Mains power recorder/monitors are expensive and not exactly common in domestinc environments.
This unit consists of three clocks in one casing, a mains-driven analog or pseudo-digital (flip-over numerals), a digital (LED) clock, and an MSF (radio-set) digital clock.
The user sets the two other
clocks to the same time as the master MSF clock.
When there's a power cut, the analog clock halts for the durarion of the outage. When the power comes back, the digital clock restarts from 0:00.
The time on the digital clock gives the elapsed time since the power resumed. The divergence between the analog clock and the master clock gives the duration of the outage.
The clocks can then be re-set for the next blip.
The system is only realy meaningful if there is only a single outage, but if the power company have an irritating habit of dropping the power supply randomly during the night, it is a useful source of substantiating data for strongly worded letters of complaint from disgruntled customers.