So...., before I get feedback about a house with 2 people and only one light on.... first person able to see and second stumbling around in the dark.... I disclose this was done for a house occupied by one person. Since one person can only be in 1 location at a given time, one light is all that is needed. Also, this was done as an emergency measure to enable use of an off-grid power source that was undersized and temporary.
This project proceeded in 3 phases. In phase one I used the existing lighting circuit (110v AC) and a small 40 watt inverter able to run a single 23 watt CFL lamp. After connecting to a 80 amp hour 12v battery specific instructions were given that only one light was able to be used at a given time.. So you have to turn off the light you are using and navigate via flashlight to turn on the next light.... Not convenient but do-able. Of course my directions were not followed two lights were turned on at once and the 40 watt inverter immediately went into reset mode. Charly would then stumble back to the inverter and reset it and then go turn on the light he wanted to use. This was not convenient but remember this was to be temporary for just a few days.
After a week and a another trip to the high desert, Charly was very disappointed with having to turn off the light he was using and then find the switch for the next light in another room.... He also left the inverter on all the time when no lights were in use..... wasting precious battery power. So I put a small NE-2 bulb and resistor in parallel with each switch in his house. He could then tell if the inverter was running from any room. Also, the light switches were easy to find in the dark. But old habits are hard to break and he would often forget that only one light at a time can be used and he would move to another room leaving the light on and then turning on another, the 12V. inverter would trip off and all the lights and neons go dark.... It was then we learned he was going to be required by the county to completely rewire his house and install a new breaker panel in a new location. Which he did not have the money to do at the time. A better long term solution was needed.
So in phase three I came up with a couple of ideas. My first thought was to make a custom light switch activated by a card or peg in a hole and only have one available. This would require turning off the light in one room and using the same card to turn on the light in the next room. Then I struck on an idea. If I used three way switches I could connect them so only one light at a time would have power. Also one of the four lights in the house would be on any time the inverter was turned on..... so at bedtime you are forced turn off the inverter to turn off all the lights, thus saving battery power. It would be a win win but require some more wires to be temporarily stapled to the walls. I had by now figured out it might be a month or more before the utility power was back on. As it turned out, it was several months before utility power was restored.
I had a large amount of 14 gauge wire from a warehouse lighting system I removed. A couple of thousand feet of wire and some plastic handy boxes from the same project. I purchased Qty 3 three way switches and did a mock up to make sure it all worked.I re-used one of the regular wall switches on the 12 volt side to turn power on/off to the inverter.
Now lets do a walk thru. Looking at the floor plan below, there are 4 lights ceiling mounted in the 4 rooms. Each ceiling light has a 23 watt CFL bulb in it. They are controlled from 4 switch locations. S-1 is by the front door which controls the 12 volt supply to the 40 watt inverter. Anytime the inverter is turned on one ceiling light in the house will be illuminated. So S-1 is the main power switch and at bedtime is turned off and a flashlight is needed to get to the bedroom. The battery and Inverter sat on the floor just inside the front door under the wall switch S-1.
S-2 is between the living room and the kitchen. In one position the light in the living room is on and in the opposite position the kitchen light is on. So as you pass from the living room to kitchen the switch is right there to illuminate your way.
S-3 is on the wall by the door to the bedroom from the kitchen. It decides whether you want lights in the Bed/Bath or Living/Kitchen. S-3 basically decides if the front half or back half of the house will have the one light on.
And finally S-4 is for the bathroom light. It is mounted on the wall by the bathroom door and turns on either the bedroom light or the bathroom light. Originally the bathroom light was a pull chain and had never had a wall switch. I just removed the pull chain from the porcelain socket in the 'ON' position.
There are shortcomings to this system. As I mentioned before a flashlight is needed at bedtime to navigate back to the bedroom. Also, if you turn on S-1 (main 12v switch) and S-3 is in the Bed/Bath position you will enter the living room in the dark. I later got a couple of stick on battery operated light fixtures at the 99 cent store and put by the front door and in the bedroom as Charly had problems finding things sometimes, his flashlight being one of them.
So did this work?? Yes, but only for a small house, on an emergency basis, and only for a couple of months. I eventually got all the material (with the help of neighbors) to rewire Charly's house to code, passed inspection, and got the utility power re-connected.