My Vaillant 12kW WP can take all the allowable current from my Multiplus II 48/5000 so I have used its relay and some Relay Assistants to disconnect the WP using its EVU input in the case of grid failure, and sound an audible alarm using the open collector output. Then I can take appropriate action depending on how full the battery is usw and if necessary put the WP into Noise Reduction mode (which consumes only 50% power) before re-starting it.