candescent light bulb. These use the most energy and are more like tiny heaters that give off light as a side effect. They will subsequently require either a large battery bank or the use of very few bulbs (up to six at the most with the entire circuit dimming with each bulb that is turned on). Compact fluorescent bulbs and LEDs are best choices as they are much more energy efficient but the fixtures have not become standardized and there will be some direct wiring needs. Concerns with CFL bulbs surface when we take into account the hazardous nature of the materials used in their construction (mercury). As for LED lights, because many of them are not manufactured in quality controlled environments, you have to be extremely careful to buy only ones that have passed the Energy Star selection process. These lights have proven quality records and won’t dim or fail like some of the cheaper varieties.
Now that you have chosen your bulbs to calculate the solar load of your system you need to determine the exact needs of your system. To do this you will need to apply Ohm's law and due some calculations based on the wattage of your bulbs. Most full house systems should run on a 200 Watts of solar panel and a single deep cycle 12 volt battery. Take into account at least a 10% drop in voltage per 50 feet of 3 gauge wire due to transmission dissipation. The longer the run of wire between the battery and the lighting unit the large gauge wire you will want to use with a maximum run of 200 feet. If your battery is more than 200 feet away from the fixture you may need to add a second battery to the system.
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Adding Extra Storage Capacity
If you are planning on using your lighting system overnight (which, let’s face it, you probably are) then you may need to add an additional battery to your system. A single 12 volt battery can handle running a house full of lights for three or four hours while adding an additional battery can give you enough backup to run for eight hours or more. This is also a good idea in case you run into an extremely cloudy day where the solar panels don’t supply enough power to both run lights and recharge the battery system. Again you will use Ohm's law to determine if a second battery is necessary by calculating the total Kilowatt hours you plan to use each day and how many can be supplied by your system. Next check the Amp hours of your battery and multiply it by the voltage (in this case 12 volts), this will give you your total Kilowatt power for a full battery. Take the battery capacity and divide it by the daily usage to determine the highest run time of your system. Adding a second battery will double your run time.
Running 12 volt interior lighting is a great way to start to wean yourself off the grid. It also is a great back-up if you live in a location that experiences frequent power outages.