Solar panel installations and their real efficiencies.

Solar panels are sold in powers of 250…310Watt but what do they offer in real operational conditions. You can also say what can we expect of our solar installations in real life. Solar is applied in two basic configurations and each of these have their own losses. The two most know configurations are:

Availability Fig 1 : Typical roof installation of a GT Solar installation
  • Grid Tied installations (GT)
  • Off The Grid Installations(OG).

The losses in these two configurations are divided in three groups. These groups are:

GT installations:

  • PV Panel loss due to cell temperature.
  • Loss due to DC/AC conversion or inverter losses.
  • Loss due to transport, Soiling, shading, panel aging and more.

OG installations:

  • PV Panel loss due to cell temperature.
  • Loss due to DC/AC conversion or inverter losses & battery storage and discharge
  • Loss due to transport, Soiling, shading, panel aging and more .
Availability Fig 2 : Effect of temperature on cell performance

As you can see the difference between the two configurations is the loss due to the use of battery storage.

The next post I will describe the losses for these two applications. All losses can be calculated using formulas. Were possible it is explained . The calculations for panel loss is left out of the scope of this paper.

PV Panel loss due to cell temperature:

A panels is constructed with cells in a array and each cell has an efficiency that goes down with the temperature going up. How much it goes down per °C is determent by a PV power Temperature Coefficient TCpmax which is around 0.47%/°C
The Cell temperature of a panel goes up as soon it receives Solar radiation in order to produce electricity. Temperatures of 55°C at 1000Watt/m2 irradiation and higher are not unusual. This means under these conditions our panel performance is not 100% as it is sold for but has reduced to 86% (sorry it is not just multiplying Temp x TCpmax). The operational panel performance at 1000watt/m2 irradiation is around 86% of the performance its sold for.

Loss due to DC/AC conversion or inverter losses:

Availability Fig 3 : Open DCAC inverter open model explained.

An DC/AC inverter exist out of two stages. Stage one is a boost stage that pumps up the panel voltage of say 40V to the needed AC line RMS voltage multiplied by the root or 2 which is 1.41. That product is the peak voltage needed to produce an AC signal with an RMS equal to your line RMS value. In Europe this voltage is 230Volt (Peak=325V) and in the Americas it is around 120V Peak=170V.

The second stage in a DC/AC inverter is what is called a “H MOSFET bridge” that inverts the boosted signal from the first stage in to a Sine wave Pulse Width Modulated (S-PWM) signal which goes though a passive LC filter. Belief it or not but a S-PWM signal at the input of a LC filter make a 230/50hz or 120V/60Hz sine wave coming out, it’s like magic.

These two stages count for a loss between 90…97%. For this exercise we use 95%, most DC/AC inverters have figures like this. keep in mind  but only for their peak efficiencies, the average is often lower.

Loss due to battery storage and discharge.

Availability Fig 4 : Voltage efficiency of a lead acid battery

To charge a battery requires more energy than you can get out of it during the discharge stage. The quotient of the output and input energy is called the efficiency of a battery. Two effects influence this efficiency and they are called the coulomb and voltage efficiency.

The current or charge (I*t=C)that you put in a battery is not the same charge that you can get out of it. The quotient of the two is called coulomb efficiency.

The voltage needed to charge a battery is not the same as the voltage you get out of a battery during discharge. The quotient of the discharge and charge voltages is called the voltage efficiency.

The product of voltage and coulomb efficiency is called the efficiency of a battery. A lead acid battery has an efficiency of 80% a Li-Ion battery has an efficiency of 99%.

System losses:

The list of variables that may influence the efficiency of solar installations other then what is discussed before is called the system losses. The system efficiency is also influenced by the following factors:


01 Soiling Dirt, leaves, bird shit and other on the panel surface 2.0%
02 Shading Effects of trees, chimneys, or other obstacles  
03 Snow Effect of snow on the panels.  
04 Cell Mismatch Effect of cell differences between panels in series 1.0%
05 DC/AC Wiring losses Transport losses of DC and AC cables 2.0%
06 DC Connector losses Loss of high currents in the connectors 0.5%
07 Light-Induced Degradation (LID) Degradation in the first period of panel use due to light. 1.0%
08 Nameplate Rating Errors due to incorrect measuring the panel performance !!!.  
09 Aging / year per year loss due to aging of the panel. 0.8%
10 Availability Loss as a result of unavailability if the grid or system maintenance  
11 Solar Orientation Factor (SOF) Loss Loss due to less than optimal orienting panels to the sun 1.0%
12 Mounting loss Losses due to positioning panel close on a roof or not 1.0%
13 Transformer losses Extra losses due to the use of external transformers.  
  Total Loss   9.3%

All these losses are estimated in percentage of their effect on the system performance. If they are considered zero they are not mentioned. The following degradation in performance can be expected is as:

Efficiency year 1 90.7%
Efficiency year 2 89.9%
Efficiency year 25 70.7%

Over all Efficiency

Now we have made and estimations of all losses in a GT or OG solar panel system we can have a look at what the total outcome is. Lets calculate the overall system efficiency and see why your seller is never telling you this. This overall system efficiency is calculated as follows. The Panel operational efficiency multiplied by the Inverter efficiency multiplied by the system efficiency.

For GTI and OGI we come out on:

1 GT : Panel x inverter x system  = 86% x 95% x 90.7% = 74%
2 OG: Panel x inverter x Lead acid battery x system  = 86% x 95% x 80% x 90.7% = 59%
3 OG: Panel x inverter x Li-Ion battery x system = 86% x 95% x 99% x 90.7% = 73%

These are the real system efficiencies that you have on your roof in the first year. They might be a little different but not that much. Meaning an GT installation like 10x 250w panels can produce on a nice summer day with 1000Watt/M2 of irradiation not 2500Watt but something closer to 1850Watt.
With lower irradiation levels the panels get less hot and there for the panel efficiency will go up a little. At the same time less irradiation also means less incoming energy to covert. The total power production with lower irradiation will always be lower.

Every year the system gets older until the 25 Year life time of the panel system. The efficiencies after 25 year will have dropped to :

1 GT : Panel x inverter x system  = 86% x 95% x 70.7% = 58%
2 OG: Panel x inverter x Lead acid battery x system  = 86% x 95% x 80% x 70.7% = 46%
3 OG: Panel x inverter x Li-Ion battery x system = 86% x 95% x 99% x 70.7% = 57%

An other side effect of this operational analysis is the conclusion that most system in location of 45° latitude or higher will have inverters which are to big for the panels. At these latitude level the sun with not often produce 1000Watt/2m irradiation. So a 300 Watt panel will maximum produce most of the time below 225 Watt of power. This means a inverter of the size of 225Watt  is sufficient for panels of 300Watt instead of a 300Watt inverter. That saves some money since the inverters are 30% of the installation cost.

If you compare the overall system efficiencies with the amount of sunlight coming in then the overall efficiency of available energy compared with actual energy used is around 10.. 12%. We still have some work to do be for we can actually use what the sun is offering us.

I hope you have given you a better idea about what your solar installation is actually doing at your roof and what we need to do to get the numbers up. Happy renewables

Oscar Goos