How ECS Calculates Panel Production

Why Panels Produce Less Than Rated

You would think that a panel rated to produce 300 watts per hour would produce a huge amount of power. With 12 hours of sunlight on a good day, you could expect up to 3,600 watts. Unfortunately, there are many factors that reduce this number. Panel angles, strength of sunlight, temperature of panels, and many other factors affect the actual output of a solar panel on any given day.

Our estimate of power production takes all of these factors and more into consideration. Listed below is an example of the calculations we do to get an accurate estimate of what your system will produce on average.

Calculating solar panel production:

There are a number of factors that need to be considered when calculating how much power a solar panel will produce on average per day. These factors include:

     Average hours of sunlight per day, for the area the panel is installed

     Size of panel

     Rated power output for the panel

     Power loss of panel

     % of power loss per degree of temperature over the optimal temp           for the panel

     Power loss through the inverters and cables

All panels are listed with a power rating. This rating (for home use panels) ranges from a low of about 250 watts to a high of about 450 watts.


In order to accurately calculate the actual power that will be produced by a panel and, more importantly, what the panel is guaranteed to produce, the factors above need to be calculated.

As an example, we can use a 300 watt panel. 300 watts is the rated power output when factory tested under optimal conditions. However, it is not what the panel is guaranteed to produce.

Most panels are guaranteed to produce (when new) 90 - 97% of their rated size. Therefore the 300 watt panel may only be guaranteed to produce 270 watts (300 – 10% = 270).

Many panels have a + or – range of up to 3%. So, a 300 watt panel is really only guaranteed to produce 261 watts (300 – 10% = 270)(300 – 3% = 9) 300 – 39 = 261.

The next calculation is the drop in production due to temperature. All panels are rated to perform best at an optimal temperature. This is 25 degrees Celsius.  


As the temperature increases, the production of the panel decreases. This is measured at the factory and is calculated as a % drop per degree of temperature rise. This is noted in all panel specifications. Each panel is different and, in general, the larger the panel, the higher the % drop per degree of temperature rise. For this example we will use 0.50% per degree of temp. rise over optimum.

So, we already have our 300 watt panel producing 261 watts. Now, we calculate the average operating temperature for the area the panel is located in. We use Chelem with an average daily temperature of 31 degrees. This is 6 degrees above the optimal temp. for the panel and = 3% drop in panel production.

300 – 3% = 9

This means the 300 watt panel will only produce 258 watts on average.

Effective hours of sunlight:

We are blessed with many hours and many days of sunshine per year. We should be able to produce massive amounts of power for hours on end each day. The problem is, how many hours of useful sunshine do we have each day? This number is calculated worldwide and these numbers are used to form the base line for each area. In Chelem, this number is 5.5 hours average per day. In effect, the 12 hours of sunshine we get per day is = to 5.5 hours of useful sunshine for power production.

So, with our 300 watt panel we can produce on average 1,350 watts of power per day or 1.35 kilowatts.

300 watt panel = 245 watts per hour X 5.5 hours = 1,347.5 watts per day.

Now, we come to power loss through the inverter and cables. No inverter will convert 100% of the dc power to ac. In fact, most will convert about 96%. This means a further power loss, including loss through cables, of about 5% or 258 – 5% = 245 watts.


Effectively a 300 watt panel will produce on average about 245 watts at peak output.

One other consideration is “clipping”. Solar panels are designed to work within a temperature range. When that temperature range is exceeded, they shut off. This is a momentary shut down and restart of the panel. This is called clipping. As the panels work, they produce heat. This heat, in combination with the surrounding temperatures, will cause clipping to occur. Our “real world” testing (done by monitoring our system) has shown this clipping will reduce panel efficiency by as much as 10%.

Using the example above, clipping will reduce the average output to about 1,210 watts of power per day.

If we ignored all these factors for calculating the output of a 300 watt panel and only considered the daily hours of sunshine, we would have an average daily output of 1,650 watts.

After consideration of all the above factors, a truer production calculation is simply put as:

(300 x .733) x 5.5 = 1,210 watts per day average.

You may produce more on any given day and you may produce more on average; however, this is all the 300 watt panel would be guaranteed to produce.

As you can see, anyone making claims that their panels will produce what the panel is advertised at, is simply not being honest with you.


Angle of panels to the sun:

The angle of the panel as related to the angle of the sun also has a great effect on overall production. These calculations are based on a fixed 20 degree panel angle. This is the angle most fixed position solar systems are set to. The reason for this is that, in our region, the optimum angle sets range from a high of 40 degrees in the winter to a low of 0 degrees in the summer. Taking the midpoint of these two angles will give the best average position for the panels to be set.

Unfortunately, fixing the panel angle at one position reduces the panel production substantially. By using a variable or adjustable panel racking system, homeowners can take advantage of seasonal angle adjustments. These seasonal adjustments can improve panel and system performance by substantial amounts.

As the sun tracks lower in the sky, the panels need to be adjusted to a higher angle and when the sun is higher in the sky, the panels need to have a lower angle.

Emerald Coast Solar is one of the few companies to do variable angle installations and we are the only company to offer 0 degree through 25+ degree angle adjustments. Our systems take greater advantage of the sun's power and generate more power on average per panel than any other company.

This video, from a company in the US, clearly demonstrates the effects of panel angle on solar production. 

Due to the nature of different construction methods, the normal sun hours in the area and the difference in installation methods, the comments about power gains based on angle for grid tie systems are incorrect for our area - we see power gains of 30% and more with seasonal angle adjustments.

   Emerald Coast Solar

Putting the power of the sun to work for you

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