Quick answer: 3 inverter manufacturers publish 3 different lists of what modified sine wave breaks. They agree on 1 item: battery chargers. A CPAP, the example everyone names, is permitted on modified sine by its own maker without a humidifier. The GFCI warning, the one that matters for safety, appears on 1 list.
This comparison is usually written as a list of “sensitive electronics” with no source attached to it. So we went and read what the people who build these inverters actually put in writing, and the interesting thing is not what the lists contain. It is how little they overlap. For the wider decision, our power station sizing guide covers every tier, and if you are shopping for a modern unit the question may already be settled — the 1,000 Wh class ships pure sine as standard.
What the inverter makers actually publish
Three documents, three manufacturers, all of them describing their own products. Xantrex, in the Freedom Marine owner’s guide, puts it in a parenthesis: the output “is compatible with most appliances, tools and other 120 V AC equipment”, and then, “Certain laser printers, breadmakers, digital clocks and small battery chargers may not operate on modified sine wave.”
PowerBright, in the ML1500-24 and ML2300-24 guide, names a completely different set. Magnum Dimensions names a third: loads “that charge batteries, produce audio and video, or operate computers or processors will not operate properly, or may be damaged”, and separately “variable speed motors, communications equipment, and certain cordless tools”.
| Named as a problem | Xantrex | PowerBright | Magnum |
|---|---|---|---|
| Battery chargers | yes | yes | yes |
| Televisions | — | yes | yes, as audio and video |
| Laser printers, breadmakers, digital clocks | yes | — | — |
| GFCI devices | — | yes | — |
| Medical equipment | — | not tested | — |
| Computers, variable-speed motors, cordless tools | — | — | yes |
One row is unanimous and the rest are not. That is worth sitting with: these are not three opinions about a controversial subject, they are three descriptions of the same electrical phenomenon by companies that sell the thing. If the effects were well characterised, the lists would match.
Both of the owner’s guides end up in the same place: they tell you to ask someone else. PowerBright: “For electrically sensitive equipment, contact the manufacturer to determine if the device you are using is compatible with modified sine wave AC.” Its troubleshooting table has an entry for TV does not turn on, and the remedy is “Contact TV manufacturer to find out if the TV is compatible with a modified sine wave.” The company that built the inverter cannot tell you.
The warning that matters most is on one list out of three
Among all of it, one line stands apart, and it is PowerBright’s: “GFCI devices may not work with modified sine wave (MSW) power.”
Everything else on these lists is about whether an appliance runs, runs hot or dies. A ground-fault circuit interrupter is not an appliance. It is the device that cuts the power when current starts flowing through a person instead of through the circuit, and it is required by code in exactly the places where you are most likely to be standing in water — kitchens, bathrooms, basements, outdoors. An extension lead with a GFCI on it, or a station plugged into a GFCI outlet, is a safety layer you may be assuming you have.
Neither of the other two manufacturers mentions it. We are not in a position to say whose document is right, and we are not going to tell you how to test one. What we can say is that of everything published on this subject, this is the line to take to an electrician rather than to a comment thread.
The appliance everybody names, and what its maker says
Search this question and a CPAP will be the headline example, usually with a flat statement that it cannot run on modified sine wave. ResMed’s own battery guide is more precise than that, and it is the only appliance document in this site’s whole source library that answers the waveform question outright:
| Configuration | What ResMed specifies |
|---|---|
| Machine without a humidifier | 150 W modified sine wave permitted |
| Any device used with a humidifier | Pure sine wave, 300 W continuous, 500 W peak |
| Certain heated humidifiers, which it names | Must not be used with an inverter at all |
So the internet’s definitive pure-sine example is a device its manufacturer explicitly permits on modified sine wave, under a stated condition, and separately forbids on any inverter in certain configurations. Both halves of that are more useful than the blanket claim, and neither survives being compressed into a list. The runtime side of the same question is in what a 500 Wh station does with a CPAP.
This is electricity, not treatment. Whether your machine and its settings can change is a question for your clinician or equipment provider, and nothing here is a reason to alter them.
Why there is no number in this article
The figure that would settle the argument is total harmonic distortion, and it circulates freely: three per cent for pure sine, twenty, thirty or forty for modified, depending on which page you read. We could not find it published in any manufacturer document we were able to open, and the pages quoting it do not attribute it. So it is not here. What Magnum does put in writing is a consequence rather than a measurement: modified sine wave inverters “often do not have UL listings because the output is altered by wave shape distortion”.
