EDF dyed the calendar three colours so France would cook dinner off-peak

France's grid, heavy on nuclear baseload, had a narrow but brutal problem: on the coldest winter evenings, electric heating across millions of homes spiked demand hard enough to require firing up expensive, rarely-used peaker plants. EDF had already tried a version of demand-shifting with its EJP tariff from 1982 — a simple two-price system, cheap almost always, expensive on roughly 22 called days a year — but EJP's plain price number gave households no easy way to plan around it.

EDF replaced the single number with three colours: every day is announced the evening before as blue, white or red, and red-day peak electricity costs roughly triple the base rate. A number requires a household to check a tariff table to know whether tonight is expensive; a colour on the calendar tells them at a glance, no arithmetic required.

Replaced the price number with a colour announced the evening before, so households could absorb the signal at a glance and plan around it.

40/kWh' versus '€0.

13/kWh' requires active comparison to register.

Any pricing signal meant to change customer behavior in the moment (dynamic pricing, surge pricing, urgency) works better encoded as an instantly readable signal — a color, a simple flag — announced i

Households shifted load off the ~22 red days a year, shaving national winter peaks without new plants.