Below is the manufacturer-supplied data which accompanies a typical 80W panel.
Specifications for 80W PV cell
|Nominal Peak Power||75.00W|
|Open Circuit Voltage||21.40V|
|Peak Power Voltage||17.00V|
|Short Circuit Current||4.75A|
|Peak Power Current||4.45A|
This information is oftentimes misleading – which in itself can lead to consumers ending up with grossly undersized arrays for their needs. The way in which we derive the actual rating for the panel is by multiplying the Peak Power Voltage (in this case, 17V) and the Peak Power Current (here it’s 4.45A) together, yielding a result of 75.65W. However, based on the formula to find power (W = V*A) many systems will claim to be capable of producing a much higher Peak Power Current than the PV cells are actually capable of (because as voltage decreases, current increases proportionally for the same total power output). This means that on a 12V system, if you fall victim to this logic your system could be more than 40% undersized for your application.
Another thing to keep in mind is that these figures are derived under STC (25ºC/100kPa) with absolutely ideal irradiance and spectrum for the panels – in the real world their performance will be much below this rating as light will be less intense, and temperature on the surface of the PV cell will be far above the ideal (as surface temperature increases, efficiency of the panel drops significantly). Your angle to the Sun will also likely be very different from the laboratory condition which introduces further losses.
To realistically approximate your system’s output capacity, the following formula should provide a fairly accurate picture. (PC * h * 0.5), where PC is the Peak Power Current of the panel, h is the total number of hours where the Sun will shine during any given day, and 0.5 being the realistic inefficiency factor of the system. This assumes a flat distribution of the panels, more ideal angling towards the Sun will yield better results. While the cooler months bring with them much shorter daylight periods, they also bring a much cooler temperature which means the panels should experience a drop in surface temperature, and also, that high-draw appliances such as compressor fridges need to kick in much less of the time. It becomes a huge game of trade-offs, which is why it’s important to get the very best system to begin with. If you suspect this may present a problem, you may well be better off looking at a 3-way absorption-type fridge which will accept 12V, 240V, and gas inputs.