These heaters store heat during the cheaper night-time tariff, then turn off at peak rate time, and the stored heat is allowed into your living space. 

For specialist heaters only, like Dimplex Duoheat see this page https://www.local-spark.co.uk/dimplex-duo-heat-storage-heater-electrician

Old Storage Heaters

Economy Seven (E7) storage heating, using cheaper off-peak electricity, certainly sounds like a good idea. More recent heaters are a good idea – but some older installations may well be in urgent need of replacement.

Older storage heaters are typically coloured brown, take up unreasonable amounts of living space, and tend to look unsightly in almost any room – especially one that has recently been decorated! There is little you can do to improve their appearance – you can’t cover them with MDF radiator covers, and they can’t be painted.