Better answer is the 1750 light-detection and measurement module.  

It connects by I2C and reads out a numeric value corresponding to the amount of light (Lux) it senses.

Very easy to set up.

I use one to measure the light of the sky so I can compare it to the output of my solar cells.

Here is a cool light meter project that uses one to sense light and displays the results on 7-segment displays:

BUILDING A SIMPLE DIGITAL LIGHT METER USING ARDUINO AND BH1750FVI SENSOR

No, I would suggest you not bother with photocell, phototransisters nor light-dependent resistors.

Better answer is the 1750 light-detection and measurement module. 

It connects by I2C and reads out a numeric value corresponding to the amount of light (Lux) it senses.

Very easy to set up.

I use one to measure the light of the sky so I can compare it to the output of my solar cells.

Here is a cool light meter project (complete with hookup instructions and code) that uses one to sense light and displays the results on a 7-segment display:

BUILDING A SIMPLE DIGITAL LIGHT METER USING ARDUINO AND BH1750FVI SENSOR