For various reasons it will be a rare event when your panels put out their full rated power. One reason for this is the power needs somewhere to go; unless you are in Bulk stage or are running big loads you will not see the system making anything like it's capacity. Even under perfect conditions.
The other big reason is that power ratings are derived from testing under specific lab conditions. The standard lab test is at:
A situation where you might get the panel's rated power (or even a bit more!) would be when the sun is directly overhead on a cold, clear day at high altitude. Least power would be produced on a hot, overcast day when the sun is low on the horizon.
MPPT controllers can run the panel at max output when needed, but are much more expensive.
Long runs of wire between the panel and controller can result in losses that make the panel appear to be putting out less power. In reality, the lost power has been converted to heat2) in the wiring. Solutions:
Solar panels are dark in color and get very hot. Unfortunately, voltage (and therefore power) output decreases as panel temperature increases. This is the reason an air gap between the panels and the camper's roof is recommended to allow cooling airflow.
The data below, derived from this calculator, show power from a 100W mono/poly panel dropping off as ambient temps rise:
|Ambient temp in F||Ambient temp in C||derated power|
So a snowbird who “chases 60” will be losing ~10% of panel output during the warmest part of the day. Snowbirds chasing 70 will be losing ~13% of panel output.
insolation - the amount of solar power reaching the panels. This can be affected by:
Latitude affects both the angle (incidence) of sun to the panels and, to a lesser degree, seasonal hours of daylight. Greater latitudes (closer to the north pole for the US) will have lower overall insolation averages than lesser latitudes. They will have more extreme variation in insolation between summer and winter. The most striking example of this is when those regions have 24hr sun in summer and 24hr night in winter.
This means any power generated at low levels of insolation will likely be trivial. Increasing panel Voc to try to get more power in marginal conditions may not be effective. Consider overpaneling instead.
Insolation maps attempt to combine the effects of the variables above to estimate hours of full sun5) equivalent (FSE) per day.
Areas with atmospheric extremes will be outliers when compared against other locations at their latitude. Oregon and Washing, for example are low insolation outliers because of the famously drizzly weather. Phoenix and the southwest are high insolation outliers because of an unusual percentage of sunny days.
You can multiply your panels' temperature derated output by the hours of full sun equivalent to get an idea of the maximum harvest you can expect from the panels.
Partial shade in good sun can have a drastic effect on panel output. This is because panels are made of strings of individual solar cells; having some strings turned “on” and some “off” (to prevent reversing current into the shaded cells) can result in dramatic power reduction.
This effect can be attenuated somewhat by panel design (bypass diodes), parallel rather than serial panel connections when using PWM controllers, use of amorphous (thin-film) panels, and the use serial rather than parallel panel connections of MPPT controllers.
According to Sandia lab testing, dusty panels cause a derating of about 5%.7)