Questions about terms of the Panel

In theory, "high noon" is 1,000 Watts per square meter of sunlight. And (crystalline) solar cells will be somewhere between 13% and 18% efficient under a thin sheet of low iron glass.

That will give you Watts per cell.

Each cell will have a Vmp (voltage maximum power) pretty close to 0.5 volts under full sun at Imp. If you have 36 cells in series, that will give you Vmp of 18 volts.

If you figure out that each cell is 2 watts (from the above sq meter per cell calculation), then:

P=V*I
Imp=Pmp/Vmp= 2 watts Pmp / 0.5 Volts Vmp = 4 amps Imp

And the basic panel specs will be:
Vmp=18 volts
Imp= 4 amps
Pmp = 18v * 4a = 72 Watt Pmp panel

For Voc and Isc, just using some standard panel numbers:
Voc=Vmp*1.22 = 18 volts * 1.22 ~ 22 Volts
Isc=Imp*1.24 = 4 amps * 1.24 ~ 4.96 Amps

Note that Voc and Vmp is fairly temperature dependent. Panels are rated at ~25C, but a working panel is a good 20+ C warmer than than in full summer sun. So, actual Vmp and Voc will be depressed by as much as 20% during summer... And conversely, Voc and Vmp will rise in sub freezing weather.

-Bill

Each cell is 0.5 Volt Vmp... two in series is 1 volt.

22 in series is 11.0 volts.

2x22=44 volts in series is 22.0 volts.

Note, you normally put the number of cells in series to match the needs of your loads/controllers/voltage converters. And that Vmp very cold Voc (voltage open circuit) to very hot (Vmp-hot, voltage maximum power hot) can have almost a 2:1 voltage range for Vmp.

Solar cells have been called solar batteries--But they are really solar current sources with a pretty wide output voltage range.

For example, the nominal "12 volt panel" to charge a 12 volt lead acid battery bank in cold to pretty hot weather is 36 cells in series for Vmp-panel~18 volts (at standard temperature conditions).

Fewer cells in series, you may have problem charging a cold battery bank on hot days. Too many cells in series, and you "waste" energy with standard PWM controllers (pulse width modulation--"on/off" switching controllers)--And with way to many cells in series, you can over voltage the PWM controller (Voc-cold).

-Bill

Remember there is resistance inside your solar panel, wiring drop, charge controller drop, temperature drop of hot cells, temperature rise of cold batteries, etc.

So, assuming these are crystalline silicon solar cells. Assuming you are going to use a charge controller. This panel is going to be mounted in a typical off grid 12 volt lead acid environment--sometimes hot, sometimes cold... You should assume ~0.50 volt per cell, and the battery will need close to 14.75 volts for "fast charging" of a flooded cell lead acid battery bank...

You will need 36 cells in series for Vmp~17.5-18.0 volts per "12 volt" panel. On a very hot/windless day, the actual Vmp-panel will be closer to 14.4-15.0 volts or so.

The "knee" of the I*V curve for Vmp and Imp is not "sharp"... You do have a volt or two that you can play with--The panel will not be producing optimum current/power, but will be producing "close enough" for daily use.

If you don't want to use a "series" type solar charge controller (directly connect the panel, and possibly use a "dump" style parallel connected charge controller in a non-hot desert environment)--You could get away with Vmp-panel in the range of 15.0 volts (30 cells in series). These use to be call "self regulating" panels (when solar cells and charge controllers where expensive and batteries where cheap).
. T
If you have "non-standard" Vmp-panels--You have two options. You can use a PWM (pulse width modulation/"on/off" series or parallel type charge controllers)--The "extra voltage" and cells do not do anything really helpful. Your solar panel operating Vmp will be around 15 volts. And if the panel is properly designed and wired, the panel will work just fine.

Efficiency wise, your panel would be around 18.0 volts / Vmp-panel efficient (example: 18v/24.96 = 0.72 ~ 72% efficient vs a 36 crll panel).

And a 12.48 volt panel will not charge a 12 volt battery bank. 12.7 volts is roughly the fully charged resting voltage of a 12 volt lead acid battery at room temperature. 13.6 volts is roughly the "float voltage" where the battery is only receiving a little bit of current to make up for self discharge. Lead Acid GEL should be charged at 14.2 volts, AGM at 14.4 volts, and Flooded Cell at 14.5 to 14.8 volts, with some industrial type lead acid flooded cell needing ~15.16 volts. And ~15-16 volts for "equalizing" the battery bank (controlled "over charging" to equalize each cell in the battery string).

Your other option is to use your 25 Volt Vmp panel with a MPPT type charge controller. These types of series charge controllers have a "switch mode buck type" power supply inside them (typically). This allow an MPPT controller to take high voltage/low current from the solar array and efficiently "down convert" it to low voltage/high current needed to charge the battery bank (with ~95% efficiency at rated current). MPPT controllers are much more expensive than PWM type charge controllers. But for larger solar systems, people use "GT type" solar panels (Vmp~30 volts typically). These large GT type solar panels are much less expensive than standard "Vmp~18 volt" panels and the cheaper solar panels can make up for the increased costs of the MPPT type charge controller. Also, with higher Vmp-array voltages, you can use smaller awg copper wire to the array, and have the array farther from the battery shed.

I don't know how much effort you plan on placing into your DIY solar panel(s)... Personally, I would highly suggest you just buy factory panels. The solar cells that you typically find on E-Bay are factory rejects from a solar panel company. And the difficulty to design, build, and install DIY solar panels, with their possible problems (including over heating and fire), other than the educational experience, I would not recommend installing DIY on your home or RV.

Just to give you an idea of what can go wrong with "home made" panels:

http://forum.solar-electric.com/discussion/3375/panel-fire-question

-Bill