The Electrim TE4 Heater Tray is specifically designed for 4 x standard demijohns. Being larger than a TE25 it is also suitable for a 25 or a 33 litre fermentation bucket. It provides a low constant heat to aid fermentation. 230v/35w output is constant with an on/off switch although we find it best to plug in a timer and set it for 15 minutes on in every hour which seems to provide about the right heat.
The tray size is 370mm x 370mm