Understanding solar panel performance

Photovoltaic solar panels are over-rated! This article explains ratings and factors affecting solar panel output. Solar panels are effective only in strong sunlight and directly facing the sun.

Solar panel ratings

Standard conditions for testing solar panels are 25 degC and 1000 W/m2 solar irradiance. Here is an example performance plot for a 12 V 120 W solar panel :

Example solar panel performance plot with basic parameters annotated. Solar panel voltage and current have been measured over a full range of loads.

Example solar panel performance plot with basic parameters annotated. Solar panel voltage and current are measured over a full range of loads.

Basic solar panel parameters are:

  • Voc = open-circuit voltage = no current (infinite load).
  • Isc = short-circuit current (zero load).
  • Pmax = maximum power.
  • Vpmax = voltage at maximum power.
  • Ipmax = current at maximum power.

Current is almost constant up to Vpmax and then drops off sharply near Voc. Power (voltage times current) increases with voltage up to Pmax. 12 V panels usually have Voc around 22 V and Vpmax around 18 V.

Temperature and solar irradiance

The standard test conditions are quite optimistic. Firstly, solar cells absorb heat and can reach temperatures of 50 to 60 degC on a warm, sunny day. Voltage decreases by about 0.4% per degC and current increases by about 0.1% per degC (parameters from panels for sale on ebay). Vpmax for a hot solar panel is around 15 to 16 V and power may be 10% less than rated.

Example solar panel performance at standard conditions and at 60 degC. Solar cells absorb heat and generate less voltage at high temperatures.

Example solar panel performance at standard conditions and at 60 degC. Solar cells absorb heat and generate less voltage at high temperatures.

Secondly, 1000 W/m2 is the power of the sun on the earth’s surface at high noon on a cloudless day at the equator. Solar irradiance is lower at higher latitudes and in winter. 800 W/m2 ratings are rarely reported.

1000 W/m2
800 W/m2
Difference
Pmax (W) 54 38 -30%
Vpmax (V) 17.4 15.3 -12%
Ipmax (A) 3.11 2.49 -20%
Temperature (degC) 25 25
Ratings from the back of my Kyocera KC50T solar panel at both 1000 W/m2 and 800 W/m2 solar irradiance.

Shading and cloud cover

I did two experiments to measure solar panel performance with shading. In the first, I shaded parts of the panel(s) with a piece of cardboard. In the second, I moved the panel(s) into the shade of a tree.

Voltage was measured with a multimeter and current with an analog direct current ammeter. Current dropped to zero when half the panel was covered (i.e. there was insufficient voltage to charge the battery). Under light tree shade, the average decrease in current was 81 per cent.

Shading effects on solar panels. Covering 25 to 50 per cent of the cells on a single panel will reduce output current to zero. Results vary depending on which cells are shaded.

Shading effects on solar panels (volts solid, amps dashed lines). Covering 25 to 50 per cent of the cells on a single panel can reduce output current to zero. Results vary depending on which cells are shaded and the load.

I also have some data from variable weather conditions. In heavy overcast weather and rain, the average decrease in solar panel current was 88 per cent.

Direction and inclination

Next, I varied the direction and inclination of a two-panel array and recorded the changes in voltage and current. The maximum current decrease was 68 per cent, when the solar panel was turned away from the sun. Tracking the sun is necessary for best solar panel performance.

Orientation Amps Decrease
Facing sun 8.7 0%
Facing + standing vertical 7.8 -10%
Facing + laying flat 6.8 -21%
Turned right 6.5 -25%
Turned left 6.7 -23%
Turned away 2.8 -68%
Changes in solar panel current with orientation. The solar array was in clear sunlight for all measurements. Voltage varied little and is not presented.

Different solar controllers

Lead-acid batteries can be connected directly to solar panels but at risk of overcharging. The main purpose of a solar controller is to limit current and voltage to the battery, especially near full charge.

There are two common solar charge controllers. Pulse Width Modulation (PWM) operates near battery voltage. Maximum Power Point Tracking (MPPT) searches for the maximum power point, which is changing throughout the day with solar irradiance, cloud, shading, etc. MPPT converts the higher voltage to a lower voltage and higher current. Systems with PWM controllers are sized in amps. Systems using MPPT are sized in Watts.

