An electrical control system for controlling a plurality of variable transmittance windows, the plurality of variable transmittance windows being electrochromic windows, the electrical control system comprising: a plurality of slave window control circuits each for controlling a transmittance of one or more of the plurality of variable transmittance windows; a plurality of user input mechanisms each coupled to one or more of said slave window control circuits; and a master control circuit coupled to said plurality of slave window control circuits, wherein: said master control circuit and said user input mechanisms are configured to supply transmittance state signals representing transmittance levels for the variable transmittance windows to said slave window control circuits; each slave window control circuit is arranged to control the transmittance of at least one of the variable transmittance windows in response to the transmittance state signals supplied by at least one of said master control circuit and one of said user input mechanisms; said master control circuit and slave window control circuits are configured such that when a transmittance state signal is provided to a slave window control circuit by said master control circuit, the transmittance state signal provided by said master control circuit is used by said slave window control circuit to determine the transmittance level of any variable transmittance window to which said slave window control circuit is coupled while overriding any transmittance state signal received from a user input mechanism; characterized in that said master control circuit causes the transmittance of groups of the variable transmittance windows to change sequentially. a plurality of slave window control circuits each for controlling a transmittance of one or more of the plurality of variable transmittance windows; a plurality of user input mechanisms each coupled to one or more of said slave window control circuits; and a master control circuit coupled to said plurality of slave window control circuits, wherein: said master control circuit and said user input mechanisms are configured to supply transmittance state signals representing transmittance levels for the variable transmittance windows to said slave window control circuits; each slave window control circuit is arranged to control the transmittance of at least one of the variable transmittance windows in response to the transmittance state signals supplied by at least one of said master control circuit and one of said user input mechanisms; said master control circuit and slave window control circuits are configured such that when a transmittance state signal is provided to a slave window control circuit by said master control circuit, the transmittance state signal provided by said master control circuit is used by said slave window control circuit to determine the transmittance level of any variable transmittance window to which said slave window control circuit is coupled while overriding any transmittance state signal received from a user input mechanism; characterized in that said master control circuit causes the transmittance of groups of the variable transmittance windows to change sequentially. said master control circuit and said user input mechanisms are configured to supply transmittance state signals representing transmittance levels for the variable transmittance windows to said slave window control circuits; each slave window control circuit is arranged to control the transmittance of at least one of the variable transmittance windows in response to the transmittance state signals supplied by at least one of said master control circuit and one of said user input mechanisms; said master control circuit and slave window control circuits are configured such that when a transmittance state signal is provided to a slave window control circuit by said master control circuit, the transmittance state signal provided by said master control circuit is used by said slave window control circuit to determine the transmittance level of any variable transmittance window to which said slave window control circuit is coupled while overriding any transmittance state signal received from a user input mechanism; characterized in that said master control circuit causes the transmittance of groups of the variable transmittance windows to change sequentially.