Calculating Solar Panel Efficiency: What You Need To Know

It’s important to understand a solar panel efficiency rating after it goes through standard test conditions to calculate how cost-effective a solar power system is. 

Several factors affect solar cell panel efficiency, as listed below:

🔑 Key takeaways

1️⃣ The type of solar cell panel is the most obvious variable. In addition, the different materials used in the construction and the density of the voltaic cells affect the solar panel efficiency.

2️⃣ For testing purposes, the panels are rated at an assumed installation on or near the equator, where a square meter of ground on the planet receives approximately 1kW (1000 watts per square meter) of sunlight daily.

3️⃣ The ambient heat has a direct impact on the efficiency of the solar cell panel. Therefore, panels are tested at 25oC (770F) to achieve a standardized rating.

With conventional energy supplies becoming stretched and the ultimate demise of traditional fossil-fueled electrical power stations, an efficient, renewable power generation system is becoming urgent.

🦾 What Is Solar Panel Efficiency?

Solar efficiency is the most common metric used to compare different solar panels.

A solar panel’s efficiency measures the amount of sunlight (energy) that is input into the solar cell panel and compares the ratio of energy which is produced by the solar cell panel.

The best way to find a solar cell panel’s efficiency is to check the manufacturer’s datasheet.

The efficiency rating is a standard figure based on the panel used at the equator (1 kW per meter squared) at a temperature of 25oC for one day.

ℹ️ Why Is Knowing Solar Cell Efficiency Important?

There are two main reasons why knowing solar cell panels’ solar efficiency is important:

  1. The price per unit of energy.
  2. The number of panels required.

The Price Per Unit Of Energy

When you know the cost of the panel, the solar efficiency, and the panel size, it is easy to calculate the average cost per watt.

With this knowledge, you can calculate the cost of the total system and the savings (and payback period), which will be achieved by moving off the grid.

The Number Of Panels 

Apart from budgetary considerations knowing the number of panels needed to meet your energy requirements means you can assess if there is enough space on the roof to mount them.

The alternative will be to use a ground-mounted solar panel frame.

♨️ What Factors Affect Solar Efficiency?

Several factors affect the ultimate cell efficiency of the solar cell panel, as follows:

  1. The type of solar panel.
  2. Insolation (Solar Radiation).
  3. Irradiance (Power Of The Sun)
  4. The ambient heat.
  5. Solar Panel Orientation.
  6. The size of the Solar panel.

The Type Of Solar Panel

With current technology standards, there are six types of solar panel technology. The solar cell efficiency of each type is listed below.

Passivated Emitter and Rear Cell (PERC) panelsRigid5% more than Monocrystalline
Monocrystalline solar panelsRigid20% and up
Solar Roof TileRoof Tile10–20%
Polycrystalline solar panelsRigid15-17%
Copper indium gallium selenide – CIGSThin-film13-15%
Cadmium telluride – CdTe Thin-film9-11%
Amorphous silicon – a-Si Thin-film6-8%

📖 Read next: Types Of Solar Panels [Pros & Cons]

Insolation (Solar Radiation)

Insolation is defined as the amount of solar energy which is produced by the sun.

The unit of insolation often used is kilowatt hours per square meter (kWh/m2).

It is affected by the panel’s position relative to the sun’s angle (Longitude), the air mass, the length of the average day, the amount of cloud cover, and the pollution level.

Insolation levels are highest at the equator and reduce the further North or South at which the solar cell panel is situated.

Irradiance (Power Of The Sun)

Irradiance is the calculation of the sun’s power (measured in watts,) which enters the solar panel and is converted to electrical energy. 

At midday, at an installation on or near the equator, a square meter of ground on the planet receives approximately 1kW (1000W) of sunlight. This is the standard that’s applied when rating a solar cell panel’s efficiency.

The 1kw value is an average and ignores the effect of changes in insolation (sun’s angle (Longitude), the air mass, the length of the average day, the amount of cloud cover, and finally, the level of pollution.)