Illustration of Pulse Width Modulation (PWM) and Maximum Power Point Tracking (MPPT) operating points.

Illustration of Pulse Width Modulation (PWM) and Maximum Power Point Tracking (MPPT) operating regions.

The difference between PWM and MPPT is not important in warm, sunny conditions where the maximum voltage of solar panels is reduced. MPPT is more helpful when the difference between maximum power voltage and battery charging voltage is large: in cold temperatures (Vmp high) and for deeply discharged batteries (charging voltage low). A 10 to 30 per cent increase in charging current is then possible with MPPT.

PWM MPPT
Low cost, low power systems High voltage panels, low voltage batteries, high power systems (> 240W)
Warm climates Cold climates
Shallow battery discharge. Deep battery discharge.
When to use PWM or MPPT solar charge controllers.

Solar panel cost

Most 12 V panels have 36 solar cells in series (36 times 0.5 V per cell = 18 V). Manufacturing labour costs are relatively constant for different module sizes if the number of cells is constant. Cost per watt therefore decreases with panel size. For example, one 200 W panel should cost much less than two 100 W panels. Big panels are less portable however.

Relationship between cost (A$/W) and power for Chinese 12 V solar panels. Data from ebay, March 2013, taking the three cheapest panels at each power rating. The equation can be used to estimate fair prices.

Relationship between cost (A$/W) and power for Chinese 12 V solar panels. Data from ebay, March 2013, taking the three cheapest panels at each power rating. The equation can be used to estimate fair prices. Solar panel prices have decreased since I collected these data!

Here’s another correlation, maximum power versus solar panel size. It can be used to check that the solar panel size is consistent with rated power (within say +/- 5%). Look out for small panels with unrealistic power ratings. Some (most) consumers never test their panels and would have trouble identifying falsely rated panels.

Relationship between solar panel size and power for Chinese 12 V solar panels. The slope is a measure of average efficiency = 15 per cent (you could multiply this by solar irradiance = 1000 W/m2). The negative intercept accounts for non-productive area in the solar panel frame, etc.

Relationship between solar panel size and power for Chinese 12 V solar panels. The slope is a measure of average efficiency = 15 per cent (you could multiply this by solar irradiance = 1000 W/m2). The negative intercept accounts for non-productive area in the solar panel frame, etc.

I recently bought some Chinese panels. They are not as good as premium panels, with lower build quality. I am yet to see them deliver more than 80 to 90 per cent of rated power although premium panels would have cost two to three times more.

Summary

For high power, solar panels should be placed in direct sunlight and facing the sun. Ammeters are recommended for monitoring solar power output.

Voltage Current Power
Heavy cloud
-88% c. -88%
Shade (tree) -7% -81% -81%
Solar irradiance (-200 W/m2) -12% -20% -30%
Direction (90 deg. from sun) c. 0% -24% c. -24%
Inclination (flat) c. 0% -21% c. -21%
Temperature (60 degC) -13% 4% -10%
Factors affecting solar power output, sorted by power loss (largest to smallest). Apart from temperature, current is more strongly affected than voltage. These results are from various sources, including my own limited testing. They are not definitive. Have fun doing your own testing!
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4 Responses to Understanding solar panel performance

  1. Outstanding post however , I was wondering if you could write a litte more on this subject?

    I’d be very grateful if you could elaborate a little bit further. Bless you!

    • bulumakao says:

      I have added some more, the watts-size correlation, and have nothing much further to say from my own research and testing. Good luck with your solar power projects!

  2. David says:

    It appears obvious that the source impedance of the cells changes with temperature which would explain why PWM controllers have trouble on hot days where the switching current pulses cannot be delivered by the panel without the voltage dropping below the ability of the PWM to regulate. At least this appears to be the case when using 60 cell panels that seem to not have sufficient voltage overhead when hot (24V system BTW). Any comments?

    • bulumakao says:

      I have recently designed a 24 V system and learnt that some 24 V panels have lower voltages and are not suitable for charging 24 V batteries. To allow for the various voltage losses detailed above, I suggest minimum 34 to 35 V at maximum power for charging. For your system, maybe you do not have enough voltage overhead. I suggest you measure the voltages and currents.

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