So we live on wheels all the time and I installed a complete solar system with approx. 1480 watts on the roof to charge the domestic batteries. This is an RV and if I don't turn on the RV to charge the starter battery which is separate from the house batteries it burns out. I wanted to put a small 100w panel on the roof to run the batteries as a battery charger. Then I came across this product. Household batteries are approximately 90% charged and are being charged. It draws power from the household batteries and charges the starter battery in the same system. It works great, the one I have is 24v to 12v. My house battery is a 24v bank system. It was much cheaper, better and easier for me to go this route, then install my own solar panel and run all the new wiring to my starter batteries. The way I set it up, once my house batteries reach 28 volts, it will start automatically, turn on the starter battery, and stop automatically when the sun goes down.