Some relatives are dealing with an unstable electricity at their home, causing various issues from lights loosing power to appliances restarting (internet router…). I would like to record the voltage and the frequency to try to pinpoint the issues and have some data for a formal complain to their provider. I have this at my home thanks to Home Assistant and some smart plugs from AVM (Fritz DECT 200). But unfortunately, my relatives have nothing like that, just a random 4G router, no smart outlets, nothing. I am therefore looking for a cheap and simple solution to record and visualize the voltage and frequency for the last few days, with a period of a few seconds. Ideally, it would be with just an outlet and a smartphone. Any idea or tip?
Il lights are loosing power, it’s voltage related, not frequency. Also, if you live in “modern country”, grid frequency is rock solid.
Shelly Em is logging voltage at 1minute interval. If it’s not enough, maybe esphome board and voltage sensor could be option.
Cheap, reliable recording? Pencil and paper is the way to go. Works well under torchlight and candles. Automation doesn’t usually work well if there is no electricity to power it.
Seriously: Complain to your electricity provider - put the onus on them. They are used to doing this all the time. Don’t re-invent the wheel. Record and show times the disruptions happened and for how long - it doesn’t have to be to the nanosecond, just approximate, within a five minute window. They may be a able to correlate that to their network faults they already know about.
If not, they will plug in a device like a Dranetz Power Monitor which records voltage, current, phase, line noise and a lot of other parameters for a week or so, and then analyse that to find the underlying problem.
The intermittent problem may be local to your premises. A loose neutral connection in the meter box can wreak havoc with a house supply. Your local electrician may have suggestions based on careful visual observation, and should tighten all wiring connections in the meter box as a precautionary measure. Oxidation of connections, partuclarly bad near seaside residences can cause heat build-up and risk of fire. Ask a professional to help, even if just to be sure in case of fire the insurance company wont be able to void their cover due to insufficient maintenance.
The problem may be where your power line connects to the pole, or even the local step down transformer. This is where it is important to discuss with you neighbours. Is it related to your premises only, your street, a few streets, or your entire suburb? Feedback to the power company will make it easier to pinpoint where.
If no luck, have you considered a UPS for critical devices that cannot cope with small power interruptions? You can buy big ones to power the while house, but often that is not necessary. Just the devices that cannot cope with small surges and interruptions.
Most third world countries rely on diesel generators or solar with batteries for reliable power - be comforted that you can connect to your passing electricity lines for 24/7/265 power.
I’m assuming the problem is not isolated to a single premises.
Your supply contract should specify the terms for power interruptions. Asking for a refund for unreliable power usually brings quick results, as it affects their profit line. Getting the neighbours onboard will make it even more urgent.
Getting custom software/hardware solutions for a one-off problem may not be a wise use of your time or money. Put the problem back where it belongs - the power company. They may also be able to inspect your premises as part of the problem resolution and offer suggestions, for a small fee or free. Worth asking about.
You’ve automatically made this thread awkward by specifying ‘-without HA’ in your title in these HA forums, but be sure to document what has happened for others that are reading to see what transpired.
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Before taking it up with the supplier get an electrician out to check all the circuits in the home and get a proper report to build from.
Possibly cheaper to complain to the utility supplier and get them to exclude their end, first. 
Aak if they will pay for the electrician callout if the problem is found to be on their end? In writing.
Talk of money and rebates usually crystallises support calls rather quickly.
Thank you all for the valuable tips!