Short wave lift

What is cheaper with the air conditioning on: lamp or sunlight? - lamp or sunlight

I have no idea how much electricity costs, except for our account is in the summer months high. I use a light of some work on my desk.

I closed the curtains and the lighting and the use of a lamp to save power
Or
I received the curtains open to sunlight and do not use the lamp.

What is more economically efficient and why? Thank you!