What Is Solar Irradiance?

Radiation may not be the first thing that comes to mind on a rare cloudless day when you’re basking in the sun…

But that’s what the sun’s energy that penetrates the Earth’s atmosphere as heat and light is…

Electromagnetic radiation, to be exact.

Without the radiation from sunlight, life on Earth would not exist.

The level of radiation a specific location receives from the sun impacts:

• Climate
• Photosynthesis
• Human Health
• Agriculture

It also directly determines the electricity output of photovoltaic (solar power) systems.

If you’re considering installing a residential solar power system at your home, estimating the available sunlight at your location is essential.

But how do you measure sunshine?

Let’s find out.

Solar irradiance measures the amount of electromagnetic radiation (sunlight) that reaches a specific location over a set period of time.

Measuring solar irradiance is essential for forecasting the performance and potential power output of solar power systems that generate electricity from sunlight.

By referencing historical data, it’s possible to make a relatively accurate prediction of how much power a specific solar panel or other photovoltaic (PV) module array can produce over time.

Investing in solar power without accounting for how much electricity you can reasonably expect a system to generate is like taking a shot in the dark.

Solar irradiance is measured with specialised instruments called radiometers that detect and quantify the various types of electromagnetic radiation at their location.
The most commonly measured types of solar irradiance are:

• Total Solar Irradiance
• Direct Normal Irradiance
• Diffuse Horizontal Irradiance
• Global Horizontal Irradiance

Global horizontal irradiance (GHI) is measured by a pyranometer and is the most commonly used metric for forecasting the electricity generation potential of solar power systems.

GHI measures the total solar radiation (direct + diffuse) a horizontal surface — like a solar panel — receives at a specific location.

Fortunately, it’s highly unlikely that you would ever need to measure the GHI at your location yourself.

Measuring and forecasting solar irradiance is essential in many fields, and historical data is readily available free of charge.

For example, governments, private companies, and learning institutions use measurements of solar irradiation for:

• Climate Monitoring
• Weather Forecasting
• Agriculture
• Building Design
• Solar Power Applications

In the UK, estimating GHI and photovoltaic potential at your location is as simple as entering your address into the Solar Wizard — a free, independent resource from the Centre for Sustainable Energy.

Additionally, Solargis provides free maps of historical GHI worldwide, including the UK:

Solar maps show historical data — not real-time information — but it’s more than sufficient to help consumers and utilities make informed decisions about the viability of photovoltaic installations at a specific location.

The above map shows global horizontal irradiation data in the UK measured in kilowatt-hours per square meter (kWh/m2). 

You’re likely familiar with kilowatt-hours (kWh) from your energy bills, as that’s how you’re charged for electricity consumption.

The map shows daily and yearly long-term averages by location for the period of 1994-2018 as measured in kWh per square meter of horizontal material.

For the purposes of evaluating solar panel performance, you would use the measurement of kWh/m2 of photovoltaic material.

This is an essential distinction because the entire surface of PV modules isn’t made up of solar cells.

As indicated by red on the map, South England receives significantly more GHI than the Northern UK, making London, for example, more viable for residential solar panel systems than Aberdeen.

High GHI typically means that you can achieve solar payback more quickly than you would with the same system in a less sunny part of the UK.

The solar payback period is the length of time it takes for your solar panel system to pay for itself through savings on electricity bills and/or participation in the Smart Export Guarantee (SEG) program.

The SEG pays homeowners for the electricity their photovoltaic system generates in excess of consumption and transmits back to the National Grid.

For many homeowners, however, a solar + storage system like EcoFlow Home Battery increases self-consumption, reduces reliance on the National Grid, and provides a better long-term return on investment.

Photovoltaic Power Potential maps like the one shown above are even more specific to solar panel systems than GHI maps.

However, they can be harder to grasp if you’re not well-versed in solar power systems.

Photovoltaic Power Potential is measured in kilowatt-hours per kilowatt-peak (kWh/kWp), which combines historical solar irradiance data with a model of how typical photovoltaic systems work.

For example, the red areas on the map indicate that a 1 kWp PV system in those locations will generate, on average, approximately 1095 kWh of electricity per year. 

To estimate the average daily production, you can divide this number by 365, though actual daily production will vary depending on the season.

Kilowatt-peak (kWp) measures the maximum power output of a photovoltaic system operating under ideal conditions, similar to peak afternoon sunlight at 25°C.

In theory, a 1kWp PV system with 4 x 250w solar panels can produce 1000w (1kw) of electricity per hour of peak sunlight. 

However, it’s essential to understand that no electrical system — including PV modules, inverters, and solar batteries — operates at 100% efficiency.

For example, solar panels from reputable manufacturers have their performance specifications — such as rated power and efficiency — measured in a lab under Standard Test Conditions.

Using Standard Test Conditions (STCs) helps ensure that consumers get accurate specs regardless of what manufacturer they purchase solar panels from…

However, it’s essential to understand that STCs simulate ideal environmental, installation, and electrical conditions that don’t occur in the real world.