There is a smaller discrepancy in plain sight too. Xantrex rates its modified sine output at 120 volts; PowerBright rates its own at 110 volts. Same waveform, same market, ten volts apart on the specification page, and neither explains it.
What this means if you are buying a power station
Mostly, that this is not your problem. Modified sine wave survives in cheap 12 V inverters that clip to a car battery, not in the portable stations this site sizes; the ones we have read specifications for, from 1,000 Wh upwards, all publish pure sine output. If you are choosing between two stations, the waveform is unlikely to be the difference between them.
The modern relative is worth knowing about, though. Boost modes that let a smaller inverter start a larger load do it by reducing the output voltage, which is a different compromise with the same shape of consequence: fine for a kettle, a question mark for anything that measures what it is being fed.
One last thing from the PowerBright guide, because it is the same habit this site keeps running into. It offers “WATTS X 2 = Starting Load for most appliances”, then immediately excepts “motorized appliances such as pumps, freezers and air conditioners”, which “can have startup loads of up to eight times the rated watts”. A multiplier with an exception that large is not a multiplier. We took that apart properly with the published figures for sump pumps, where the real ratio ranges from 1.54 to 2.88 inside one manufacturer’s own line, and the general case is starting watts versus running watts.
How we read this
Every item in the table above is quoted from a document published by the company that makes the product, opened and read in September 2026, and linked here so you can check it. A dash means that document is silent, which is not the same as a manufacturer saying an item is fine. We have not tested an inverter, we do not recommend one, and no product was tested by us and none is recommended. Our method is described at how we test.
What we deliberately did not do is merge the three lists into one and present it as the answer. They disagree, and the disagreement is the finding. A combined list would read as authoritative and would be no better sourced than the ones already circulating.
Before you rely on any of this
One manufacturer states that GFCI devices may not work on modified sine wave power. Do not assume ground-fault protection is present because an outlet or a lead has a test button on it. If that protection matters where you are working, ask a licensed electrician rather than testing it yourself.
PowerBright asks you to monitor a lead-acid battery charger for about ten minutes on modified sine wave and disconnect it if it becomes abnormally warm. It also states that its inverter is not tested for use with medical equipment.
Do not wire an inverter or a station into your home’s circuits, and never feed a house through a wall socket — that is backfeeding and it can kill line workers. A transfer switch is electrician work.
Frequently asked questions
Which appliances genuinely need pure sine wave?
On published manufacturer documents rather than repetition: battery chargers are the only item all three inverter makers name. Beyond that each names things the others do not, so the honest answer for any particular appliance is to check whether its maker says anything — and most say nothing at all.
Will a television work on a modified sine wave inverter?
PowerBright does not know. Its own troubleshooting table lists “TV does not turn on” and gives one remedy: contact the television’s manufacturer. Magnum lists audio and video equipment among the loads that may not work properly or may be damaged. Xantrex does not mention televisions.
Can a CPAP run on modified sine wave?
ResMed permits a 150 W modified sine wave inverter for the machine without a humidifier, requires pure sine at 300 W continuous and 500 W peak when a humidifier is in use, and names certain heated humidifiers that must not be used with an inverter at all. Read that page for your own model rather than a general answer, including this one.
Do portable power stations use modified sine wave?
The units whose specifications we have read publish pure sine output, so in practice this is a question about cheap 12 V inverters rather than about power stations. A UPS on battery is the likelier stepped waveform at a desk. Where a station advertises a much larger figure than its continuous rating, check what that figure means before assuming it is a clean surge.
The short version
If you are buying a portable power station, buy it for its capacity and its continuous output; the waveform is almost certainly pure sine already. If you are using a cheap modified sine inverter, treat battery chargers as the known problem, treat the GFCI warning as the serious one, and take the specific question about your own appliance to the company that built it — which is exactly what the inverter manufacturers tell you to do, because they cannot answer it either.
This article is general information, not an installation guide and not a substitute for a licensed electrician. Electrical work on a service panel, a transfer switch or a utility connection is regulated in most jurisdictions and normally requires a permit and an inspection. Fuel-burning equipment produces carbon monoxide and must never be run indoors, in a garage or near an opening into the home.
