My intentionally minimalist design with PICOscope measured 8% THD does not comply with the THD specifications for grid tie inverters (5%). My primary goal for an off grid inverter was to have the lowest possible EMI, while being able to run single phase induction motors efficiently. I intentionally sacrificed THD to allow for slow switching times and reduced EMI, as long as motor efficiency was not significantly affected; keeping induction motors happy was my requirement. Most generators are in the 9-15% THD range. My ST-3 THD measured at 12%. So "generator" class THD was sufficient for my needs, and I did beat that, just, when I got rid of the waveform distortions of slow switching a 7 step sine and went to the simpler 5 step.
The large toroidal transformers will always be costly, and this will always pressure designers away from this approach, especially when EMI is not even considered.
For the low THD required for GTI inverters, the designer has no choice but to do 100x faster switching, even while using a low frequency design per the original 3 transformer Trace SW series. The sine wave must be built with many small steps; Trace used 32 steps per half wave. The same Trace SW approach with a modern microcontroller can be done, but even with very well designed snubbing, the faster switching design will require substantially more passive filtering of the AC and DC. AC filtering in particular is costly in efficiency. For example, only 10 microfarads of total capacitance line to line at 230VAC draws 1 amp (230 watts) continuously. Capacitance to ground in microfarad levels is largely a "beat the test" farce since the earthing system in the real world is NOT a copper plane from which all measurements are referenced. Capacitance to ground in larger values, for AC or DC typically just turns the entire grounding system into an EMI radiator. I combat that problem by spending a fortune on the earthing system; I use copper flashing, buried, with rods connected, and copper flashing extending all the way to the filter enclosure, typically.
The large toroidal transformers used in my design are heavy and expensive. The mass market for GTI's has driven them to make them smaller, cheaper, lighter. Low frequency, transformer isolated designs were well proven as being the most durable and reliable.
My usual plea to those adding inverters to their homes:
To avoid developing Electrical Sensitivity and to greatly reduce the risk of accelerated cancer growth and autoimmune diseases (arthritis, thyroid, diabetes, cardiac) I urge everyone with an inverter (GTI or off grid) to add a commercial grade two stage common mode choke filter to both PV input and AC output. This will greatly reduce the EMI radiating from the home and PV wiring. These passive filters are available at any big electronics component supplier, and they run $70-$250 each depending on size. They will not adversely affect inverter efficiency or performance. They are likely to provide -20 to -30dB worth of filtering up to 50MHz, where must of the switching EMI energy will be. DO NOT buy plug in "filters" such as Stetzer or Greenwave. They are nothing but 22uF motor run capacitors and will void the warranty of your inverter, will likely cause it to fail, dramatically reduce it's efficiency and will provide absolutely zero common mode filtering. Anyone promoting them is utterly incompetent, knows nothing of the engineering field of EMC, and should be given a good tar and feather job.