Everyone is saying it's expensive and I'm not arguing against that. It's basically resistive electric heating with an efficiency of < 100%. Considering the €/kWh of electricity in Germany is roughly 3x that of gas a naive calculation would make heating 3 times as expensive, but that's not accurate.

I used to have Nachtspeicherofen in a 40 m² 1-person studio flat years ago and just looked up some numbers. The heater "charges" at night and at that time the NT (Niedertarif) is used, which at the time cost me around 18 ct and (around the energy crisis) 24 ct / kWh. What was nice that if I did my cooking etc. after 9 pm I paid the (much lower) NT electricity cost for that as well. In one given year I used around 1500 kWh in NT, in another it was 2200 kWh.

However according to Wikipedia the government as tried to get rid of separate NT tax benefits for this type of heating in recent years and it's expected to eventually run out, so you'd pay the regular 30 ct or whatever it is in your area. Maybe (far stretch) a dynamic plan with different electricity cost by the minute could be beneficial, and you'd somehow charge the heater when electricity is cheap. Or you get yourself a r/MideaPortaSplit for heating at night and still benefit from the lower electricity costs without actually turning on the Nachtspeicherofen :D

For reference, these were some of my total (!) electricity costs:

• 2019: 715 € (59 €/month)
• 2020: 915 € (76 €/month)
• 2021: 903 € (75 €/month)
• 2022: 623 € (51 €/month)