It’s better to assume that you will require at least 20% more rated power than the spec listed on PV modules to regularly achieve the combined electricity output of the solar panels in your array.

For example, for a 1kWp system, 3 x 400w solar panels (1200w of combined maximum output) are more likely to generate 1000w of electricity regularly during peak sun hours than 4 x 250w PV modules with a combined rated power output of 1000w.

As always, ensure that you don’t exceed the maximum DC power input of your solar inverter or charge controller for systems with battery storage.

A reputable solar installer can help ensure your photovoltaic system meets your electricity generation goals.

It may be counterintuitive, but solar panels actually become less efficient in temperatures above 25°C.

That might not seem like a big issue in England, where the average summer temperatures are 20-25°C, but the outdoor temperature isn’t the only factor.

Most residential solar panel systems are rooftop installations designed to receive the maximum amount of sunlight during the brightest — and hottest — hours of the day.

However, it’s essential to understand that the photovoltaic effect — which generates direct current (DC) electricity from sunlight — relies on photon energy from light, not heat.

In many parts of the world that seem ideal for solar power — such as deserts with extreme heat — cooling systems are required to maximise solar panel efficiency.

That’s highly unlikely to be necessary in the UK… 

However, a well-designed installation that allows for spacing between the PV modules and the roof can maximise natural ventilation, minimising the heat of the solar panel array and ensuring you get the most out of your system, especially on hot summer days.

A multitude of factors —both astronomical and here on Earth — determine how much electromagnetic radiation from the sun reaches a specific location at any given time.

Rather than getting too cosmic, let’s focus on the terrestrial factors that impact solar irradiance, focusing on those you can avoid or minimise the effects on your photovoltaic system.

It goes without saying, for example, that geographic location plays a huge role in how much solar irradiance you receive, but let’s assume you don’t want to move to the equator just to generate more electricity from solar panels.

Let’s drill down on the factors related to how much solar irradiation your PV array receives that you can mitigate or control.

Most modern solar panels are easy to install and connect for anyone with basic DIY skills.

However, positioning them optimally to maximise electricity generation is an art unto itself.

Particularly with mounted rooftop installations, the orientation and tilt of the solar panels must be precise to capture the most light during peak sun hours each day.

A slight miscalculation can significantly reduce your solar return on investment over time.

Unless you’re using portable solar panels for off-grid adventures that are easy to adjust, it’s worth the additional upfront cost to work with a professional installer.

If you’re connecting to the National Grid with a grid-tied or hybrid inverter, working with a professional installer and electrician isn’t optional — the regulations require it.

Solar panels and other PV modules don’t work on their own.

They must be connected to each other with cables or wiring and to the balance of system (BoS).

Balance of system refers to all the components other than PV modules required to make the system work.

The most common balance of system components for residential photovoltaic solutions include:

• Solar inverter
• Solar battery*
• MPPT or PWM charge controller*
• Battery Management System*
• Automatic transfer switch+
• Cables and wiring
• Mounting hardware

If any of the balance of system components are inefficient or faulty, electricity generation from available solar irradiance will be reduced.

Most solar panels are remarkably resilient and often last 25-30 years before needing to be replaced.

That’s longer than many home roofs.

However, like everything else, PV modules do degrade with age.

Most solar panels lose about 0.5% of their efficiency per year, meaning they generate less electricity with the same amount of available sunlight.

Solar panels are durable and typically require little maintenance, particularly in the UK, where rain does a good job of keeping them clean.

However, anything that decreases the level of solar irradiation reaching the photovoltaic cells — such as heavy dirt or snow — should be removed to optimise performance.

Many home rooftops experience shade from neighbouring buildings, trees, and other potentially immovable objects during certain times of day.

A skilled installer can help minimise the negative impact of unavoidable obstructions and shade.

How PV modules are connected — in series, parallel, or a hybrid of both — can also mitigate the negative impact of shade.

You can’t control the weather, but you can take historic conditions at your location into account.

Solar panels still generate electricity on cloudy days, but not as much as in direct sunlight.

If you live in an area with high air pollution, your PV modules should be cleaned more frequently.

Understanding how weather and pollution patterns work in your location can help you form reasonable expectations for your solar panel system and improve performance.

Many people have never heard of solar irradiance, but almost everyone knows we rely on heat and light from the sun to sustain life on Earth.

If you’re considering switching to solar power to generate some or all of your household power, an understanding of solar irradiance fundamentals can help you make an informed purchase decision and determine your likely return on investment.

Hybrid solar + storage solutions like EcoFlow PowerOcean increase self-consumption and reduce your family’s dependence on the National Grid.

And unlike grid-tied photovoltaic systems without battery storage, EcoFlow PowerOcean continues to generate and supply electricity during power cuts.

Contact EcoFlow Home Battery for a free consultation today.

*Not required for grid-tied systems without storage
+Not required for off-grid systems