They will produce electricity intermittently. When the thermal store is up
to temp and no heat is demanded from CH and DHW, then you draw from the
grid. It is claimed that over 50% of all electricity will be generated in
the home using these units and 25% cheaper to run in both gas and
electricity, as you also sell electricity back to the power company.

A bit of careful design with the heatbank could get better than this. If you
have space to oversize the cylinder, you could get it to only demand heat
when less than 1/2 full. Then it could be available for electricity until
full. I haven't seen the specs, but I presume the CHP's heat output when
demanding electricity only would be around 3kW when producing 1kW of
electricity, so you could get several hours of electricity out of it before
the heat bank is 100% satisfied, requiring the motor to be shut down.

Christian.