NOCO BLSolar2 2W and 5W panels:First things first with solar panels. Even the very best solar cells are generally 15%-20% efficient. I've been hearing about how solar panels will become the dominant alternative energy source in "the next 5 years" for the past 20+ years. I started using the NOCO 2W charger knowing that based on my calculations, it probably won't be powerful enough to maintain a car battery, and that a 10W is a more appropriate size for a maintainer. After using this NOCO 2W charger for over a week, and taking down some data along the way, my conclusion is that the 2W isn't enough unless you are living closer to the equator with the panel directly under the sun with longer periods of daylight. The 5W is more useful as it can provide enough current to overcome a vehicle's standby current draw AND maintain the battery's charge.I'm in Michigan and tested this panel in the sun (as much as we can get in February/March) but we mainly had cloudy days and the amount of daylight is fairly restricted during this time. I started off on a E-series van that has minimal amount of electronics and standby current draw/parasitic draw. The initial voltage as 12.05 VDC. After using the solar charger for 2 days, the voltage actually dropped to 12.03 VDC. At first I thought it was the LED on the cigarette lighter adapter, but further experimentation showed that there is about 10-20 mA of current draw when the panel is not being hit with sunlight. I did a second trial run for a period of 4 days. The starting voltage was 11.93 VDC at 32F, but 4 days later the voltage dropped down to 11.88 VDC at 25F.The typical parasitic draw for a car is about 30-60 mA, so a battery maintainer has to overcome that current draw and provide enough capacity to actually charge the battery. I normally use 800mA Battery Tenders, which are "smart" chargers that can provide a meager amount of charging power but are more intelligent when it comes to maintaining. The NOCO 2W provides up to 21 VDC, but I'm skeptical about the charging current.So I grabbed an older car battery and left it right under the sun. I used a clamp DC ammeter and observed that the panel would provide a maximum of 30mA when there was direct sunlight. If a cloud went overhead, or I turned the panel over, it would draw up to 20mA of current.I took the 5W panel and ran it through the same tests. It provides 120mA of current in direct sunlight which is roughly 25% of the maximum advertised charging current. When the panel was flipped over, I only saw a peak of 10mA current draw from the panel itself.Doing some math, a 10W panel would be more appropriate for all conditions as the larger panel would work better in conditions like Michigan, at least being able to overcome parasitic draw during the day. I'd recommend disconnecting it at night or when it is cloudy though.Overall: 4/5, great build quality but the 2W is just too small of charging capacity for a car battery. If you live near the equator and have plenty of bright sunlight during the day, this may work for a smaller capacity battery. The 5W is more reasonable, but a 10W is what I'd recommend. You still need to park your car in a way the panel can directly face the sun and when you expect lots of daylight. It is a green way of maintaining your car battery or charging a separate battery.UPDATE 13 APR 2013:I have been using the 5W on the E-250 van. When we had sunny days, the 5W panel would actually charge up the battery. I saw it go from 11.98 VDC to 12.01 VDC over the span of just a few hours.Once we started to get non-stop cloudy and rainy days (it has been raining and cloudy for over 8 days straight), the 5W panel cannot provide sufficient current to the battery. The open terminal voltage at the solar panel is 20-21 VDC, but the current is too small to maintain the battery. As a result, the battery voltage is steadily dropping by 0.02-0.03 VDC per day with the panel hooked up. I removed the LED on the cigarette light plug, and that slowed down the discharge to 0.01 VDC a day. The bottom line is that IT NEEDS DIRECT SUNLIGHT during the day, or else it isn't providing a sufficient charge. The 2W, even in direct sunlight, is providing a mediocre amount of current that is insufficient to overcome parasitic draw and self-discharge of the battery. Get at least the 5W, but a 10W will work a lot better.