How is it different from an electric boiler?

The ZEB ‘charges up’ using electricity from the grid at times when it is cheapest and greenest, storing that energy as heat to be used on demand when you need it.

Conventional electric boilers, by contrast, draw electricity from the grid whenever you call for heat, which can make them significantly more expensive to run because they are often using peak-rate electricity that is both costlier and higher in carbon intensity i.e. it's dirtier