Electric storage water heaters #ESWH can be a major form of energy storage in places that already use them (which includes much of Europe).
Not in the sense of long-term energy storage, but definitely in the sense of reducing power consumption "rush hours".
“Interesting report on power storage potential of electrical storage water heaters #ESWH (hot water tanks) by @BeritTennbakk et al. Households need hot water, but this is also an energy storage that could be used to balance out electricity consumption peaks. ESWH in blue below.”
Jonny Axelsson