When I started tinkering with off-grid solar one of the first questions I asked myself was “how long is it going to take to charge this battery?” Or, similarly, “how much power will I produce in a day”? Initially, the answer seemed easy: I’ve got a 100-watt panel, Minnesota gets about 4 hours of peak sun on average per day, so I’ll get 400 watt hours per day! A 960 watt hour battery should be charged in two and a half days!
Wrong. Way off. Maybe more like 250 watt hours per day, and more like four days to fully charged.
I realized this quickly when attempting to charge a fully depleted battery. In reality, it took into the fourth day for the charge controller to switch to battery maintenance mode (showing it was completely full).
Under perfect operating conditions and when grid-tied, you may actually get close to that nameplate 100 watts. However, when your system is connected to a battery the voltage drops. Many charge controllers (except for the really good expensive ones) will match the voltage of the panel to that of the battery to facilitate charging, which is almost always a lower voltage than the panels potential peak. The rest of that potential is wasted. So while my panel will produce 5.29 amps at 18.9 volts under optimum conditions (5.29 amps x 18.9 volts = 100 watts), when connected to my battery, it will probably only produce 5.29 amps at between 11 and 13 amps (5.29 amps x 12 volts = 63 watts).
Panels are built this way on purpose to make sure power can continue to flow to the battery even during overcast conditions when voltage may drop a little (gotta make sure that water flows downhill!). Grid-tied panels don’t have this problem, since the grid can usually accommodate your voltage, and in a grid-tied system you’ll probably opt for one of the fancier MPPT controllers (see link above).
So, in summary, I’m probably getting 65 – 70% of my nameplate wattage (by design), and a realistic estimate for charging a fully-depleted 80 amp-hour battery from my 100 watt panel is 3.5 – 4 days.
Another takeaway for this amateur: it’s about amps, not watts. The websites where you shop for panels always have the watts in large font, but the small print tells you the optimum operating current, or amps. This number times hours of sun gives a much better estimate of the output (5.29 amps x 4 hours = 21 amp hours per day) for a battery-tied system.