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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 are running big loads the power isn't needed so the power isn't generated.

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. The lower the battery voltage (Vbatt) the lower the panel voltage (Vpanel), therefore the lower the power output.2) The output can be increased somewhat by tweaking battery voltages higher.

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

wiring losses

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 heat3) in the wiring. Solutions:

  • use shorter runs
  • use thicker wire
  • if multiple panels, run them in serial instead of parallel to decrease carried current

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:

Ambient temp in F Ambient temp in C derated power
32F 0.0C 97w
40F 4.4C 95w
50F 10.C 92w
60F 15.6C 90w
70F 21.1C 87w
80F 26.7C 84w
90F 32.2C 82w
100F 37.8C 79w
110F 43.3C 76w

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.

zenith angle

The sun will climb in the sky until it reaches its highest point for the day (local solar noon), then will start dropping again. For a given latitude and time of day the sun's location in the sky is calculable and can give you the cosine of solar zenith angle (“cosine” hereafter).4). You can use the cosine to understand how much power your system might put out.

Examples: if you have 200w of panels, your mppt controller typically yields 83% after derating, and the calculated cosine is .70 then you might expect ~116w in clear conditions at that time in that location. (200 x .83 x .7 = 116.2).

Another way of thinking about this is that panel ratings are given for 1,000w/meter2. At that time and place only 700w/meter2 land on the panel. (1000 x .70 = 700)


Insolation5) is the 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:
  • altitude (total irradiance ~+2.67%/1000')8)
  • humidity9)

img.mousetrap.net_misc_wattsunderconditions.jpgBased on lux measurements, we can estimate how sky clarity/brightness affects the amount of power theoretically available to the panel. Takeaways: overcast skies greatly reduce output, and there is no meaningful power available at sunrise/sunset.

Also see these anecdotal observations.

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 very low levels of insolation10) 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 sun11) 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.

worst month

US insolation zones

cities FSE by month

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.



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.

solar magnifiers

Some conditions can cause an effective magnification of solar power, the opposite of shade in a way. This is usually caused by reflection of previously-uncaptured light onto the panels – the panels are receiving both normal direct light and the additional reflected light at the same time. Since panels are rated at a lab standard 1000w/meter2 , this multiplication of available light can cause the panels to make more than their rated power, and can cause Voc to rise beyond rated specs.


  • reflection from bodies of water
  • intentional reflection from reflectors (as on a solar farm)
  • reflection off snow
  • edge-of-cloud effect, where sunlight that would otherwise land far away elsewhere is reflected by clouds “near” the sun onto the panels. This is a relatively rare phenomenon, most often observable in overcast when there is a small break in thick clouds between the sun and panels.

panel tilt

Insolation figures are given for flat surfaces13) but you can increase yield by keeping the panel more perpendicular to the sun. The downsides are setup and takedown time/effort, and (with van-mounted panels) the requirement to park the van at a particular angle in the campsite. Tilt might be more practical for people who stay in one place for longer periods rather than moving every couple of days.

Optimum aiming involves both tracking the suns elevation above the horizon (zenith) and tracking the sun's east-to-west travel (azimuth). Doing both can increase yield ~30% but requires frequent repositioning.

Leaving panel tilt in a reasonable default is more common and has milder yield improvments. In this approach the panel is aimed at true14) south15) at a particular angle from perpendicular16) depending on time of year and latitude.

Tilt angles calculated by

30 degrees latitude is near the southern border of the US; 45 degrees latitude is near the northern border.


According to Sandia lab testing, dusty panels cause a derating of about 5%.17)

further information

This is what a panel's efficiency refers to: the percentage of 1000w the panel can harvest per square meter of surface area.
Vpanel * ~Imp = P
from resistance
the complement of elevation angle
Incoming Solar Radiation
sunrise, sunset, full moon, streetlights
1000W/square meter
watts to amps constant for 12v nominal system
like flat-mounted panels
not magnetic
in the northern hemisphere
ie, flat panel
FIXME citation
electrical/solar/output.txt · Last modified: 2021/04/28 15:18 by frater_secessus