Pure Sine Wave vs Modified Sine Wave Inverters
A pure sine wave inverter makes a smooth, rolling output. A modified sine wave inverter makes a blocky, stepped one. Both turn battery power into AC. The difference is in the shape.
What a sine wave is
Mains power in a wall outlet follows a sine wave. Wikipedia's page on alternating current shows a sinusoidal alternating voltage rising and falling in a smooth curve over one cycle1. Most appliances were designed with that shape in mind.
An inverter can only copy it. Some copy it closely and some roughly.
How modified sine wave is built
The simpler design is the modified sine wave. It's "the sum of two square waves", one delayed against the other2. The output steps through a repeated sequence: "zero, peak positive, zero, peak negative, and again zero"2. It's a rough approximation, drawn with straight steps.
The same source says most inexpensive consumer power inverters produce a modified sine wave rather than a pure sine wave2. Price is not a reliable guide to the type, so check the spec sheet.
How pure sine wave is built
Makers often use the phrase "pure sine wave inverter" to mark inverters whose output has much less distortion than the stepped design2. Wikipedia adds a caution: almost all consumer inverters sold as "pure sine wave" "do not produce a smooth sine wave output at all"2. The source describes the output as "just a less choppy output" than the two-step and three-step designs2.
So the label isn't a guarantee of a smooth shape. Check whether the maker publishes a distortion figure, often written as THD, and what conditions it's measured under.
What the shape changes
Several effects follow from the stepped shape.
- Extra harmonics, which are unwanted frequencies mixed into the output.
- Some equipment may hum or run less efficiently on stepped output.
- The same source says pure sine wave inverters "may provide significantly higher efficiency" than modified ones, because a maker's rated conversion efficiency doesn't account for harmonics2.
Which devices actually care is covered on Which Devices Care About Waveform.
A buyer's checklist
Don't guess the type from the price or the shape of the case. Look for it in writing.
- Find the words "pure sine wave" or "modified sine wave" in the manual or spec sheet.
- If the listing doesn't say, treat the type as unknown and ask the maker.
- Look for a distortion figure and what it was measured against.
- List the equipment you plan to run, then check each item's own manual for any waveform advice.
- Note whether the rating is for continuous or surge output. The rating units are explained on this page about watts and VA.
Where the equipment maker says "pure sine wave only", believe them. The manual for the device you're powering outranks general advice on a website like this one.
Battery systems and waveform
Home battery systems and larger power stations vary. Some state their output type clearly, and some leave it to the data sheet. Treat a missing statement as a question to put to the seller or maker, not as a good sign.
Permanent installations are an electrician's job, and an installer can tell you what waveform the product delivers. Don't wire anything into a house yourself.
Where the facts come from
- Alternating current, Wikipedia
- Power inverter, Wikipedia
More on inverters
What an Inverter Does in a Battery SystemThe inverter turns stored battery power into the kind of electricity your outlets use. Here's what it does and what to look at on one.
DC and AC Power: Why Batteries Need an InverterBatteries store DC, but household outlets and appliances use AC. Here's the difference and why a battery can't power them directly.
Which Devices Care About WaveformSome equipment runs fine on a modified sine wave and some doesn't. Here's how to judge what's likely to cope and how to check.