This is my very simplistic description of the flow of power, as controlled by a Solar Charge Controller (SCC) from the solar panels (source) to the battery bank (storage) to the output (load) in the course of a typical 24-hour day and night. For this simplified example the only load is "lights" that come on after dark and stay on all night if possible.
Let's start with Dawn on a sunny day:
dawn breaks: the lights were on last night all night and the battery is depleted down to it's "cutoff voltage"
if the battery's not depleted then the SCC shuts off the lights anyway when it detects daylight
the sun comes up; it's a sunny day and the power flows from the panels into the battery until it's fully recharged then shut off
sunset; the panels sense that it's dark out now and turn on the lights
the lights continue to burn: either till dawn, or for a number of hours preset on the SCC, or 'till the battery is drained down to its cutoff voltage
OK, now for a cloudy day;
dawn again and the batteries are depleted from the night before
it's sunrise but a cloudy, overcast day so the panels aren't providing much energy to re-charge the battery
sunset: the SCC senses it's dark out now and turn on the lights
but they don't burn long before the battery is depleted down to it's cutoff voltage and the SCC turns off the lights to protect the battery
Hope this helps. This is the general sequence. The next step would be to add some math relating the power flow: solar panels are measured in Watts, batteries in Amp hours (Ah) and load for lights in Watts. Consider the time in Hours for each case. Think of it as water flowing into a bucket.
OK, now let's go thru it again with some typical but simplified numbers. Let's say that the capacities of the panels, battery and load are as follows:
PV panels are rated at 12v and can produce up to 2.08A for a power input rating of 12x2.08 = 25 watts per hour
Battery is 12v and 8.3Ah for a storage capacity of 12x8.3 = 100Wh or watt-hours
SCC is set to only allow the panels to continue charging the battery until it reaches its max capacity of 100Wh and then stop charging
SCC is set to turn off the load lights when the battery is 50% drained.
The load lights are rated at 12v and consume less than 1a (.83a) for a power consumption of 12x0.83 = 10 watts per hour
So now the sequence goes like this (my numbers were chosen mainly to make the math easier and may not be entirely realistic)
dawn breaks and let's say the lights have drained the battery overnight to 50% so now it stands at 50Wh
it's a sunny day so the batteries are replenished again to 100% of their capacity in 2 hours (50Wh + 2x25Wh = 100Wh)
it's sunny for more than 2 hours but the SCC stops charging the battery because it's reached its maximum capacity of 100Wh
night comes and the SCC turns on the lights which then continue to burn for 5 hours overnight. 100Wh - 5hx10W = 50Wh
at this point the SCC turns off the lights because the battery has reached 50% capacity. They stay off 'till dawn of the next day and we begin again
And if it's an overcast or wintery day with weak, low sun then the batteries may not fully recharge and the lights will not be on as long the next night.
From this simple example you can see what would happen if you changed the size of one of the elements:
if you increase the size of the panels you can increase the charging rate and replenish your batteries faster and under less favourable lighting conditions
if you increase the size of your battery you can increase the time your load lights can stay on, power more loads (lights) or even enable the lights to come on for several nights in succession despite little daytime sun.
if you change the size of the load, you can increase or decrease the time available. Or you can re-allocate the power available to several different loads.
The possibilities are endless. But you can see how the three things relate - like water flowing in and out of a bucket.
