Photovoltaic array monitoring

A previous post covered how the inverter is an integral part of our 26 module, 8.58 kW photovoltaic array. Both, the inverter and the array, service our 4,500 sf building with its three apartments/households.

The inverter allows us to monitor our kilowatt hour (kWh) production in real time, either through an android/iOS or web app. The app interface also gives us access to historical production data, such as the past three years (2020 – 2022).

During those three years, the lowest monthly production on record was for December 2022 with a meager 273 kWh. Our worst production day was January 31, 2021 with 1 Wh due to a foot of snow on the modules.

June 2022 gave us the highest monthly production with 1,495 kWh, and the best daily yield so far fell on May 29, 2021 with 68.7 kWh.

Our array averaged 11,000 kWh in annual production from 2020 through to 2022, which is within 4% of what we were aiming for (11,459 kWh).

The 11,000 kWh for our 4,500 sf building breaks down into 3,667 kWh of self-produced electricity per household per year, or 305.6 kWh per household per month.

Annual production overview

Even though some months are sunnier than others, and despite the solar irradiation varying each year for any given month, the production across the year is remarkably consistent with an average of 11,000 kWh per year.

The overall small production decline since 2020 reflects the estimated annual system decrease, which is specified to average 0.26% per year for our Panasonic 330 watt modules.

While planning the system with our solar installer Lisa Albrecht from All Bright Solar, we targeted an annual production of 11,459 kWh. 11,000 kWh per year brought us within 4% of that goal. Some shading during the winter months from an adjacent tree was likely responsible for the 4% shortfall.

The “Estimated energy” is the minimum amount of energy we need to produce to fulfill our contractual obligations for the state solar incentive we received. The state incentive is based on how much kilowatt hours our 8.58 kW photovoltaic array is predicted to produce over the first 15 years. This number was calculated by our solar installer, and determined our total state incentive amount.

Monthly production overview

The data for the past three years on a monthly basis shows the expected seasonality in our production. We can also see the varying yield each year for any given month, a product of a given month being more overcast or sunny in one year than in another.

We are not always meeting our production goals between the months from October through to January. We have yet to meet the production goal for November, and January isn’t far behind. We suspect that the shading of the adjacent tree during these months is partially responsible for the lower than predicted production.

However, Lisa Albrecht mentioned that this is a trend across her portfolio and she assumes the region. Current forecasting models use 30 years of historic weather data to predict future performance. However, climate models suggest we may experience heavier cloud cover in Q1 & Q4 in our region as our winters warm.

The good news is that based on our net metering agreement the production goals are tallied by solar year, and the excess production during summer makes up for the underproduction in winter.

Worst production

Not necessarily a meaningful data point, but an interesting one. The absence of solar irradiation this past December (2022) manifested itself in a meager output of 273 kWh. Did snow cover have a negative impact? No. We only had two days that produced measurable snow, and I quickly cleared the snow from the array.

Out of the 31 days in December 2022, we had five sunny days (>20 kWh), two mostly sunny days (>15 kWh), and we had 15 gloomy days (<5 kWh). We had to turn lights on during the day for half of that month because it was so overcast.

The worst production day in the past three years was on January 31st, 2021 with a meager 1 Wh. We had a big snow storm moving through and there was no point clearing the snow off the modules until the storm system had left the area the next day and the wind had subsided.

Best production

No need to look any further than the height of summer. June 2022 with 1,495 kWh was the month with the highest yield so far, closely followed by July 2020 with 1,487 kWh.

The high solar irradiance in June of 2020 resulted in 19 mostly sunny days (>50 kWh) and only four mostly overcast days (< 30 kWh). Another 5 kWh in production would have given us a monthly total of 1.5 MWh.

Our highest yielding day in the past three years was on May 29th, 2021 at 68.7 kWh. That came as a surprise, because it was still a good three weeks away from the summer solstice. Humidity levels and the associated haziness around mid to late June must have kept yields below that of May 29th, 2021 for the past three years.

The above production data will set the scene for the next post, where we will compare it against our electrical use.

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About Marcus de la fleur

Marcus is a Registered Landscape Architect with a horticultural degree from the School of Horticulture at the Royal Botanic Gardens, Kew, and a Masters in Landscape Architecture from the University of Sheffield, UK. He developed a landscape based sustainable pilot project at 168 Elm Ave. in 2002, and has expanded his skill set to building science. Starting in 2009, Marcus applied the newly acquired expertise to the deep energy retrofit of his 100+ year old home in Chicago.

2 thoughts on “Photovoltaic array monitoring

  1. “Both, the inverter and the array, service our 4,500 sf building with its three apartments/households.“

    I was wondering how you are splitting the energy generation across the different units? Do you have individual meters for each unit, a single meter for the building, or large draw appliances on a single meter?

    That also leads to, how was your annual usage calculated when sizing your solar system? Using a multi-unit combination or a single meter?

    I have a 3 unit building as well and I’m trying to solve this for myself.

  2. Brian, our building has a single meter. The annual usage for the sizing of the solar array was calculated based on previous utility bill records. All this was very new when we installed the system in 2019, including all the rules and regulations. I don’t know if they are still the same today. I think ComEd made some adjustments since.

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