With only 5W of solar power, efficiency is going to be very important. A resistor, diode or even the Arduino's onboard regulator are going to waste about 60% of the input power. That would be expressed as 40% efficient.
A switching regulator, such as this one from Pololu, will be at least 80% efficient. You will get more than twice as much useful power from your solar panel. Connect it directly to the Arduino's 5V pin, bypassing the onboard regulator.