Enter the total number of photons and the total wattage (watts) into the Calculator. The calculator will evaluate the Photons Per Watt. 

Photons Per Watt Formula

PPW = P / W


  • PPW is the Photons Per Watt (photons/watt)
  • P is the total number of photons
  • W is the total wattage (watts)

To calculate Photons Per Watt, divide the number of photons by the total wattage.

How to Calculate Photons Per Watt?

The following steps outline how to calculate the Photons Per Watt.

  1. First, determine the total number of photons. 
  2. Next, determine the total wattage (watts). 
  3. Next, gather the formula from above = PPW = P / W.
  4. Finally, calculate the Photons Per Watt.
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

total number of photons = 1500

total wattage (watts) = 400


What is the significance of calculating Photons Per Watt?

Calculating Photons Per Watt is crucial in various scientific and engineering fields, especially in photovoltaics and lighting design. It helps in assessing the efficiency of light sources in converting electrical energy into light energy, which is fundamental in optimizing energy consumption and improving the design of lighting systems and solar panels.

Can the Photons Per Watt calculation be applied to any light source?

Yes, the Photons Per Watt calculation can be applied to any light source, including LEDs, incandescent bulbs, and fluorescent lights. However, the efficiency and output can vary significantly between different types of light sources, making this calculation a valuable tool for comparing their performance.

How does the wavelength of light affect the number of photons per watt?

The wavelength of light directly affects the energy of individual photons, as described by the Planck-Einstein relation. Light with shorter wavelengths (higher frequency) has more energetic photons compared to light with longer wavelengths. Therefore, for a given amount of energy (watts), a light source emitting at a shorter wavelength will produce fewer photons than one emitting at a longer wavelength. This is an important consideration in applications requiring specific light characteristics.

Are there any limitations or considerations when using the Photons Per Watt formula?

While the Photons Per Watt formula provides a straightforward way to calculate the efficiency of light sources in terms of photon emission, it does not account for other factors that may affect the overall efficiency and usability of a light source, such as the spectral distribution of light, the directionality of light emission, and the color rendering index (CRI). Additionally, practical considerations like the operational environment and the presence of any optical components that might affect the transmission of light should also be taken into account.