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Influence of Maximum Power Point Tracking

Manufacturers of solar panels are optimistic when calculating the power rating of their panels. Thus, the power that is effectively produced by a panel is significantly lower than claimed on the data sheet. The power rating is only achieved at a certain voltage, at a panel temperature of 20 degrees Celsius and at a sun radiation of 1000 Watt per square meter. This is not realistic because a solar panel gets really hot at 1000 Watt radiation per square meter. High temperature reduces the effective power output of a panel. There is not much that can be done about it apart from keeping in mind that a panel never achieves the claimed power rating.

The influence of the panel output voltage is more important to consider in a autonomous system. If a simple charging regulator is used, the voltage in the panel drops down to the level of the battery voltage. A solar panel may have the best efficiency at 18 Volts -it may produce 1 Ampere at 300 Watt/m at 30 degrees Celsius. This point of best efficiency is called Maximum Power Point or MPP.

Thus, our panel would produce:

18 Watt = 18 Volt * 1 Ampere

If this panel is connected to a battery at 12.3 Volt the current will be slightly higher than in the MPP, maybe 1.1 Ampere, but the panel voltage will drop down to the level of the battery:

13.5 Watt = 12.3 Volt * 1.1 Ampere

The efficiency in our example would be only 75% with a simple charging regulator. This problem could be addressed by using a solar regulator with maximum power point tracking. A well designed MPP-regulator achieves an efficiency of 90%. A system with a simple regulator may never achieve more than 70% of the power rating given by the manufacturer.

Last Update: 2007-01-25