The acid test for output performance of ANY generator is a RESISTIVE load... an electric heater.
A guy can easily MAKE a resistive load network by gathering up a bunch of 1200w heaters. FIVE of them is 6,000w.
Get a volt meter, a frequency meter, and if you have access, an oscilloscope.
start the generator, get it warmed up, and start adding heaters. The generator's initial frequency will be around 61-62hz, and your output voltage should be in the high side of an expected range (110-125v or 220-250v). As you add load, the voltage will come down slightly, the frequency will come down slightly, and the throttle will start to open.
The range between no load, and the point where the frequency drops AND the throttle responds, is called DROOP.
Once you've reached DROOP point, the voltage and frequency should NOT drop much further with added load, until you're out of throttle.
Along this entire range, the output waveform should be sinusoidal, with very little in terms of ripple or spikes. That's a smooth form, that doesn't look 'fuzzy'.
I'll politely differ from Steve's assertion that older generators are 'noisyier' than newer. With exception of my portables and welder-generators, ALL my 'real' generators are much older than I am, and they're all dead-on beautiful... but they also have had proper maintenance- the exciter brushes are clean and free-moving, the bypass capacitors are all fresh, and high-quality. This is really the only thing that will introduce 'noise' into a generator, and it is frequently the least maintained element of the system.
Small units that do NOT have a dedicated exciter, do not have brushes, which gives them a small advantage in that respect, however, they're extremely limited in their ability to self-reguate, and generate wimpy output and poor transient load response, so you get what you pay for.
Air-cooled portable generators are good when you need portability. Likewise, PTO generators are great when you need to bring a LOT of power to an otherwise inaccessable area (across a muddy field, or through deep snow), because they're usually hanging on the 3-point or tongue of a tractor.
Portables are generally weak, and being air-cooled, they are louder, and do not have the service life of a liquid-cooled unit. This is a natural consequence of portability.
PTO generators are frequently poorly maintained because they have to be very substantially closed-in in order to prevent the ingress of dirt... they're always outside, or sitting in a dusty shed.
Air-cooled units tend to accumulate beechnuts, walnuts, straw, and bits of chewed up cloth, loss of wire insulation, as well as substantial urine-corrosion of sensitive components due to the myriad of lovely places that rodents can invade. When you need that air-cooled generator the MOST, sit in the cold, under candlelight, and take the shroud off the flywheel and dig out the mess, then clean it all out before attempting to fire it up.
The PTO equivalent very well may be equipped with heavy hardware-cloth screen layers to reject rodential residence... but mud-daubers often move in, so be careful with that too... mud-clods aren't made of flint, but they do disservice to a generator head when they start racing around the end-bells as they're battered into powder.
Air cooled portable and semi-portable units have simple governors, and they're NOT going to be very 'tight' in regulating frequency. PTO units rely on the tractor as their prime-mover, and tractor governors are NOT as sensitive to load change as a synchronous generator SHOULD be.
If you want a reliable backup system, START by doing a proper survey of your NECESSARY loads. Put a Kill-A-Watt meter on your freezers, and measure what they actually DRAW, and then get an idea of how often they actually RUN.
(You can unplug a freezer for a WEEK, and the contents will still be frozen... )
Add up all your critical loads, multiply that by two, and get a generator of about-that-capacity. Look for a liquid-cooled Kohler, or an Onan JB genset, from the '40s to 70's vintage. Make a small shed into which you can run propane or natural gas, put lights in it, add an exhaust outlet and ventilation that mice cannot get through. Keep spare parts, tools, and extra oil and coolant mix on hand, as well as battery-powered overhead lighting and more-than-one starting battery.
You can use a transfer switch, or a 'transfer panel', so that your generator is isolated from incoming mains, AND so that your emergency loads, and occasional convenience loads are the only thing applied as load when you have an outage.
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