Yes and no to both of the preceding posts (#15 and #16). Voc is around 0.6V/cell at 25C, and exact Voc can be found on just about every panel data sheet.
Voc is dependent on temperature, and increases with lower temperature. Panels are rated at 25C, and the TCVoc (Temperature Coefficient of Open Circuit Voltage) is around -0.4%/C (Voc will drop by 0.4% for every degree C increase in temperature >= increase for a drop in temperature). If you are pushing the limits of your system then you need to calculate Voc at the lowest temperature, at 0C it makes about a 10% difference (again, exact values can be found on most datasheets). The highest operating voltage is generally seen early in the morning when the sun starts to get bright and the panel temperature is still low, it drops quickly thereafter, but if your controller or inverter
limits are exceeded you can burn things out even though the total power is actually very low.
Edge effects on clouds can greatly increase irradiance values for a short time period. We have measured increases from 1000 W/m2 (generally considered the reasonable maximum irradiance value) to 1400 W/m2 with passing cloud edge effects. Voc does change slightly with irradiance (typically about 0.5V from 600 W/m2 to 1000 W/m2 for a 60-cell panel), the amount of change is generally insignificant. In addition, if you are near 1000 W/m2 it is very unlikely that your panels are operating at their lowest calculated temperature, as they quickly increase in temperature at that kind of irradiance level. Irradiance values can generally be ignored when calculating Voc from the published panel datasheet.