Inverter vs conventional generators
The short answer: a conventional generator runs its engine at a fixed speed to make 60 Hz power directly. An inverter generator makes power at any speed, then converts it electronically to clean 60 Hz. That lets an inverter slow down at light load, so it's quieter and uses less fuel. A conventional generator usually gives you more watts for the money and copes better with big motor starts.
How each one makes power
Conventional: the engine turns the alternator directly, and the output frequency depends on engine speed. To make 60 Hz, a typical portable runs at a steady 3,600 rpm whether you're powering one lamp or the whole load. A governor holds that speed as the load changes.
Inverter: the alternator makes AC at whatever frequency the engine speed gives. Electronics turn that into DC, then build a fresh 60 Hz sine wave from it. Because the output no longer depends on engine speed, the engine can idle down when the load is light and speed up when it isn't.
Think of a car without a gearbox versus one with a continuously variable transmission. The first has to rev the engine to a set speed to go anywhere; the second can cruise at low revs and only rev up when you ask for power.
What the difference means for you
Noise and fuel at light load
An inverter's biggest practical advantage. Overnight, a fridge and a few lights are a small load, and an inverter generator can run its engine slowly for them. A conventional generator still runs at full speed. That's also why inverters tend to use less fuel on a light load. Many have an economy mode that controls this.
Power quality
An inverter builds its sine wave electronically, so its output usually has low distortion. A conventional generator's waveform depends on its alternator design. Good ones are clean; cheap ones can be rougher, and their voltage and frequency wander more as loads switch on and off.
Most modern electronics, like phone chargers, laptops and TVs, tolerate a less-than-perfect waveform. Equipment with sensitive electronic controls can be fussier. If an appliance's manual says what generator power it needs, follow it. The spec to compare is total harmonic distortion (THD): lower is cleaner.
Starting motors
An inverter's surge is limited by its electronics. It can exceed its running rating only briefly, and when the limit is hit, it protects itself by cutting out. A conventional generator's alternator can push a large current for a moment, and its voltage dips instead. So for the same running rating, a conventional generator often starts a well pump or central air conditioner more easily.
Whichever you choose, check its starting rating against your biggest motor. See running watts vs starting watts and locked-rotor amps. If an inverter has an economy mode, owner's manuals commonly advise turning it off when you need to start large motors, because the engine has to speed up first.
Size, 240 V and price
- Small inverter generators are usually lighter and more compact than conventional ones of the same rating.
- Many small inverters have 120 V outlets only. Running a well pump or central air needs a 240 V outlet, which is more common on larger units of both types.
- Many inverter models can be paired with a second identical unit using a parallel kit, to double the output.
- Per watt, conventional generators usually cost less, especially at larger sizes.
Which one to choose
- Mostly light loads, quiet matters, or you'll run it overnight near people: an inverter.
- Big motor loads, 240 V, or the most watts for your money: a conventional generator, or a large inverter with a 240 V outlet and enough starting rating.
Either way, the size comes from your load. Work it out in the calculator. Battery power stations are inverters too, just without an engine. See how long a power station will run a fridge.
Both types make carbon monoxide. Run them outside only, at least 20 feet from the house, never in a garage. See what size generator do I need for the safety rules.
Show the math
Frequency from a conventional alternator f (Hz) = poles × rpm ÷ 120 A 2-pole alternator at 3,600 rpm f = 2 × 3,600 ÷ 120 = 60 Hz A 4-pole alternator at 1,800 rpm f = 4 × 1,800 ÷ 120 = 60 Hz If the engine slows by 5% (3,600 → 3,420 rpm), a 2-pole conventional generator's output drops to f = 2 × 3,420 ÷ 120 = 57 Hz An inverter's output stays at 60 Hz, because the electronics set it. Total harmonic distortion THD = √(V₂² + V₃² + V₄² + …) ÷ V₁ V₁ = the 60 Hz part of the voltage V₂, V₃, … = the parts at 2×, 3×, … 60 Hz
References
- ISO 8528-1:2018, Reciprocating internal combustion engine driven alternating current generating sets, Part 1: Application, ratings and performance.
- ISO 8528-5, Reciprocating internal combustion engine driven alternating current generating sets, Part 5: Generating sets (frequency and voltage performance when loads change).
- NEMA MG 1, Motors and Generators (synchronous generators, including the speed, poles and frequency relationship).
- IEEE Std 519-2022, IEEE Standard for Harmonic Control in Electric Power Systems (definition of total harmonic distortion).
- IEEE Std 1159-2019, IEEE Recommended Practice for Monitoring Electric Power Quality.
- IEEE Std 3002.7-2018, Recommended Practice for Conducting Motor-Starting Studies and Analysis of Industrial and Commercial Power Systems.
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