Solar Panel Rated vs. Actual Output – Why is it Lower?

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Is your solar panel not producing the output it is rated for? Maybe you have a 250W system but only getting 230W-240W even during clear skies? Perhaps you are thinking the panel is defective. Actually even a high end solar panel cannot maintain peak production for various reasons, and that is what this guide will explain.

Solar panel production is affected by several factors including efficiency rating, orientation, dirt and the angle of the sun. Solar panels with high efficiency and capacity ratings will produce at or close to their rated output in ideal conditions, 85%-100%.

Solar Panel Ratings and Efficiency Explained

Before going over the factors that affect solar panel production, some terms need to be explained. Knowing what these words mean will make it easier for you to understand why solar panels are not 100% efficient.

Rated Output / Power Output Capacity. This is the amount of power a solar panel can generate. Power is measured in watts (W) or kilowatts (kwh). A 100W solar panel can produce 100W per hour under ideal weather conditions, a 300W solar can produce 300 watts an hour and so on.

Efficiency. The efficiency rating measures the amount of energy the panel is converting into energy. Most solar panels have an efficiency rating between 17%-23%. A solar panel with a 21% efficiency rating means it converts 21% of the sun’s energy striking it into electricity.

Do not confuse the efficiency rating with the rated output. A 23% efficiency rating does not mean the panel will only produce 23% of its rated output in watts. However, the higher the rated output the greater the production.

A 300W solar panel will outperform a 250W solar panel even if both have a 2% efficiency rating. The larger panel has the advantage because it has more cells to convert solar energy. if both are 300W but one has higher efficiency rating, then it will generate more power.

17%-23% seems like a low efficiency rating, but it is a significant improvement considering that 15% was the average years ago. And again, that efficiency rating is sufficient for homes and RVs, especially if the solar panel is large enough.

Factors That Affect Solar Panel Output

Before you buy a solar panel, you have to know the biggest factors that affect its performance. It is a given that results will vary, but knowing what causes these will help you prepare. If you can make portable solar charger work faster, you can do the same with solar panels.

System Losses

Solar power systems incur energy losses during the conversion. Inverters may lose up to 10% energy, and cables could lose 2% or more. Even if your solar panel produces at its rated output, energy losses in other parts of your solar system will reduce the electricity that reaches the battery and your appliances.

The best way to deal with this is get a solar panel with a high efficiency rating and output. Some inverters have 95% efficiency so those are worth checking out as well. The extra amount you may pay will be repaid in terms of more electricity.

Dirt and Grime

Solar panels are low maintenance. Rain showers could easily wipe the dirt off and you don’t have to do a thing. But if you live in a dry climate you should clean the modules at least every six months. Look out for bird droppings or worse, bird nests. Even bits of foliage could lead to performance reduction. Consider a professional solar cleaning service if you have a rooftop solar array.

Strong Wind Gusts

A gentle breeze isn’t going to cause problems, but strong gusts could send leaves and other debris into the panel. Keep an eye on your solar system performance monitor. If you notice a dip in production during periods of strong wind gusts, debris might be all over the panel.


This will come as a surprise to some people. The way solar energy works, one would assume the hotter it gets, the better for solar panels right? Actually, solar panels produce less electricity when the temperature starts climbing.

Solar panels need energy from the sun, not the heat. PV modules are designed to run only under specific temperatures, and when it gets too hot the conversion rate goes down. The temperature requirement varies per solar panel so check the product specs. So don’t expect solar panel output to increase during the hottest days of the year.

Haze, Clouds, Fog, Mist

A passing cloud will hamper solar output, and even more so with haze or mist. Any atmospheric phenomena that blocks the sun could significantly reduce the output regardless of capacity or efficiency.


True south is the best orientation for solar panels in the northern hemisphere. In the southern hemisphere it is true north. Solar panels will still generate power regardless of orientation, but if you want the best possible location, set the panel to true south or north, depending where you live.

Sun Angle

When the sun is low, solar panels will generate less power. You cannot change this, but you can predict the sun’s angle movement by the day and the passing of the season. You can prepare ahead and reserve energy in your battery bank.

Why Your Solar Panel Output is Low – an Example

You installed a new 1000W (1 kwh) solar panel and have it oriented at true south. It is three in the afternoon and the sun is out. You check the system and everything is working. But then you see the output is only 800W. What happened?

There are many reasons why this can happen as pointed out earlier. Any or all of those factors can cut down output. The other factor is the season. Depending on where you are and the date, you might have shorter afternoons so the sun goes down faster.

