As the sun rise in the morning the voltages rise also in a solar panel. Charging requires the solar voltage to be only slightly higher than the battery bank. Reaching enough potential takes at least a couple hours when the sun is low on the horizon. Or under low light conditions a solar panel will not be able to charge your storage batteries.
To over come this, issue the solar panels will produce a higher voltage than that required to charge. Thus speeding up the wait time in the mornings and evenings. Also low ambient light will assist in charge times.
PWM Charge Controllers
The original solar charge controllers (PWM) are unable to use the excess voltage. So this excess voltage that the panels produce at noon is unusable. When the panels are producing at the greatest. This unusable energy amounts to over 30 percent of which the panels are capable. Thus a larger solar array than needed, added to the cost of early solar power systems. As a result, smaller 12 volt systems still use this technology. PWM controllers can only regulate the charge voltage. Remote cabins, or well sites where efficiency is not as important as solar costs.
MPPT Charge Controllers
Small data collectors feed information to a specialized on board computer. Data such as the actual energy the panels are producing along with the state of charge of the battery bank. MPPT controllers can change the collected energy into a perfect charge voltage. The excess energy now converted to charge current. Increasing or decreasing the charge current to the batteries as produced. Therefore, the 30% excess energy is available for battery charging. Thus MPPT controllers earn their reputation Maximum Power Point Tracking. Electronic tracking of the suns energy instead of an expensive array tracking.
Modern controllers found in an off grid solar power system, have further advanced. Solar array now a looped in high voltage strings of up to 150 or more volts. Solar array will now start producing use able energy much earlier in the day. Also in lower light conditions more usable energy becomes available.
It is common to see array voltages of 90 volts producing only 5 amps of energy now charging at over 20 amps. No its not magic or free energy. It is the conversion of energy into a more useful form.