Underfire:
In reality, systems and backup systems are designed to protect for reduced power (brownouts) and lack of power (blackouts) and power spikes, such as lightening strikes...BUT power surges, which may last several minutes, overloading primary, and depending on duration, backup systems, are obviously a different issue.
While a loss of power or power reduction is protected within backup systems, sustained power surges are not typically protected, as a redirect of power is difficult.
It will be interesting to see, while the BA issue was newsworthy, what other systems were affected, if any.
Lets get one thing clear. The BA systems are whats called "mission critical" - meaning they are expected to be designed to work no matter what the failure, short of a nuclear attack.
Hence talk of brown outs, power surges, leaking/failing batteries, momentary interruptions, etc. as possible unforeseen causes is irrelevant. A mission critical backup system is supposed to work no matter what the cause of the failure of the primary system, no ifs, no buts.
To put that another way, a truck running into a power pole,, a ham handed excavator driver, an intern hitting the wrong button, ice on the points, floods, fire, etc. are not sufficient excuse for the failure of a mission critical backup system, in fact, those events are some of the particular conditions to be designed for.
There is no excuse for the failure of the BA IT systems except poor design and execution.
..But what does one expect from a country that is continuously surprised every winter when there is a small snowfall.