A report on solar energy at Prairie Hill
John McGonegle gives a detailed report of how solar panels are working at Prairie Hill
As of December, 2021, there are ten Prairie Hill units with photovoltaic (PV) systems installed for a total capacity of 33 kilowatts. The PV systems range from 5 to 12 roof-mounted panels. The systems are tied to the electrical grid and net metered. If the PV system is producing more energy than the unit is using, the excess energy is delivered to the utility grid and banked to the user’s account. The roof area on the four-unit stacked flat is divided to allow each owner to purchase their own PV system. Some of the PV systems have power optimizers with string inverters and some have micro-inverters.
How it works
For example, in 2020, one of our 3.8 kilowatt (kWh) PV systems produced 5,247 kWh and the unit’s usage was 4,546 kWh. The surplus 1304 kWh was credited to the user’s account. This 3.8 kW PV system has generated over 10,000 kWh since its commissioning in February, 2019. That's the equivalent of 126 trees saved and 16,755 lbs. of CO2 not released into the atmosphere.
The PV systems shut down when the normal power is lost, so they do not provide emergency power. Systems with batteries and transfer switches can be designed to interface to the utility and provide emergency power from the batteries when the normal power is lost, but these systems are very expensive.
Costs are going down and panels are improving
The cost of basic solar systems has been decreasing at the same time that power density and the efficiency of PV panels have been improving. This means even units with small roof areas can produce more energy. In the last three years, the standard panel has changed from 345 watts to 420 watts, a 23% increase in power density and 11% improvement in efficiency.
Prairie Hill Net Zero Energy Building
Our Prairie Hill cohousing units are equipped with high efficiency systems. We have managed to achieve our goal of building sustainable homes with a low carbon footprint. With solar photovoltaic panels installed, the units can be near net zero energy (NNZE) or net zero energy building (NZEB). A net zero energy building produces enough renewable energy to meet its own annual energy consumption requirements, thereby reducing the use of nonrenewable energy in the building sector.
Details from John McGonegle's usage logs
It's interesting to track your energy usage. In 2020, my 1290-square-foot 2-story duplex had a Net Zero Energy Building (NZEB) usage year. I used 4,546 kWh for heating, cooling, lighting, ventilation, water heating and plug loads during the twelve-month period. The Energy Usage Intensity (EUI) for the 12 month period was 12.88 kBTU/ square foot. The offsite energy purchased was 9.59 kBTU/ square foot. This compares to a US mean residential multi-family housing site usage of 59.6 kBTU/ square foot. The energy production from the 3.795 kW photovoltaic system was 5.247 megaWatts during the year. The photovoltaic system produced 115% of the electrical needs for the unit.
Low energy bills
My total MidAmerican Energy annual bill for 2020 was $136.31 with only February and March bills exceeding the $8.59 monthly meter charge. The water bill for 2020 of $243.64 exceeded the electric bill. The 2020 surplus energy (1304 kWH) may offset the 2021 winter usage, and I would have 12 months of minimum payments of $8.59! The 4.546 megaWatts electrical usage would have cost approximately $558, so the estimated saving is $422.
The PV system production displaced 7,047 lbs. of CO2 and is equivalent to 53 trees planted in 2020. The purchased electricity gives the unit an equivalent carbon footprint of 0 lbs. of C02, compared to a 5,455 lbs. average for a one-person household per year.