup. Size the continuous rating to your steady load and the peak rating to your worst startup surge; a full treatment is in continuous power, peak power, and surge loads.

VA vs watts (and power factor)

Inverters are often rated in volt-amperes (VA), not only in watts — Victron's smaller Phoenix range, for instance, is listed as "250 VA – 1600 VA." The two units measure different things:

  • VA (apparent power) is voltage × current — the full electrical demand the inverter must supply.
  • Watts (real power) is the part that does useful work.

They are linked by the power factor (PF):

Real power (W) = Apparent power (VA) × Power factor

For a plain resistive load the power factor is about 1, so VA and watts are nearly equal. But reactive loads like motors have a power factor below 1 (often 0.6–0.85), so they demand more VA than watts. A motor drawing 700 W of real power at a power factor of 0.7 presents 700 ÷ 0.7 = 1 000 VA to the inverter — which therefore needs 1 000 VA of capacity even though only 700 W does work. Size on VA for reactive loads, or you will undersize the inverter.

Efficiency and idle draw

Because conversion is active work, an inverter is never 100% efficient. A good pure sine inverter is typically around 90–95% efficient (confirm the exact figure on the datasheet — this is precisely the kind of number the data-sheet reading guide teaches you to check). The missing few percent leaves as heat, which is why inverters have heatsinks and need ventilation.

Two idle behaviours matter for an off-grid battery:

  • No-load draw. An inverter consumes some power just being switched on, even with nothing plugged in — a small but 24-hour drain on your battery.
  • ECO / search mode. To limit that, many inverters offer a low-power standby that wakes when a load appears. Victron's units expose a configurable "ECO mode on/off and ECO mode sense level." Useful, but note it must sense the load to wake, so very small loads can be missed.

Why the DC current is large

Energy is conserved, so the power drawn from the battery is the AC output power plus the conversion losses:

DC input power   = AC output power ÷ efficiency
DC input current = DC input power ÷ battery voltage

Work it through for a 1 000 W load at 90% efficiency:

DC input power = 1 000 W ÷ 0.90 = 1 111 W
On 12 V: 1 111 ÷ 12 ≈ 93 A
On 24 V: 1 111 ÷ 24 ≈ 46 A
On 48 V: 1 111 ÷ 48 ≈ 23 A

Check the direction: doubling the system voltage halves the current, exactly as the arithmetic shows. Ninety-three amps is a lot — it is close to the rated discharge current of a typical 12 V, 100 Ah lithium battery, and it needs thick, well-fused cable. This is the core reason larger systems move to 24 V or 48 V: the same power flows at a quarter or an eighth of the current, so wiring is thinner, losses are lower, and the battery is less stressed. The battery and cable, not just the inverter, must be rated for this current — see battery storage fundamentals.

Protections and limits

A decent inverter protects itself and your system. Victron's datasheet describes units that are "short circuit proof and protected against overheating, whether due to overload or high ambient temperature," with a "dynamic cut-off" whose low-battery shutdown level depends on the load. These protections are features to look for, not guarantees to lean on: they respond to faults, they do not prevent you from undersizing the inverter or its cabling in the first place.

Safety essentials

  • The inverter's AC output is live mains voltage and can be lethal — treat it exactly like grid power, and leave AC wiring to methods your local code and the manufacturer specify.
  • The DC input current is large (tens to ~100 A). Conductors and overcurrent protection must be sized for the real current by the manufacturer's instructions and local code; undersized DC cable can overheat and cause a fire.
  • Give the inverter the ventilation its heat demands, and do not exceed its continuous rating expecting the peak rating to cover you.

Key takeaways

  • An inverter switches battery DC through a bridge and transformer to make mains AC; it is never 100% efficient, and the loss becomes heat.
  • Choose pure sine unless every load is simple and resistive.
  • Read continuous and peak power separately, and size reactive loads on VA, not just watts.
  • The DC input current is large and grows as system voltage falls — a key reason to choose 24 V or 48 V for bigger systems, with cable and protection sized to match.

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