For each day of the five-week test, the system captured a full megawatt hour — enough to power more than 100 homes for a year.
This doesn't add up. I'd like to believe what they mean to say is "enough to power more than 100 homes", but the average UK home uses about 4.6 megawatt-hours per year, or 12.6 kWh per day. This is, more accurately "enough to power less than 80 homes each day".
Perhaps that's per-train, and I don't doubt that this is a significant energy (and heat) savings. But this is a surprising inaccuracy from the BBC.
The whole energy-vs-power confusion seems problematic. This is a grid-connected system; there are no batteries. Thus it seems average power would be a more helpful measure than energy, when talking about train-brakes and households both. If one insists on discussing energy, why not quote it in joules or BTUs or whatever? One rarely sees average power used in these contexts however, so there must be something about it that the power companies don't like.
Retail electricity is sold by the kWh, consequently that is the unit that most readers will be familiar with. Of the readers who prefer to use strictly Si units in their daily lives, those people, aside from frequently converting to/from various units, are aware or can quicky find that 1kWh is 3.6MJ. For anyone who wants to use BTUs, well, it's a free country, I suppose.
At least in the US, electrical energy is generally measured in kWh. Joules would be annoying to the reader, even though it's simpler.
I think the constant confusion just comes about from journalists trying to write emotionally compelling articles. Saying that all 5 days recouped enough energy to power a single home for a year sounds pretty unimpressive. So the interviewee extrapolates out to if the system ran for the whole year or otherwise were scaled up. But having no technical background (and thus little respect for technical details), the writer edits their phrasing to be much simpler. Treating "power" and "energy" as synonyms, we end up with gross misstatements like in the article.
(And then of course the fact that such gross inaccuracies get past the editors and actually published shows you just how detail oriented the modern news org actually is. Just think of that any time you read any thing!)
(Mega)watt hour is a common unit of energy in electrical power because it's convenient. (For example, my power company reports my power usage in kWh (~100 kWh for my family currently)) A 60-watt lightbulb will use 60 watt hours over an hour.
Perhaps that's per-train, and I don't doubt that this is a significant energy (and heat) savings. But this is a surprising inaccuracy from the BBC.