Pure sine wave inverters: what the output spec means

What the inverter is doing

The cells inside a power station deliver direct current at a steady voltage. A wall socket delivers alternating current whose voltage swings smoothly from one polarity to the other and back, sixty times a second. The inverter's job is to manufacture that swing from a flat direct supply.

It does it by switching. A set of transistors arranged in an H bridge flips the direction of the current through the output, thousands of times faster than the mains frequency, varying how long each pulse stays on. Wide pulses near the peak of the wave, narrow pulses near the zero crossing. A filter of inductors and capacitors then smooths that pulse train into a curve. The closer the result sits to a true sine, the less the appliance downstream can tell the difference from a socket.

A modified sine wave inverter skips almost all of that. It switches between a positive level, zero and a negative level, producing a stepped block wave. The average energy per cycle can be made correct, but the shape is not, and the harmonics that make up the difference are what causes the trouble described below.

What tolerates a stepped waveform, and what does not

Indifferent. Anything that just turns electricity into heat: a kettle, a hotplate, a space heater, an incandescent bulb, a soldering iron. A resistor does not care about waveform shape, only about how much energy arrives.

Damaged slowly. Induction motors in fridges, pumps, fans and power tools. The harmonics in a stepped wave produce currents that do no useful mechanical work but still heat the windings. A motor on a modified sine runs hotter, louder and shorter-lived, and a compressor may struggle to start at all. Transformers behave the same way, which affects any appliance with an old-style plug-in adapter, plus microwaves, which run their magnetron from a transformer.

Do not risk it. Medical equipment. CPAP machines, oxygen concentrators and anything with a mains-referenced sensor can misread, alarm or refuse to run. Variable-speed tools, some laser printers, mains-dimmed lighting and audio equipment fall in the same bracket, the last because the harmonics arrive as an audible buzz.

This is why the category settled on pure sine and why it has stopped being a differentiator between the models on the model index. It is still worth checking when you look at the very cheapest boxes outside this catalog.

Conversion efficiency, and why it is never published

Nothing about that switching is free. Transistors drop voltage while conducting and burn energy every time they change state. The filter inductors have resistance and magnetic losses. The control electronics need feeding. Put together, an appliance drawing a given wattage from the socket removes noticeably more than that from the battery.

Not one of the four brands in this catalog publishes a conversion efficiency figure. It is not in their specification tables, it is not in the manuals, and it varies with load anyway: an inverter is most efficient near its comfortable working range and least efficient at very low load, which is where a great many real appliances sit.

So this site assumes 85 percent for AC loads, states that it is an assumption, and shows it in the run time calculator rather than hiding it inside a result. Any runtime claim that does not tell you its inverter assumption is quietly choosing a flattering one. The method page sets out the rest of the assumptions this site makes.

Idle draw: the loss that empties a battery overnight

An inverter that is switched on but powering nothing still consumes power. It is running its switching stage, its control board, its display and often its fan. On units of this size that standing draw is typically a few watts to a few tens of watts, and it is not published either.

Over a night it adds up to a real share of a small pack. The classic mistake is leaving the AC output enabled to charge a phone through a plug adapter: the inverter runs all night to serve a load that a USB socket would have handled directly from the battery, without any conversion to alternating current at all.

Two habits fix it. Turn the AC output off when nothing needs it, and use the DC ports for anything that will accept them. Phones, tablets, laptops with USB-C charging, camera batteries, most camping lights and many fridges built for vehicles all run from DC. On a multi-day trip that choice moves the arithmetic more than the capacity difference between two neighbouring models does.

Surge is not one number, and two brands redefined it

A conventional surge rating is a brief tolerance to inrush current. A motor at the instant of starting looks almost like a short circuit, and draws several times its running current for a fraction of a second until it spins up. A surge figure says the inverter will supply that spike without shutting down, at full voltage, for a short moment.

BLUETTI's Lifting Power and EcoFlow's X-Boost are a different mechanism wearing similar words. They run an appliance rated above the unit's continuous output by lowering the output voltage. Less voltage means less power drawn, so the load fits inside the inverter's real capability.

For a resistive load that is a sensible trick: a kettle at reduced voltage simply heats more slowly, and nothing is harmed. For a motor it is the opposite of helpful. An undervolted motor draws more current to deliver the same torque, heats its windings, and can stall outright. For electronics and anything medical it is not worth attempting.

So the two figures must never be compared as though they were equivalent. BLUETTI Elite 200 V2 publishes 3,900 W (Lifting Power) alongside 2,600 W of continuous output. EcoFlow DELTA 3 publishes 3,600 W of surge and a separate X-Boost figure of 2,200 W on top of 1,800 W continuous. Read the continuous rating as the honest one, and treat the rest as a bonus that applies to heaters and kettles.

Reading the output section of a specification table

Continuous output is the number that decides what the unit will run. Outside the largest units such as Anker SOLIX F3800, most household appliances that matter fall under a couple of kilowatts, and 2,600 W or 4,000 W covers them comfortably. Anything approaching that ceiling should be checked against the appliance's own label rather than a generic table.

The outlets themselves are usually unnamed. Most makers state a number of AC sockets and a voltage, and stop. Whether a port is a standard household outlet or a higher-current one, which USB-C port carries the highest wattage when several are fitted, and which socket is live in pass-through mode are frequently absent from the published table. Where a maker does not state it, this site does not invent it.

Efficiency, idle draw and waveform distortion are absent almost everywhere. Their absence is not a small omission: between them they decide how much of a rated capacity actually reaches an appliance. Treat every published runtime as an upper bound, and see what the cells themselves contribute for the other half of that story.