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