I've been wondering exactly what is meant by a poor quality mains voltage, specifically in places like Africa, where the mains voltage is considered worse than say the UK. Having done some research, one of the issues is the fact that power cuts off entirely, especially in the summer (presumably A/C usage/overheating of supply components?). Having said that, are there other factors to consider? For example, is the voltage supplied noisy with lots of small fluctuations, be it in amplitude or frequency, or is the only issue the fact that the it can cut out entirely?
Good mains voltage is a clean sinusoidal signal (or several phases thereof, as the case may be) free of dips, spikes and excessive distortion, whose frequency is stable over time, and whose amplitude stays within several percent (say, five) of the nominal value. Also, on good mains voltage, an appliance can draw the maximum rated current for the circuit it is plugged into, without experiencing a voltage drop which takes the voltage out of range.
Good mains voltage is a function of the ability of the grid, as well as the local circuit, to handle the load which is placed upon it.
It is also a function of the quality of the local wiring installation in a building, and also of the quality of the devices that are plugged in. Devices can degrade power quality seen by other devices by spewing noise into the wiring.
availability - as in how often and for how long the power is gone;
voltage sags and dips - due to e.g. high line impedance, shorts;
too low or too high voltage - typical in suburban areas. If you are close to the MV/LV transformer you have too high voltage. The voltage drops with the distance to the transformer; frequency stability - some things depend on it. Pump speeds, clocks;
overvoltages - when part of the network is energized or de-energized voltage surges may occur;
harmonic content - impacts e.g. capacitor lifetime and motor performance.