The most significant difference lies in the name. In the solid oxide fuel cell (SOFC), the electrolyte that separates the positive and negative poles from each other consists of a ceramic, while in the PEM fuel cell it consists of a proton-conducting membrane.
Different properties are derived from this. The PEM fuel cell works at 70°C and can therefore start or stop quickly (approx. 1 hour) or modulate the output within a few minutes to adapt to the needs of the house.
The SOFC works at around 700°C and has better electrical efficiency. However, it takes 24 to 48 hours to start and takes significantly longer to modulate the power. A SOFC optimally starts at the beginning of the heating season and is stopped again at the end of the heating season.