A 10kw heating element costs 250 euros, lets double that to make it able to switch off by the smart meter. Now if you used it 1% of the year and saved 5ct per kWh you would gain 43 eur a year. Not a super convincing ROI but compared to your 10kw peak solar array a very small investment. If you used it just 2% of the year it would already be pretty good and the switch might become very cheap in the future if most houses use load steering for all kinds of things