If you have not done so already I'd recommend starting this
project by looking into how big a fan(s) you need. I have a couple 12 inch dia Hayden fan here. They are rated for 800 cu ft per minute free air flow, 12 VDC 8.8 amps. Twelve volts x 8.8 amps = 105.6 watts. We need to know the watts consumed.
If the fan was to be run for one hour it would consume 105 watt-hours; 2 hours run time = 105 x 2 = 210 watt hours.
Batteries are rated in amp-hours. A typical 6 volt golf cart battery has a rating of 210 - 224 amp-hours. If we use 50% as the maximum amount we can discharge the battery that gives us 105 amp-hours at 6 volts. We would need two batteries wired in series; negative to positive in a chain, to provide 12 volts. That would provide total capacity of 12 x 210 = 2520 watt hours. We should only use half; = 1260 watt hours. 1260 divided by 105 = 12 hours run time (approx, in theory).
I assume the fan would only be required to cool the
greenhouse; is that right? If correct the PV cells/modules would be providing power while the sun was shining, so if the PV cells supplied enough power they could stay ahead of the batteries. If the charge current was less than the draw the battery drain would be slowed.
Lead acid batteries are not perfect, there is somewhere between 10 to 20% loss in power when the charge/discharge cycle is considered. That means the PV source should be sized about 20% larger than what may be drawn out. Very rough numbers.
Next the task would be to calculate how many cells would be required to supply the power needed to recharge the batteries before the next use cycle.
Different approach. I have a solar air collector. Actually two. I power those fans directly from a store bought solar module. No batteries, no charge controller. When the sun shines the panel is making power. When the air collector heats up a simple thermostat turns on the fan. It runs until the collector is cooler or the sun goes away. That works for that application quite well. Whether or not you can use a similar idea depends on if the fan only needs to run when the sun shines. The fan runs slower if a cloud suddenly obscures the sun, but overall it works and require no batteries and no battery maintenance.
Note that successful off grid power systems will normally be sized to provide three days of autonomous, no sun. power. Again, if this is for cooling, if their is not much sun the power requirements may be reduced as well, so that may not be a big deal.
Now the hard part, some of us will see this as an economically viable project. Other not so. I belong to the second camp. Yes a home made panel will cost less in dollar outlay, but requires a lot of labor. With the requirements of mounting the cells in a weatherproof assembly, something the sun will not degrade and that will keep out the
water and humidity it's just too big a task to assemble enough cells to have meaningful power. Fans draw a lot of power. But that's just my take on it.
If you Goggle diy solar cells or something like that many results will come up. A lot of them want to sell
books, instructions, etc. or kits. Caveat emptor.
Decades ago I assembled a small PV panel. It powered a radio; very small drain. It worked.