I'll add in my $0.02 for what it's worth on the subject. To answer the OPs question I don't concern myself with EPQ measurements of any significance. For me, conditioning the incoming power is a very basic requirement to cleanup the harmonics and spikes you receive at the house. I use legacy Monster brand conditioners on my primary and secondary audio systems. These read a constant 120.1 vac all day long, and I'm good with that...condition it, fahgettaboutit!
Is your incoming dirty? Absolutely. Is it a pure sine wave and distortion free? Ha, no way! Here's why.
Power starts with a turbine/generator at your local station, leaves the building at 23 kv or so at a rock solid 60 Hertz. From there it feeds numerous auxiliary/station service transformers (tx) and hundreds of motors back inside the plant. When it leaves the building it hits generator step up tx units taking it typically to 230 kv, then 500 kv in the switch yard for transmission.
Every tx adds in a bit of influence, and harmonic distortion especially in the 1st and 3rd orders up to the 21st order or so. New tx included, but even more so with aged units you also have internal partial discharge and external corona over a wide band of frequency ranges . IEEE/IEC standards dictate what level is acceptable for the particular vintage, class, kv and size of the tx . Anything over 21 kv to ground will go into air corona unless shields are installed. Count in dozens of switchyard breakers, each with internal and external connections, as well as bushings, connectors, buss conductors, tap changers, wire, terminations, lightning arresters, insulators, switches, ground connections, cap and inductor banks, so forth and so on.
Leaving the switchyard down the transmission corridors the feed travels for miles; in my state there are over 30,000 miles of transmission and many hundreds of feeder substations to step down the power for distribution at low voltage, 25/14.4 kv and12/7.2 kv are common. In these stations... more switches, breakers, tx, connections, etc. and miles of wire on the street. I`m lucky, the closest generating station is < 20 miles from the house, and the closest substation is <6 miles away.
Regulators (regulating tx) constantly jog the distribution lines based on loading to keep voltages at your neighborhood, when stepped down by those little green boxes (or grey garbage cans on the pole) close to 240/120 volts. These can be set to narrow or wide tolerances but keep the incoming between 115-125 vac at the kitchen outlet. Is that dirty? Yes indeed. Will you ever receive a pure sine wave off the street? Nah.
If you hang an Agilent or other such really high Q analyzer on your lines you would likely see flat tops and transient spikes in the voltage trace. Harmonic distortion in the current trace will look more like a sawtooth than a sine wave. Google spends millions on power conditioning at their data centers. As an audio aficionado I spend a little.
Every large utility will have an EPQ group, 8 or 10 guys and gals to investigate significant power quality issues. Hospitals and industrials come first followed by commercial, the lonely chicken rancher who has
5 or 10 acres and 10,000 chickens that wont lay eggs, then Mr. & Mrs. Residential. If you have a critical medical device in the home you just got upgraded. Reach out, be cordial, ask for help. It's what they do.
Most significant issues will likely be found in the ground connections.
I hope I've added some value to the discussion. Every time I flip a light switch, and the light comes on, I just smile a little bit. Freaking miracle that is!
