How much energy does photosynthesis need per O2 molecule produced?

This is not simple. Fasten your seat belt.

It takes 4 electrons to convert one CO2 to carbohydrate (the final product of photosynthesis). We know that at least 2 photons of visible light are used to boost each electron to an energy level capable of reducing CO2. We also know (as you can see from your equation) that for each CO2 fixed the plant releases one O2. Thus, we need to put in 6 x 4 x 2 = 48 photons of visible light to produce the 6 O2 molecules, theoretically. Under ideal conditions some inevitable losses occur so that a more realistic estimate for the photon requirement might be 60 (this is in agreement with some of the most respected values in the photosynthesis literature). So, now we need to calculate the energy content in those 60 photons of visible light.

A good median estimate for the wavelength of visible light (w) would be 550 nanometers (nm). Energy content of a photon (E) is given by Planck's Law E = hc/w where h is Planck's constant (6.63 x 10^-34 Joule-sec) and c is the speed of light (3 x 10^17 nm/sec). So, for a single photon of green (550 nm) light the energy content is

E = (6.63 x 10^-34) x (3 x 10^17)/(550) = 3.63 x 10^-19 Joule.

The energy content in 60 green photons is

60 x 3.63 x 10^-19 = 2.17 x 10^-17 Joule.

Energy Required For Photosynthesis

Firstly a Watt is not a unit of energy. A Watt is the rate of energy usage. The appropriate unit is the Joule.

Since each molecule of CO2 processed requires about 8 to 12 photons. Assuming that our photons are 170nm then for 6 molecules and

6*10*h*c/170nm = 7 × 10-17 joules

But bear in mind that for real plants, 94% of photons are wasted.

The efficiency of the photosynthesis processes combined is roughly equivalent to a Diesel engine's, approx 35% of total of received energy, under ideal conditions.

Efficiency = energy in sugar / energy absorbed

So find out the heat of oxydation for glucose, you know the number of oxygen molecules generated, and the efficiency of the process.

well the procces of photosynthesis is where the plant absorbs the sunlight and converts it into useable energy through photosynthesis, I'm almost positive it stores this "useable energy as chlorophyll in a "pocket" called the chloroplasts. This pocket is a feature the plant cell has that an animal cell doesn't. Depending on the colour of the chlorophyll will determine the colour of the plant for example a carrott has orange chlorophyll where as an apple has either red or green depending on the type.