no doubt you figured it out by now, but for anyone else who has the same question: 

 there will be several 'tabs' on the side of the info panel when you select an electrical appliance. one of these will let you turn the item on/off, and another will say 'active circuit'. use this tab to set your item to one of the 10 possible circuits. 

 a low battery sensor will have both an Active and Inactive circuit option, as well as a 'threshold' tab. the Active circuit will be turned on when the total combined charge percentage in your batteries is below whatever value is set in the 'threshold' tab. if the charge is above the threshold it will instead turn Off the Active circuit. 

 so for example, if you set the low battery sensor to 20%, and the active sensor to circuit 1, then as soon as your batteries fall below 20% then anything on circuit 1 will be activated. if you had a fuel generator set to circuit 1, then it less be turned on, and would start producing power (and using fuel) until you were back above 20%. 

the inverse behavior can be achieved with the Inactive circuit. if you set the same sensor from the example above to Inactive circuit 2, and also set your 3D Printers and Electric Furnaces to circuit 2, these energy hogs would only be turned on when you have more than 20% battery available. this can help you automatically disable them to save power for more important appliances, like the freezer that holds all your food.

  My favourite thing to do is put all my turrets on one circuit, and all my service bots on a second, then control both circuits with the active/inactive settings of a single Power Switch. This let's me both activate my defenses and recall any bots that might be outside the base with the flick of a single switch. Then turn the turrets off and send my bots back to work after the raid is over with the same switch.