Today is the autumn equinox. The sun rises due east, sets due west, and spends almost exactly twelve hours above the horizon. On 20 March it did precisely the same thing. Astronomically, the two days are as close to identical as the calendar gets.
Ask a heat pump, though, and it will tell you they are six months and a world apart. On 20 March the average British home needed 8.6 heating degree days. Today it needs 2.4. The same sun, the same day length, three and a half times the heating.
Put it another way, which I find harder to shake: today your heating system thinks it is the 5th of June. On the spring equinox it thought it was the 19th of November.
That gap is not a curiosity. It decides when a solar roof starts paying for itself, how violent the evening ramp gets, and whether a battery has anything useful to do. And almost none of it lines up with the dates we actually mark.
I pulled 35 years of hourly reanalysis weather for sixteen British cities, weighted by population, and ran a physically-modelled all-electric home against every hour of it. Then I put that next to fifteen years of NESO's half-hourly demand data. What follows is what the two equinoxes actually look like from the inside.