I set up a solar system using two 100w solar panels and a 125 hour deep cycle battery with a 1000w cobra inverter. It works fine for powering plugs. But, when I ran power to a motion sensor security light it works for a couple of days then stops working. Any ideas why?
if your lights are 220/110V through the inverter, then it is a different thing. but i guess, that your battery is running low. did you check the voltage of the battery, when the lights go out? does the inverter shut down? they normally give a sound signal, when they reach low voltage and shut down.
Inverters draw current if they are on, even if you draw no power. Efficiency is very low if you draw small amounts of power, and rises as you reach your inverter's peak operating point. If you are running an AC motion sensor and lighting, it will rapidly drain your battery. Simply flip to a 12V DC motion sensor and light; you'll gain a tremendous level of efficiency. You could leave the inverter connected to power outlets, provided its current draw is minimal when AC power usage is zero. You should turn the inverter off when you are out of the building.
You can actually wire an inverter to only supply current if the emergency lighting triggers. This will save you money if your emergency lighting is AC. Simply install a DC motion sensor and wire it's output to the switching side of a relay. Wire the high output side of the relay to an inverter and when the motion sensor triggers, it will activate the relay to enable AC. Relays with a DC activation circuit and high amperage DC power output circuit are available and cheap... about $10. Use a small inverter for this so that you at its efficiency point. The large inverter would remain directly connected to the battery so you can have outlet power.
Along with the power utilization issues mentioned above, look at your energy production. I see shading very near to your panels in the picture. Do shadows cross your panels throughout the course of the day? Even a small amount of panel shading (a few percent) will significantly decrease panel output.
As a final suggestion, you might invest in a DC power analyzer. Connect it inline in different places to test power input and power draw when the system is in use. I use one by RC Pro; Watt's Up is another brand you can look at. If you shop around, you should get one online for about $30.