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electrical:solar:output

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Factors affecting solar panel output

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 have a big load on the system there is no point in making big power.

The other big reason is that power ratings are derived from testing under specific lab conditions. The standard lab test is at:

  1. cell temperature of 25C, which would be an ambient temperature of about 0C (freezing). Panel voltage decreases significantly as cell temperature increases.
  2. 1000w of insolation per square meter of panel. That is the amount of sun when the sun is directly overhead on a clear day. Insolation maps will estimate how much sun various regions get.1)

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.

charge controller type

Due to their design, PWM and shunt charge controllers will very rarely allow the panel to run at max output for given conditions.

MPPT controllers can run the panel at max output when needed, but are much more expensive.

panel temperature

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:

Temp in FTemp in CTemp derated power
32F0.0C97w
40F4.4C95w
50F10.C92w
60F15.6C90w
70F21.1C87w
80F26.7C84w
90F32.2C82w
100F37.8C79w
110F43.3C76w

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.

Note: that radiated heat from the underside of panels can raise temperatures inside the camper.

insolation

insolation - the amount of solar power reaching the panels. This can be affected by:

  • hours of seasonal daylight
  • clouds
  • rain, fog
  • angle at which rays strikes the panel (angle of incidence). At low angles effectively less panel area is exposed to sunlight.
  • amount of atmosphere the rays have to penetrate (less at solar noon, more at other times or anytime sun is relatively lower on the horizon)
  • air quality:
    smoke2),3)

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.

effects of insolation on power output

Poor insolation affects panel amps (Ipanel) radically but panel volts (Vpanel) stay stable until insolation is very low (like ⇐20%).

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

Insolation maps attempt to combine the effects of the variables above to estimate hours of full sun4) 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.

http://solarinsolation.org/wp-content/uploads/2012/01/world_solar_insolation.gif

worst month http://solarinsolation.org/wp-content/uploads/2012/01/usa_insolation_map.gif

average US https://ww3.wholesalesolar.com/grid.package_folder/insolatn.gif

cities per month http://solarinsolation.org/wp-content/uploads/2012/01/Solar_insolation.jpg

Derated output X FSE

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.

Example:

shade

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 cell) can

This effect can be attenuated somewhat by panel design (bypass diodes), parallel rather than serial panel connections, use of amorphous panels, and the use of MPPT controllers.

cleanliness

Dusty panels cause a derating of about 5%. FIXME

further information

1)
This is what a panel's efficiency refers to: the percentage of 1000w the panel can harvest per square meter of surface area.
4)
1000W/square meter
5)
watts to amps constant for 12v nominal system
electrical/solar/output.1550093010.txt.gz · Last modified: 2020/10/11 19:48 (external edit)