I'm an experienced EE by profession and education, a mixed-signal RF/analog/digital guy, and I tested this unit six ways to Sunday (i.e. extensively, see images). It does everything the brochure says it will do, and fully loaded (okay, almost fully, 253 watts of incandescent load), it runs quietly and stays cool as a cucumber. Make that a fresh refrigerated cucumber from where? Whole Foods, of course.Output voltage regulation is excellent; I tested it from 10 VDC in to 14.4 VDC, and it sags only a little, from 114 VAC RMS down to 110 VAC RMS. Your attached appliances will never know the difference.Mine clocked in at 59.99 Hz from 14V in, 60.02 Hz at 10V in; again, your appliances will never know the frequency difference, as frequency regulation is excellent. I ran FFT on the sinusoidal output to check harmonics; the highest was the 4th at 40 dB down. The sinusoidal output is very pure, very clean.Ever the curious one, I took a look inside. Four TO-220 nicely heat-sinked switching FETs (presumably) operating in tandem to drive a series toroidal inductor/capacitor low-pass (shaping) filter. The switching frequency is right at 25 kHz, and pulses range from 760 nS up to whatever width is required to maintain the waveform, depending on load and phase angle. I measured this on the driven side of the L/C output filter, see pics.This unit does generate some hash in the AM radio band, no surprise. It might generate RF noise at higher frequencies including the HF bands (I didn’t test that; hey, this is a free product review), but if you’re a ham radio operator, you’re probably running off of batteries if you need low noise. It’s probably within FCC Part 15 unintentional radiator limits; I didn’t test conducted or radiated emissions either.I bought this unit because my DJI drone battery chargers (3 different drones, 3 different chargers) refused to recognize my older “modified” sine inverters as providing useful AC power. I don’t blame them; whoever named it “modified sine wave” definitely worked in marketing, not engineering. And they’re rude. All of my DJI chargers work fine with this Bestek Sine unit, and why wouldn’t they? They’re being fed a swell, 60-Hz sine wave, just like at home.I highly recommend this unit. It’s internally fused, so you’ll have to do some soldering if you pop the fuse. You need to remember that 300 watts at 120 VAC is only 2.5 amps, but at your 12V input, it’s 25 amps assuming 100% conversion efficiency. Assuming a realistic conversion efficiency north of 80%, that’s 30 amps.Many automotive cigar lighters are fused at 10A. Never replace a 10A fuse with a 30A fuse to keep it from popping. There’s a reason auto manufacturers made that a 10A fuse, and it’s the wiring between your battery/charging system and the cigar outlet. Don’t turn your car into a crispy critter by upping any fuse, certainly not this one. If you need 300W, either make sure your car or truck cigar lighter fuse can handle the 30A, or buy some healthy alligator clips and attach this unit, via dual inline 30A fuses, directly to your battery. It’s also worth noting that most automotive alternators will produce about 50 amps at idle.I plugged three 100W incandescent bulbs to my Bestek 300 W Sine Wave inverter and read 253 watts on my handy Kill A Watt P3 test instrument. I attached it to my 36 AH fishing-motor battery and ran it for several minutes with no discernible heating of the inverter casing or the air at its vents. I also attached my iPhone to the USB output, and the USB outputs on this unit are indeed wired to charge iPhones (proper resistor-divider values on the USB data pins). It measured 1.56A into my iPhone, probably current limited by the phone, for a nice fast charge.The fan seems to have two speeds: modest and off. When operating, it’s fairly quiet. As you might expect, whether the fan runs or not is not so much affected by load; most of the power dissipated in switch FETS is related to moving charge and intermediate resistance value during transitions, not to Rdson losses. Assuming these are FETs (highly likely).I tested low-voltage and overvoltage cutoff points, and found that the unit had hysteresis at both ends, as any good design would. The unit I examined would cut off at 9.77 volts and would stay off until voltage rose to 10.994 volts. For overvoltage, the unit would trip off at 15.57 volts and start back up when voltage dropped below 15.000. Unlike another reviewer, I did not note any beeping sound from the unit when input voltage dropped too low. Note that common wisdom for lead-acid batteries and gel cells says not to take them below 11.6 volts/no load. I’m not sure how that translates to loaded voltage, but exercise caution if running this thing at full load for an extended time repeatedly from a battery that is not being charged.I’m going to give this unit an A+, and I’m a hard grader. And $45.99? At that price point, you may not expect much: you’ll be pleasantly surprised with this Bestek unit. Worth every penny. And no, I’m not on the company’s payroll; just pleasantly surprised and impressed with this unit’s design excellence and manufacturing quality. That and I was a little bored today, thought I would take some measurements and share my insights. Can you tell? :)