Do Solar Panels Need Direct Sunlight? Solar Panel Efficiency

Do Solar Panels Need Direct Sunlight_featured image

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Can solar panels work in direct sunlight?

Is full sunlight best for solar panel operation?

Solar panels are not particularly efficient. The best can be up to 20% efficient, which means that for every 1000 watts of the sun’s energy falling onto a solar panel it will generate about 200 watts of electrical power.

It isn’t a very good deal! The amount of sunshine energy available for energy conversion is called irradiance and is measured in kilowatt-hours per square meter per day or year (kWh/m2/day).

It’s also known as peak-sun-hours and solar installers often use an average of 4 peak-sun-hours per day for making quick estimates when sizing solar systems.

Solar panel power output is directly proportional to the strength of the sunlight falling on its surface.

Do solar panels need direct sunlight?

Solar panels absorb light particles from the sun rays known as photons and convert them into usable electrical energy. Direct sunlight is the most efficient but some particles will still get to the panels even when there is indirect sunlight. Solar panels don’t necessarily require direct sunlight to function, as indirect, diffuse and reflected sunlight can all be converted into energy.

Solar panels can operate in a variety of weather conditions, including fog, clouds, rain and snow!

As you can imagine, the amount of power generated when sunlight is restricted in this way is very low – perhaps 10% of the power produced in direct sunlight.

Video – Do solar panels only work in direct sunlight?

How much sunlight does a solar panel need?

The amount of sunlight that a solar panel requires to perform optimally is determined by various factors:

What is the best roof angle for solar panels?

The table below shows the difference in irradiance levels (peak-sun-hours) in Las Vegas comparing flat panels against a title of 54 degrees:

Insolation by month for Las Vegas, Nv by month (kWh/m2/day - also are known as Peak-Sun-Hours)

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Flat

2.83

3.64

5.18

6.50

6.73

7.36

7.62

6.22

5.41

4.29

3.14

2.17

54 °

4.63

5.11

6.32

6.70

6.74

6.33

6.27

6.06

6.15

5.85

4.99

4.48

In winter time the tilted panels can generate almost twice as much power when tilted than if mounted flat.

As a general rule, the best tilt angle for any location in the summer months is the location latitude -15 degrees and +15 degrees in the winter.

How does solar irradiance affect solar panels?

Irradiance, or the amount of sun energy, is by far the biggest factor affecting solar power generation. It varies by location across the globe and the differences can be significant – see table below:

City

Chicago, Il

London, UK

Glasgow, UK

Houston, Tx

San Fran., Ca

Irradiance (peak-sun-hours)

4.043

2.374

1.860

4.253

5.699

A solar array in San Francisico generates 3 times more power than a similar power system in Glasgow, UK. Chicago has the average US irradiance of 4 peak-sun-hours.

Does orientation of solar panels matter?

Orientation means which direction the panels are facing. The very best direction in the Northern hemisphere is due South, and the very worst is due North. (In the southern hemsiphere it’s the opposite way round!)

However, due to the angles of most roofs and the effect of diffuse sunlight, the reduction in power output due to orientation isn’t as much as you might think.

North-facing solar panels suffer from a reduction in output of 16% to 20%, while East or West-facing panels generally see a drop of about 15%.

The impact of shading on solar panel power generation

Shading can have a huge effect on power output and this is where the solar power system topology becomes important.

Topology is the way the system is designed. It indicates how many strings of solar panels are used, how many panels per string and what kind of inverters are used.

If one or more solar panels become covered by a passing cloud or branch of a tree, it can severely reduce the output of all the panels grouped together – called a ‘string’.

A central inverter, or even a string inverter, will not help in minimizing a big reduction in power generation (although a string inverter will do better than a central inverter.)

Modern solar power systems often use micro-inverters to reduce power drop due to intermittent shading.

These devices convert DC to AC at the individual panel level and so if one panel output is down, it doesn’t affect the rest of the solar array.

What affects the output of a solar panel?

There are many factors affecting the power output of a solar panel system.