Ambient Temperature 

Terrestrial solar panels (as used on earth) are calibrated according to the rAM1.5 conditions while at a temperature of 25°C

The efficiency of solar panels is affected negatively by temperature increases.

The effect of heat on a solar panel is linear – it is so predictable that it is possible, in theory, to use the solar panel as a temperature gauge.

For every degree (centigrade) that the temperature increases over 25°C, the maximum potential power of the solar panel reduces by 0.258%. 

The opposite is also true, in that for every degree the temperature drops below 25°C, the power produced by the panel increases by 0.258%.

In locations where the temperature is very high, steps should be taken to reduce the effect of the heat, as follows.

  1. The solar panels should be installed with a gap behind them to allow airflow around the panel.
  2. Use panels that are made with light-colored materials to reduce the amount of heat that is absorbed
  3. The systems components (inverters, combiners, charge controllers, batteries) should be installed in cool areas which are well-ventilated and away from sunlight.

Solar Panel Orientation

The orientation of the solar panels to the sun is a significant influencer on the performance.

South of the equator, the panels must point North, and the North of the equator, panels, point South.

🙋‍♂️ Other FAQs

What Is The Standard Efficiency Of A Solar Panel? 

The standard efficiency of a solar panel is calculated as the amount of the sun’s energy converted into electric energy at a temperature of 25oC over a panel measuring 1m2.

For every degree of temperature change, the electrical energy produced by the panel increases (as it gets colder) and decreases (as it gets hotter) by 0.258%.

Knowing the temperature means that the watts produced can be measured over 1m2 of solar panel surface area.

The standard efficiency of a solar panel typically ranges from 15% to 20%. This means that a solar panel with an efficiency of 15% will convert 15% of the sunlight that hits it into usable electricity.

Will Solar Panels Ever Reach 100% Efficiency? 

The technology does not exist to manufacture a solar panel with 100% module efficiency.

Silicon is the primary material used in solar panels. The theoretical limit of silicon solar panels’ efficiency is approximately 32%.

Unfortunately, although silicon is the best material used in conventional solar panel manufacture, it is not perfect.

Most mass-produced solar panels experience additional power loss (due to inefficiencies in the manufacturing process). These include the quality of the connections between individual cells, reflective metal contacts, and gaps between the cells.

The result is that a maximum efficiency of approximately 25% is achieved in ideal conditions.

What Does 20% Efficiency On A Solar Panel Mean?

A solar panel rated at 20% efficiency means that 20% of the energy (sunlight) which enters the panel is converted to current (electricity).

Assuming the following variables:

  1. The temperature is 25°C (standard temperature.) 
  2. The irradiance produced by the sun is 1,000 (1kw) W/m2.
  3. The solar panel measures 1m2 
  4. The solar panel is rated at 20% efficiency.

Under these conditions, the solar panel would produce 200 Watts (0.2kw.)

🔑 Key Takeaways

1️⃣ Knowing the solar efficiency of the solar cell panels you purchase ensures you get the “most bang for your buck” with your purchase, as you want the highest solar system performance.

2️⃣ It is particularly important to know in locations with compromised solar conditions where the sun’s irradiance may be limited.

3️⃣ When you are considering which solar panels to purchase, don’t be fooled by nominal power ratings (the number of watts produced by the panel.) 

Remember, this value is calculated by the manufacturer using ideal conditions. Therefore probably unrealistic.

4️⃣ Always check the manufacturer’s datasheet to establish the solar efficiency. Once you know this, ensure it aligns with the expected efficiencies for each solar panel type.

This useful solar panel efficiency calculator should help you with your future system setup requirements. And visit our knowledge hub for more articles on solar panels.

I’m the website operator and editor here at ALTA Devices. The solar revolution is the most exciting thing to happen in a generation! I’ve written extensively on solar, electric vehicles, and the electrification of the marine industry. You can find out more on LinkedIn below: