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At the time of writing this blog post there is a huge trend towards installing solar power, raising many basic questions among home-owners. Some are complex, but most focus on the first principles of electric PV (photovoltaic) production.
Different types of solar panels have different efficiencies but they are all governed by the same basic rules – light is converted to energy and more light equals more energy. Unfortunately, it’s really difficult to determine exactly how much power a solar panel array is going to generate at any particular time.
As a general rule solar panels start working as soon as their is light to convert into energy. However, the amount of energy produced depends on the strength and angle of incident of the sun’s rays. In the early morning, when energy production starts, the amount of energy generated is very low.
- What time do solar panels stop working?
- What time of day do solar panels work best?
- How many hours a day do solar panels work?
- When do solar panels generate electricity?
- When are solar panels most efficient?
- When are solar panels worth it?
- Do solar panels work at night?
- Can solar panels store energy?
What time do solar panels stop working?
The best way to illustrate the times at which solar panels start and stop working for any average day is by showing a production curve – see below:
The above curve compares the amount of power generated by a solar power system compared to a home’s energy usage. Solar panels start working around 6 am but the output is extremely low, gradually building up towards mid-day, when it’s at its maximum.
Similarly, at the end of the day solar panels stop working in the evening around 6 to 7 pm depending on the region. It’s impossible to estimate what the power production would be at these times, which is why historical irradiance records are generally used to estimate a solar system’s output.
Best solar panel for low light conditions
The best low-light solar cells are the type with the highest efficiency, which is monocrystalline. The table below shows the efficiencies of the different cell technologies and the most efficient brands:
Thin silicon layers
Up to 22%
Maximum of 10%
25 to 30 years
20 to 25 years
15 to 20 years
Solar Panel Efficiency (Max %)
Country of Manufacture
What time of day do solar panels work best?
Solar installers use a rule of thumb for assessing solar panel production, using the 4 hours in the middle of the day for sizing solar systems. A value called Peak-Sun-Hours is used and is multiplied by the kilowatts of solar panels installed to roughly assess the amount of power that the system will generate.
Peak-sun-hours is also known as the insolation for a particular location. It is measured in kWh/m2/day (kilowatt hours per square meter per day) and is a measure of the amount of energy falling on solar panels. It can also be expressed as a yearly value.
That’s not the whole story, however. Solar panel output is impacted by the direction in which it they point (orientation), also the tilt angle they are mounted at and various other factors. Insolation and irradiance values for any location can be found by using sites like https://globalsolaratlas.info/
Simply enter you state and town to view historical solar data.
Then multiply your solar system power rating by Peak-sun-hours to give the total energy production over the year.
How many hours a day do solar panels work?
Solar panels work for as many hours of the day as there is sunlight, which is the main factor affecting overall energy production. The strength of the sun’s rays varies a great deal throughout the day, weakest in early morning and late evening, and strongest during the 4 hours in the middle of the day.
Solar installers tend to ignore production at the either end of the day, preferring to ass solar panel output using the average Peak-Sun-Hours found in historical solar data. However, strictly speaking, solar panels will work all day as long as there is sunlight.
When do solar panels generate electricity?
Solar panels generate electricity during daylight hours Spring, Summer, Winter and Fall. However, the amount of solar power generated varies considerably throughout the day and also throughout the seasons. The table below shows the variation in peak-sun-hours, or sun-strength, for each month in Algarve, Portugal:
Month - Location: Algarve, Portugal
Peak Sun Hours (kWh/m2/day)
Instantaneous assessments of solar power production are not really useful – it’s the average value over time that is the important data.
When are solar panels most efficient?
All solar panels are most efficient when mounted under direct sunlight at an angle of 90 degrees to the sun’s rays in the middle of the day. A good angle to mount panels for optimum production throughout the year is 60 degrees.
The table below shows the difference in peak-sun-hours for solar panels mounted flat compared to an angle of 60 degrees:
Insolation by month for Houston, Texas by month (kWh/m2/day - also are known as Peak-Sun-Hours)
What affects solar panel efficiency?
Solar panel efficiency is determined by the following factors:
- solar panel type – monocrystalline is the most efficient
- shading should be eliminated
- heat reduces solar panel efficiency – 0.5% for every every degree C above 25
- regular cleaning can improve efficiency by 10 to 20%
- location and the amount of peak-sun-hours for your area
- orientation – the direction in which the solar panels point
- tilt angle – the incident angle of the sun’s rays affect solar panel efficiency
- use of MPPT technology (Maximum Power Point Tracking)
Is solar tracking worth it?
Solar tracking refers to the practice of automatically following the sun’s movement across the sky during the day in order to maximize irradiance and therefore power production.
During the day the sun moves across the sky from East to West horizontally (in the Northern hemisphere) and also up and down (vertically). The amount of movement varies across the seasons.
A sun-tracker which follows the sun’s horizontal movement is called a single-axis tracker and one which follows both horizontal and vertical movement is called a dual-axis tracker.
The above image shows how a sun-tracker recovers a lot of energy in the morning and evening, when the sun’s energy is very low. At these times, aligning solar panels at an angle of 90 degrees to the sun’s rays makes a big difference to overall energy production.
Unfortunately, auto-trackers are expensive, costing up to 30% of the total solar installation cost. A dual-axis system can recover up to 40% of the solar energy lost during the times when the sun’s angle is very low but is it worth the cost?
My feeling is that it may be worthwhile only in special circumstances. For example, if roof mounting is not possible and general insolation is low for that region. In these cases it may be cost-effective but it depends on an evaluation carried out by a professional installer.
The big game change over the last 10 years is the tumbling prices of solar panels, which have reduced by 80% since 2010. It’s very often much more effective to simply install a few more solar panels, if you have the roof space.
When are solar panels worth it?
Solar panels are worth it when the energy production matches your home energy consumption and the irradiance in your area gives a reasonable payback period of between 9 to 12 years.
Installation payback time is easily calculated by dividing your solar installation costs by your annual solar energy production savings – see calculation example below:
- Installation cost = $12000
- Energy saving per year = $1150
- Payback period = 12000/ 1150 = 10.43 years
Do solar panels work at night?
Solar panels cannot work at night because there is no sunlight to convert into electrical energy. While it’s true that a certain amount artificial light can be converted, the process is very inefficient, as not all light frequencies are converted to electrical power at the same rate.
Plus, artificial light sources tend to come from one source rather than be diffused like sunshine. The only light available at night-time is the reflected sun’s energy from the moon. The maximum amount is reflected at full moon on a clear night.
However, the amount of reflected moonlight is extremely low and cannot be converted into any meaningful amounts of solar power. Moonlight is 10 million times weaker than sunshine, which is an indication of it’s power generating capabilities.
Can solar panels store energy?
Solar panels cannot store energy in any form. Solar cells are basically semi-conductor junctions which convert the sun’s energy into electrical energy. They have no circuitry or chemical composition that would allow them to store energy.
Light energy in itself cannot be stored in any form, but must be converted to electricity or heat before it can be conserved for later use.
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