Heres the math- To figure out how many hours it will take to charge the battery, first figure out the output of the solar panel by dividing its WATTS by its Volts, which will give you its output in AMPS. So. .
a 15 watt panel DIVIDED by 12v = 1.25 amps
Next, read the Amp Hour rating on your 12v battery. If we use a 50 amp hour battery for this example. . .
DIVIDE 50 amp hours [required to fully charge the battery] by the 1.25 amp output of the solar panel = 40 hours of direct sunlight to charge the battery. Then add another 10% of time to his number, so figure 44 hours.
Some more examples:
5watt/12vpanel=.42amp and 50amp hr/.42amp=119 +10% = 131 hours of sun
30watt panel works out to 22hours. 50watt=13hrs 80watt=about 8 hrs.
Of course all of these times are based a 50amp hour 12v battery.
BTW, Im no expert¦ i just found this equation online 5 mins ago because i was wondering the same thing myself. . Now could anyone tell me how figure out quickly my computer or light or whatever will drain that 12v battery??
To determine how quickly your battery will drain, lets assume that same 50AH battery. You wish to run 2-5W 12V lamps for about four hours each evening and use your small laptop for an hour which has a 50W draw at 18V, so about 80W at 12V. Your use will be 80W/12V=6.7AH + ((2*5W)/12V)*4 hours=3.2AH, or a total of about 10AH per evening. If you allow the battery to discharge to 50% before wanting to recharge it, you can run as described for about 3-3 1/2 evenings per week. If you concurrently have a 30W solar panel charging the battery for 4 hours per day, you will be keeping the battery fully charged it (you will be putting back 10AH per day). This allows for a couple of cloudy days.
Hope this answers your question. Remember, watts/volts=amps. Multiply each load by the number of hours you need it to run to get ampere-hours. Add all these results together to determine the total draw on the battery.