You are correct, 2.3 amps at 12 volts does not equal 40watts.
From a practical perspective you can expect to get about 25 to 30 watts from this panel. For example, you could expect somewhere around 2.3 amps at 13 working volts, which yields 30 watts of practical output.
This is not a fault with the panel and you will find similar results with other panels (ie. a reduction from the stated theoretical power output).
answered 2 years, 1 month ago
In order to have an effective charge, the solar panel operates at 17.1V. 40W / 17.1V = 2.3 Amps.
answered 2 years, 3 months ago