How is it different from an electric boiler?

The ZEB ‘charges up’ with electricity from the grid at times when it’s cheapest and greenest, storing it as heat to be used on demand when you need it. Electric boilers on the other hand take electricity from the grid whenever you need heat, which makes them significantly more expensive to run, as it’s using peak electricity, which is more expensive and has a higher carbon intensity i.e. it’s dirtier.