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How do solar panels make electricity?

Solar power systems like the one on our roof, produce power directly from the sun. Sunlight is converted directly into electricity by each solar cell in the panels. There are many factors which affect how well the cells do that.

The best demonstration of solar power in action that I have ever seen, is at the Children’s Museum of Pittsburgh. When we visited there, I took some photos and a video of the simple working system they have in the middle of their playground.

How the sunlight turns into electricity

The solar cells are made of silicon, that has been combined with other materials in such a way that there are extra electrons in one part of the cell, and missing electrons in another part of the cell.

When the sunlight strikes the cell, photons in the light knock some of the extra electrons loose from the silicon, and they flow to the part of the cell that is missing electrons. This flow produces an electrical current that eventually reaches the inverter, where it gets converted into usable electricity.

The page that explains what the “Current status” numbers mean, goes into more detail about the way the electrical current works and what the inverter does.

Regular solar cells, like the ones on our roof, can only convert around 15% of the sunlight that strikes them into electricity. They are still a new and developing technology (modern solar cells were only invented in 1954). The theoretical limit for regular cells is 30%. More advanced solar cells can reach higher conversion rates, over 60% in theory.

Orientation to the sun

A cell’s orientation to the sun is one of the most fundamental factors affecting how much electricity it can produce.

In the northern hemisphere, solar cells need to face generally south for best results, although cells facing east or west can still generate power, just for a shorter period of time.

Some systems employ trackers that turn the cells to directly face the sun as it moves across the sky. This can greatly increase the amount of electricity that a solar cell can produce. It also introduces more complicated moving parts into the system which require maintenance and repair from time to time.

The angle of the solar cells is important as well. For optimal output year round, based on the geometry of the earth’s rotation and relative position to the sun, solar cells should be raised on an angle equal to the latitude at which they are installed. Toronto is at roughly 43 degrees north, so generally speaking, the ideal angle for solar cells would be a 43 degree angle. However, according to research published by the City of Toronto regarding the 100 kW array on the Horse Palace building at the CNE grounds, 32 degrees is a better angle, taking into account Toronto’s normal weather patterns and the difference in potential output between summer and winter.

To truly maximize output, the angle should be adjusted throughout the year, though in practice most systems are installed at a single angle, usually flush with the roof, or raised at a certain angle from a flat roof.

Shade and the Maximum Power Point

Shade, whether from buildings, trees, clouds or something else, is bad for power output. It obviously stops sunlight from reaching a solar cell.

In general, the amount of light striking a solar cell will vary during the day, even if there is no shade, simply because the angle of the sun relative to the cells is changing throughout the day (unless you have a tracker).

The performance characteristics of solar cells mean that as the amount of sunlight varies, the voltages through the cell should be varied as well, in order to generate the most power. Solar power systems employ a technique called maximum power point tracking (MPPT) to determine the optimal voltage at any moment.

In systems with a central inverter (like ours), the maximum power point is determined for the entire array of solar panels as a unit, which can reduce the output of the entire array when only part of it is shaded.

Some systems now have multiple inverters, sometimes one per solar panel. The benefit of this is that if the array is partially shaded, each panel will be set to its optimal voltage.

For small residential systems, the footprint of the array is usually small enough that good candidate locations won’t have any shade to worry about. But for larger systems, dealing with different amounts of sunlight across the entire array is an important consideration.