During winter, the sun’s angle is lower so solar production will be limited. The opposite happens during the summer. Another factor is the panel design. A poorly made solar panel will be susceptible to heat even if the temperature is not that high. If the modules get too hot the output could drop by up to 10%.

The bottom line is there are a lot of reasons why solar panels do not produce their rated output. But knowing why this happens is critical in helping you get ready and take remedial steps.

How to Monitor Solar Panel Performance

If you bought a solar panel kit it probably comes with a performance tracker. If not, your inverter has a performance indicator, ether on the device or online. From here you can see how well – or poorly – the solar panel is running The best time to check is when the sun is over the panels. If your solar panel faces true south and performance is at 80% or more, the system is fine.

If the performance is below expectations, it does not necessarily mean the system is damaged. It could be due to any of the factors above. Even a small bit of shading could have a significant impact on the output.

Factors That Affect Solar Panel Efficiency

Solar power efficiency is affected by many of the same factors that determine output. In addition to those, the following elements play a role in deciding the efficiency of a PV module.

Material Composition

Solar panels are made of either monocrystalline or polycrystalline cells. Both work the same way but monocrystalline cells are more efficient. While poly solar panels have made strides, mono panels are still the premium option if you want the highest efficiency possible.

Rigid vs. Flexible Solar Panels

Flexible solar panels have improved a lot over the years, but rigid PV modules are still more efficient. Flexible panels have their and are best suited for irregular surfaces. If you want a portable solar panel then it is a good choice. But for efficiency, rigid panels are better.

Thermodynamics and Reflectance

Thermodynamics is the highest efficiency attainable. A good number is about 85%-86% though it can vary depending on the solar panel design. Reflectance is the degree of sunlight reflected instead of being converted. The lower the reflectance the better.

Frequently Asked Questions

Do Solar Panels Lose Efficiency?

Solar panels undergo degradation, the rate by which efficiency is reduced. The percentage can be 1% a year or more depending on the warranty. If you want the maximum efficiency, get a solar panel with a long warranty from a reputable manufacturer. The average warranty length is 25 years, so after that time, solar panels will have a 75% efficiency rate.

Can Solar Panels Ever be 100% Efficient?

Currently that is not possible because semiconductor materials have various spectral ranges, and some solar energy is inevitably lost. Every year researchers are finding ways to improve PV efficiency, but there is always some photon energy that turns into heat that convertible power.

What Time of Day are Solar Panels Most Efficient?

The best time is midday from 10:00 AM until 2-3;00 PM. This is true not just for summer but other seasons as well. If there are no clouds it is possible for your solar panel to produce its rated output for hours.

Do Solar Panels Work in the Rain?

Solar panels still function during a rainfall, but output will be very low. If it is only a drizzle and the sun is just partly covered by clouds, you can still expect production especially if it is a high efficient panel. Rain however is good for solar panels because it washes away the dirt.

Do Solar Panels Work Better in the Cold?

Yes, solar panels can work in the cold and winter. It is a myth to assume that solar panels cannot function in cold temperatures. As long as there is sunlight the panel will convert energy into electricity. In fact many solar panels at their best when the temperature is somewhat cool.

At What Temperatures Do Solar Panels Stop Working?

This will vary from manufacturer to manufacturer, but majority of solar panels today are designed to operate between 59 F / 15 C to 95 F / 35 C. The temperature may reach up to 149 F / 65 C on very hot days and in extreme weather, and this could affect efficiency.

What Happens When Solar Panels Get Too Hot?

Solar panels increase output as the temperature rises. However the voltage goes down and so does the electricity that goes in the system. While searing temperatures may seem ideal for solar power, in fact it is more detrimental than anything

Do Solar Panels Need to Warm Up?

No, in fact solar panels do not need that much heat at all. Solar panels work by converting sun light, not sun heat, into electricity. Hotter temperatures do not improve a solar panel’s performance. If it is too hot the cells will actually transform less energy into electricity.

How Do You Cool a Solar Panel?

Air is enough to cool down a solar panel. A wind gust can bring down solar panel temperature by 40 F / 4.7 C and improves efficiency by 2.5%. Water is even better, reducing PV module temperatures by 46.4 F / 8 C and boosting efficiency by 3%.


There are a lot of factors that affect solar panel production, so anyone who uses solar power needs to watch their power usage carefully. But advances in solar technology will only lead to greater efficiency in the future, so things will only get better.