If you are referring to the electrical night storage heaters that are still popular in certain types of dwellings in the UK, I don't think there is a ready-made component for it in E+. What I did last time is to use a water tank paired with a connector or radiator to represent each unit (see image below), and use schedules to control the charging and heating cycles, to mimic the process. Apart from the water tank volume being absurdly large, the effect is fairly realistic, I think.