We know that solar panels have about 20% efficiency. To calculate the solar panel or solar cell efficiency, we use the **solar efficiency equation**. We will look at how you can use this efficiency formula to calculate solar panel efficiency. It’s not exactly the easiest thing to calculate, but we will show you how to do the math *step-by-step*.

Namely, solar efficiency is expressed as the percentage of sunlight solar panels are able to turn into **useful electricity**. *Example:* If the irradiance of the sun shining on our solar panel is 100 watts per square foot, and the panels can produce 17.25 watts per square foot, that means the solar efficiency is 17.25%.

## Solar Cell Efficiency Equation

To drive a formula for solar cell efficiency, we start by using this basic solar efficiency equation:

**P _{max}= V_{OC} × I_{SC} × FF**

Based on this equation, we can write the formula for calculating the efficiency of solar panels like this:

**η (Solar Panel Efficiency) = V _{OC} × I_{SC} × FF / P_{in}**

Here we need to be familiar with all these variables (all of them are listed on solar panel specification sheets):

**V**stands for_{OC}. This is the maximum voltage a solar panel can give (in an open circuit = at 0 current (0 amps)).**O**pen-**C**ircuit**V**oltage**I**stands for_{SC}. This is the maximum amperage a solar panel can give (at 0 voltage).**S**hort-**C**ircuit**C**urrent**FF**stands for. This FF is the ratio that helps us determine the maximum power a solar panel can give (it ranges from 0 to 1).**F**ill**F**actor**P**stands for_{in}. This is the power of solar irradiance; we usually take**In**put**P**ower**100 watts per square foot**for most calculations.

Let’s solve an example to illustrate how to calculate solar panel efficiency using this equation:

Let’s say we have a normal silicon 100W solar panel. Normal silicon has an 80% fill factor (or 0.8 factor). The listed short circuit current (I_{SC}) is 5.20 amps, and the listed open circuit voltage (V_{OC}) is 24.1 volts. The panel is 41.8×20.9 inch dimensions and has a 6.07 sq ft area. At 100 watts per sq ft, that’s an input power (P_{in}) of 607 watts.

What is the efficiency of this solar panel? We put everything in the equation like this:

**η (100W Solar Panel Efficiency)** = 24.1 Volts × 5.20 Amps × 0.8 / 607 Watts = **16.52%**

With this equation, you can calculate the efficiency of any solar cell or solar panel (array of solar cells). Admittedly, there are quite a lot of variables you have to find, but eventually, you can find the solar efficiency of all solar panels just by using this one equation.