Why Two Solar Systems with Identical Equipment can’t be Compared

Posted by YES on February 27, 2020 in Design Installs Solar Solar Inverters Solar Monitoring Solar Panels Support

Two neighbors can get solar systems with identical equipment installed. However, how much actual electricity their respective systems produce can differ.

As an example, we recently installed rooftop solar systems for two neighbors in Pleasanton, California. They purchased identical equipment, consisting of:

  • LG 365-watt DC rated solar panels
  • Enphase IQ7+ Microinverters

We will refer to these neighbors as Amy and Kathy.

The issue

Kathy recently called asking why her solar system isn’t as productive as Amy’s.

Below is the actual per-panel production for each system for the past week (2/20/20 – 2/26/20). Comparing the two systems, Kathy is correct.

Kathy’s Solar System

Average per-panel production: 10.7 kWh

Amy’s Solar System

Average per-panel production: 12.3 kWh

The question: Why is Amy’s solar production almost 15% greater than Kathy’s? They both have identical equipment.

Factors that determine solar production

In addition to the efficiency of the solar panels themselves and shading, there are other factors that determine actual solar electric production:

  • Geographic Location – Certain geographical regions yield better solar production. For example, a solar system installed in sunny Los Angeles will outperform an identical system installed in the more cloudy and rainy Seattle.
  • Roof Angle – For rooftop solar systems, we install based on the contour/angle of the existing roof. Certain roof angles yield higher solar production.
  • Roof Azimuth – In Earth’s Northern Hemisphere, solar panels that face the South (180º) yield the highest solar production because they receive direct sunlight all day. East and West facing solar panels yield decent production as well, but these angles are not as productive as the South. North facing panels are the least productive.

Why is Kathy’s system underperforming?

Not the Geographic Location

Since Amy and Kathy are neighbors, we know the cause is not the geographical location of the solar system. Being neighbors, both their homes experience the same weather.

Not the Roof Angle

Both solar systems have the same roof angle, at 20º.

It is the Roof Azimuth

Kathy’s solar system faces 245º, which is close to West facing.

Amy’s solar system faces 155º, which is close to South facing.

As we explained earlier in Roof Azimuth, South facing solar panels are most productive.


Both solar systems are performing well. However, Amy’s system has higher solar production because her system has a more ideal roof azimuth.

Because Roof Azimuth and Roof Angle differ between houses, two solar systems cannot be directly compared, even if they are neighbors with identical equipment.

Instead, a reputable solar installer will perform a detailed Site Survey to determine the ideal location for the solar panels in order to maximize production. The installer will take into account Roof Azimuth and Roof Angle to estimate solar production. This estimate determines how much actual electricity the solar system will generate.

The installer will guarantee production based on this estimate. If actual production is beneath the guarantee, a reputable installer will make things right. Once the system is installed, the homeowner can compare her actual production with the guaranteed production.

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