Panels are rated by manufacturers accord to the STC (Standard Test Conditions). Among other things, it specifies panel output at an irradiance of 1000kWh/m2 at a temperature of 25 degrees C.

Basically, these are ideal laboratory conditions and quite rare in real-life situations.

In fact, a commercial or domestic solar power system has quite a few losses, some are inherent to the system design, while others can be reduced by the user.

The inforgraphic below shows the details of 10 major solar PV system losses:

Energy losses for domestic solar panels sizing

Infographic showing solar panels losses for home power system

You can expect a power output of around 75% of the solar panel ratings in a practical system.

Do solar panels work with Moonlight?

No, moonlight does not have enough energy to run a solar panel. Solar panels are designed to operate at maximum efficiency in full direct sunlight.

Although the moon will shine bright during the night, it does not generate any light – it reflects light from the sun.

Moonlight is 10 million times less powerful than the sun’s energy.

How are solar peak hours calculated?

A solar peak hour is the amount of time in which the intensity of the solar irradiance reaches an average of 1,000 watts (W) of energy per square meter.

Simply put, it’s calculated as follows: One solar peak hour is equal to 1000 W/m² of sunlight per hour.

Solar installers use this value to estimate the power output of solar panel arrays, often using a US average value of 4 peak-sun-hours.

Irradiance values can be taken from historical data tables such as those found on websites such as Global Solar Info – see image below:

Irradiance level for Tampa Florida

Daily peak-sun-hours is the same as irradiance value in kWh/m2/day

Do solar panels work at night?

Solar panels cannot work at night as there is no sunlight available.

Solar cells generate (convert) the sun’s energy into electrical power, but cannot store it for later use. Off-grid solar power systems use extensive energy storage batteries to provide night-time electricity.

What time of day are solar panels most efficient?

Your location has a great impact on how efficient your solar panels can be. 

The sun’s intensity varies throughout the day. It’s very low early morning and in the evening, while at its strongest during the 4 hours around mid-day.

The image below shows typical power output during daylight hours:

Solar panels tracking the sun to maximize output in watts

The blue line denotes the power output with a fixed panels system. Maximum power generation as at the solar mid-day point and ouput in the early morning and late evening is very low.

The red line shows the effect of solar tracking, where the panels turn to follow the sun’s journey across the sky. 

Duel-axis solar tracking can improve power output by up to 40%.

How many hours of the sun should you get a day?

You should know the average peak sun hours of the region you live in before going the solar route. The state you live in is a great determinant.

For instance, different areas in the United States can have an irradiance level between 3 to 6 peak sun hours – that’s quite a difference in possible power generation levels.

Irtradiance also varies across the months of the year depending on the season – again, this difference can be considerable.

The table below shows irradiance over 12 months for Houston, Texas:

Insolation by month for Houston, Texas by month (kWh/m2/day - also are known as Peak-Sun-Hours)

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

2.70

3.23

4.25

4.98

5.61

5.79

5.94

5.44

4.88

4

3.03

2.56

Do solar panels last forever?

Just like any other device, a solar panel will deteriorate in normal use.

Though it doesn’t have any moving parts, the chemicals break down and deteriorate over time.  Debris and storms might damage the panels as well.

The majority of solar panel manufacturers give a warranty of 25 to 30 years. However, panels can continue to generate power for up to 60 years!

A typical manufacturer’s guarantee will promise up to 90% of the initial power rating after 20 years. In general, power output reduces by o.8% for each year of operation.

What time do solar panels start working?

Though a solar panel can start working at 6 am (if the sun is shining), its output will be very low.

The power produced gradually builds up and reaches maximum between 11 am to 3 pm, when the sun is at its maximum energy.

Depending on the location, you can expect a solar panel to stop working at around 6 to 7 pm, or whenever the sun begins to go down.

Do solar panels work in the rain?

Yes, the rainy season does impact the output of your solar panel, and moreso if there’s a lot of cloud cover with it.

Therefore, anytime the clouds darken the sky and block the sun rays, the output becomes 40–90 % lower.

Though precipitation could lead to short-term output losses, the rains have positive side effects that clean the panels, especially in a dusty region.


 

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