The idea of a storage radiator is that the heating unit charges itself up overnight when some electricity tariffs are less expensive and it then emits the heat during the day when electricity is at the full rate. Electric underfloor heating systems, and in particular the 7mm in-screed cable system, can operate as storage radiators if correctly set up with the thermostat.
To use an underfloor heating system in this way there are two main considerations. Firstly you must ensure a programmable thermostat timer is installed so you can ensure the system turns on and off when you desire. Secondly you should ensure your ‘radiator’ is as large as possible. With a screed system you can ‘charge up’ the whole screed layer early in the morning and it will then let out it’s heat slowly throughout the day. The main downside with these types of system is they have a high thermal lag, you cannot quickly turn them on and off, but they do offer a relatively stable temperature.