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Photovoltaic Efficiency - Inherent and System

First let's consider what we mean by efficiency. Efficiency is a measurement of the output divided by a certain factor. When talking about solar cell or panel efficiency, we are gerally looking at output per given area.
Therefore a more efficient panel will either give more power or be smaller than a less efficient one. This does not mean that a more efficient panel is more economic, in-fact the reverse may be the case.
More efficient panels may be easier to mount, particularly if space is limited, or a tracking system is being used.

There are several separate areas to consider here concerning the efficiency of pv solar panels, concerning the inherent efficiency of the cells in the panel you buy, the panel construction, and how your installation may affect panel efficiency.

For home owners setting up there own system, efficiency is likely to be less important than the cost per nominal watt output.

Solar Cell Efficiency

Researchers are continually striving to improve the efficiency with which solar cells convert light energy to electrical energy. The record at the moment stands at an efficiency of around 40%, using multi junction cells (multiple layers of silicon), each layer tuned to trap different frequencies (colours) of light. This type of cell will however be expensive to produce and in the past has been mainly used in space where efficiency may be more important.
Researchers also strive to increase the amount of light entering the cell. Silicon is a naturally shiney substance and cells are coated with non-reflective layers to ensure that as much light as possible enters the cell.
Cells used in photovoltaic panels for electricity production are usually single junction type with an efficiency of somewhere around 15%.

Solar Panel Construction

Solar panels will normally have a layer of glass (protecting the cells) through which the light must pass before entering cells. There is scope for reducing light reflected from the glass and also to trap light that is not striking the panel at 90o.
Some research centres on trapping light from a large area from various angles and directing it onto a relatively small cell.

Panel Mounting and Positioning

The two important factors here are the angle at which the panels are mounted and the amount of sunshine falling on the particular location/avoidance of shade. Small areas of shade on a panel can have a significant effect on the power produced.

You will find more practical information on mounting solar panels here

Effects of Temperature

The output of a solar cell, and therefore a solar panel, is affected by its temperature. As a result the power output will be reduced by between 0.25%(amorphous cells) and 0.5%(most crystalline cells) for each degree C of temperature rise.
Panel temperatures in the summer in warm climates can easily reach 50oC resulting in a 12% reduction in output compared to the rated output at 25oC.
This reduction in efficiency may be important to you if you have a high electricity demand in the summer.

Partial Shading

Solar panels obviously produce less power when they are shaded and should idealy be situated where there wil never be any shadows on them.
There may be situations where this cannot be avoided, and the effects of partial shading should be considered.
A shadow falling on a small part of a panel can have a surprisingly large effect on output. Not only will the cells that are shaded be producing less power, but as the cells within a panel are normally all wired in series, the shaded cells affect the current flow of the whole panel.
If the affected panel is wired in series (in a string) with other panels, then the output of all those panels will be affected by the partial shading of one panel. Therefore in a situation where partial shading cannot be avoided, there may be a case for not having the panels wired in series to produce the higher voltages that can be used with some inverters.

How Your System Affects Panel Efficiency

Graph of solar cell electrical output showing sharp drop off at 0.4 volts

Apart from positioning and angling your panels in order to capture as much light energy as possible, there is more.
The inherent characteristics of solar cells results in current produced by a particular light level being virtually constant up to a voltage of 0.4 volts. A solar panel with a nominal voltage of 24 volts would normally have 72 cells, resulting a constant current up to 28.8 volts. Above this voltage, current drops off rapidly, resulting in maximum power output being produced at around 28.8 volts. However, below this voltage the power output will be proportionally lower, due to the current remaining constant (power = voltage x current).
When the panel is connected to the battery via a simple charge regulator, the voltage will be pulled down to near that of the battery. The result is that the panel will only be able to produce it's maximum power when the battery is near to being fully charged.

This is where an MPPT (Maximum Power Point Tracking) Charge Controller can play a part by maintaining the panels at their optimum voltage while producing the voltage required by the battery.

What about if you are not using an MPPT controller? Well if you are in a situation (possibly in winter) where your batteries rarely become fully charged, adding an extra panel will not only add the extra rated power potential, but the rest of your panels will be more efficient as they will be running at a higher voltage.
This effect will to some degree be negated by the battery being less efficient at higher levels of charge but remember that it is beneficial to battery performance and life to be fully charged on occasions - see information on Battery Charging