Solar Module Efficiency

Solar cell efficiency is the ratio of the electrical output of a solar cell to the incident energy in the form of sunlight. The energy conversion efficiency (η) of a solar cell is the percentage of the solar energy to which the cell is exposed that is converted into electrical energy. This is calculated by dividing a cell’s power output (in watts) at its maximum power point (Pm) by the input light (E, in W/m2) and the surface area of the solar cell (Ac in m2).

Solar panel efficiency is a measure of how much energy the panel produces per square metre. The efficiency rating measures what percentage of sunlight hitting a panel gets turned into electricity that you can use.

Most solar panels are around 11-15% efficient. The higher the efficiency, the less surface area you’ll need in your solar panels. So, a more efficient 250 Watt panel will be smaller than a less efficient 250W panel and take up less space on your roof. However, in the end both will produce 250 watts of power.

Out of the solar panels on the market, SunPower makes some of the most efficient–one of their models is 19% efficient. They’ve reached that number by using several techniques, including a reflective coating that can capture more light from an angle. They also offer a line of panels that’s 18% efficient. Sanyo, another solar panel manufacturer, offers efficient models as well.

High efficiency doesn’t mean better, it just means your panels use less space on your roof. Efficiency isn’t usually a critical concern unless you have an unusually small space for your solar panels. However, most efficient panels are a lot more expensive so that you may be paying a large amount extra for just a few extra units of electricity.

Better, more useful measures of “efficiency” are:

  • Cost per kW – the cost per kilowatt of installed power.
  • kWh per kW – number of units of electricity (measured in kWh) generated per year per kilowatt of installed power.
  •  Cost per kWh – the overall price per unit of electricity generated over the lifetime of the system.

Factors that affect solar array efficiency include:

Panel Orientation

It is always important that your panels must be oriented in the direction where it can receive maximum sunlight but a quality design can often compensate for other directions.

Roof and Panel Pitch

The “pitch” or tilt of your roof can affect the number of hours of sunlight you receive in an average day throughout the year. Large commercial systems have solar tracking systems that automatically follow the sun’s tilt through the day. These are expensive than the fixed ones.


Some panels like it hot but most don’t. So, panels typically need to be installed a few inches above the roof with enough air flow to cool them down. Some photovoltaic panels are designed to be more efficient in hotter climates.


Basically, shade is the enemy of solar power. With poor solar design, even a little shade on one panel can shut down energy production on all of your other panels (like a bad bulb in a string of Christmas lights). Before you design a system for your home, you will have to conduct a detailed shading analysis of your roof to reveal its patterns of shade and sunlight throughout the year. This is just one of many reasons to work with a highly experienced solar provider.