Two years have passed since my last review of our photovoltaic roof array, which we activated December 24, 2019. And since we produced our first surplus during the solar year 2023, followed by another surplus for 2024, I feel it’s time to brag – a little.
Consumption
In the five years since we had our 26 module, 8.58 kW photovoltaic roof array installed, the electrical consumption for the three apartments in our 4,500 sf building ranged from 13,428 kWh at the high end per SY to 10,574 kWh at the low end.
I attribute the rather high electrical use of 13,428 kWh during solar year 2020 to the fact that all heating was provided by our air source heat pumps, and a cold spell that swept through Chicago with temperature in the single digits during the month of February.
The lower annual consumption during solar year 2022 and 2023 was partially due to user behavior (using the natural gas powered radiators for heating rather than the air source heat pumps), and milder weather conditions during the heating season.
Production
The electrical production varied over the past five solar years due to a number of factors. It is, however, difficult to exactly quantify what factors may have had what impact.
The solar modules lose some efficiency over the years. Our modules, the Panasonic N330/N325, have a specified annual degradation rate of 0.26%. This would be hard to notice or measure year over year.
It is much more likely that weather and/or shading conditions as well as soiling of the panels are the driving factor for the variation in production. That said, it does not explain the decline from 11,390 kWh during solar year 2020 down to 11,052 kWh for solar year 2021, considering that the percent of possible sunshine increased from 56% to 58%.
The drop in percent of possible sunshine between solar year 2021 and solar year 2022 from 58% down to 50% may, however, explain the production drop from 11,052 kWh to 10,319 kWh.
The uncharacteristic jump in production to 11,633 kWh for solar year 2023 and 11,801 kWh for 2024, can be partially attributed to the rise of the percent of possible sunshine to 57% and 58% respectively. However, production was still higher than solar year 2020 and 2021, which had almost identical amounts of percent of possible sunshine.
What may account for the difference was the removal of a dead Ash tree [LINK – Photovoltaic roof array shade study] on the adjacent property. Prior to the removal (solar year 2020, 2021, and 2022) that Ash tree partially shaded our array in late fall, winter, and early spring (see also video below).
Surplus
For the first three solar years our electrical consumption outpaced the electrical production, and we ran small deficits.
That changed during the solar year 2023 with a production surplus of 995 kWh, followed by a surplus of 715 kWh in 2024. This surplus does coincide with the above mentioned jump in electrical production. But it probably also needs to be attributed, at least partially, to user behavior and weather conditions.
For example: The smaller surplus for solar year 2024 is due to the increased electrical consumption, which I attributed to a slightly higher air conditioning load compared to solar year 2023.