Is a 5 Watt Solar Charger Enough to charge a Car Battery?
Do the math. Typical car battery is 70 amp hours (your battery will say). 1 watt = 1 volt x 1 amp. 70 amp hours @ 12 volts = 840 watt-hours.
So your 5-watt solar charger will take 168 hours of full-on sunlight to fully charge that battery. 168 hours is exactly 1 week (24 x 7 hours). However most of the time the sun’s not out, and even if it is, the charger may not be working at its full 5 watts, because it’s a Harbor Freight piece of junk made in China.
However, it works in your favor that you probably won’t drain that battery all the way down. (regular car batteries can’t survive being deep cycled, about 10 deep drains and the battery is ruined.) So if you only use 25% of the battery’s capacity, you should be fine. That’s 210 watt-hours or 50 watts for 4 hours. (I’m assuming you won’t have lights/music while you’re sleeping.)