Daily Solar

On Apr 23, 2020 the first clear day we saw up to 12 kWh peak generation.

One month later on May 18, 2020, the peak generation was less than 11 kWh.

If we look into more detail, we can see the role of the angle of the sun relative to the panel face which is fixed to our southeast facing roof with a 45 degree pitch. While the early elevation of the sun is only 14 degrees at 740am, the solar panels on the southeast face still can generate 100W with a heading angle of 14 degrees to the panels. As the sun heading becomes perpendicular to the panel facing, the generation peaks around 310W by 1215pm when the elevation is 70 degrees. While the sun stays relatively high in the sky in the late afternoon (32 degrees by 541pm), the generation drops significantly (20 kWh at 541pm) as the sun moves behind the panels face. The sun angle actually overshoots the 45 degree tilt of the roof by up to 32 degrees, but the component of the generation along the solar panel axis varies as the cosine of the angle difference and is only ~15% less. The bottom line is that with a little elevation, the panels can still generate significant generation even at poor heading angles.

The peak solar rate is highly correlated with when the heading angle is directly facing the solar panel. At 1247pm the southeast-facing panels generate 310 W when the sun in directly facing the panels, while the southwest-facing panels generate 330 W at 240pm at the optimal header (20W difference; 6.5%). The main difference between the two panel orientations is that the azimuth is higher (74 degrees; 29 degrees above a 45 degree pitch roof) vs the lower (68 degrees 23 degrees above a 45 degree pitch roof). The 6% difference lower solar angle predicts a 5% greater value in peak solar generation which is very close to the 6.5% observed on this day.

Taking a step back, the daily generation values have been up to 89 kWh.

Clouds have had a dramatic impact on generation:

After the solar install was complete we installed Solar Sense to give us real-time access to generation and usage. On most days of May, our solar generation was greater than our usage.

Here is what a full clear day looks like with the AC spikes being around 6000 kWh for our downstairs AC and 4000 kWh for our upstairs AC for 15-30 minute periods.

Here are a few hours of usage:

Ironically, the peak generation actual is not in the summer. This is mostly due to the solar angle being too high for the 45 degree tilt of my roof. Here is what one of the nearby solar systems shows for peak clear days throughout the year:

A solar system in Dallas peak solar noon during summer 2019.
A solar system in Dallas peak production on a clear day each month showing peak production when angles are closer to 45 degrees (the same angle as the pitch of steep roofs) in spring and fall 2019.
Daily solar generation (Kwh) from Apr-Oct 2020.

Other Pages:
– Nov 14-15, 2020 cloud edge vs lensing
– Nov 12, 2020 elevated cloud lensing
– Nov 9, 2020 solar & cloud observations
– Nov 8, 2020 cloud lensing examples
– Oct 22, 2020 44.9 degree altitude solar noon and more cloud lensing
– Oct 2020 solar altitude and air temperature effects on solar generation
– Sep 2020 home improvements energy savings and solar generation
– May 2020 installation