I bought it to keep my car battery in good condition. It doesn't work miracles, nor does it produce the 4W it claims, unless you constantly keep it facing the sun in the Sahara. Keeping it inside the car, connected to the cigarette lighter, I measure a current of up to 100mA, which corresponds to 1.2W. Outside the car (maybe because the windshield has UV protection), it goes up to more than double that. The measurements were taken in March, so maybe in June-July it will reach 4W. At night, I measure a small reverse current flow (4-5mA). So be careful, if the sun doesn't hit it directly for at least one or two hours a day, instead of charging the battery, it drains it! But if you make sure it gets sunlight for more than two hours, then yes, it manages to maintain the battery in its current condition. Tip for quarantine: Remove the battery, fully charge it with a charger, put it back in, and then it will be able to maintain it for a long time.