Three years ago I used 10w solar panels from a different supplier to maintain the batteries I use on Shooting Ranges. These proved inadequate. I found these 20w units and they work excellent. I have them installed in OK through the winter and they have done their job admirably. They maintain the battery in peak condition and I don't have to charge the batteries using a normal battery charger as I did before with the 10w units. These units provide just the correct power to maintain the batteries, they don't bring a battery back from total discharge (I haven't found any that do), but for maintaining the batteries they work great. They do not require a clear blue sky with full sunshine, they work fine in overcast cloudy days as well. I purchased 66 units and I'm just placing an order for another batch. Excellent value for the cost. The Controller that comes with them works fine as well. When working on the range, my workers like the feature on being able to charge their cellphones from the USB port on the side of the controller
