We installed a 22 panel (400W each) solar system to our house in July 2023. This article provides some real world data on the performance of the system through a twelve month period.
To provide the necessary context: the house is a mostly standard three bedroom single family house with almost all electric appliances, such as heat pumps, an energy recovery ventilator, dehumidifier, sump pump, water heater, refrigerator etc. It is located in Massachusetts around 42 N latitude. Panels are installed on a south facing roof as a single block and connected to a SolarEdge inverter rated at 7.6 kWh AC output. There is a Powerwall battery. We also switched to an EV car around last February which is charged solely at home.
Let's start with the electrical balance sheet :)
Left side of each bar is the electricity produced by solar panels (yellow) and bought from the city grid (red). The right side shows the consumption breakdown by house usage (blue), EV car charging (green) and the surplus (gray) which was sold back to the grid.
Solar production and the surplus happen during the daylight time. The battery stores some of the surplus and reduces the need for grid backup during the night time but its effects are ignored here. See the battery section below for the details.
Overall, the system produced around 12 MWh over a year which was slightly above of the total consumption. This was also very close to the design spec which proves that the model calculations based on the satellite data are quite accurate.
You can see that the monthly solar production goes below the consumption (when blue and green lines top is above the yellow line) in 6 months. The one in the July is due to the consumption increase and probably caused by extra AC usage due to hot and humid weather. To understand the winter months' production decrease, let's look at the external factors.
Solar Performance Factors
You can see the daily electric production, how many hours of daylight that day had, and the average temperature of the day in graphs below.
First observation here is that there is no day with zero production. Even if it is very dark day, or panels were covered under few inches of snow, some energy from the sun reached to the panels.
Second observation is the strong correlation with the sunlight hours of the day. This makes a lot of sense and kind of expected, but it also means the other factors are almost irrelevant in the big picture.
The efficiency of the panels decrease with heat, so temperature has a negative correlation, and combined with the snow cover and clouds, create the variance in the daily production as the secondary factor.
Battery
Besides providing energy during the outages, a battery allows direct use of the produced solar energy. The stats provided by the gateway shows that the house was on direct solar power 41% of the time during the one year period, on battery for the 16% of the time, and on grid for the remaining 43% of the time.
We have configured the gateway to always keep the battery at least 40% capacity to provide backup during the grid outages. System also receives information about storm warnings and auto charges from grid to 100% capacity until the storm passes.
The house usually uses about 1-3 KW during any moment, but sometimes it goes up due to multiple machines running at the same time. Biggest consumer is the electric water heater which might draw up to 4 KW by itself when tank is cold and outside temperature is below zero. Maximum draw we have observed so far was around 7 KW.
Biggest reason for selecting the Powerwall battery was its peak output. Other batteries have higher total capacity and cheaper prices, but they can't support this much of instant load with a single battery unit. The offers we have seen always involved building a separate subpanel with only four or eight circuits and only backing those up. This would be costly due to the extra electrical work and quite inconvenient.
We weren't even aware of several outages happened during the year. I only became aware of them when looking at the statistics on the system for writing this article.
It could be possible for us to go completely off-grid by installing a second battery and switching to a more efficient heat-pump water heater with a larger tank, but that would be neither cheap nor really needed.
Cost
The installation work was done by
Revision Energy which is an employee owned company. They have done an excellent job, paying attention to keeping the high power lines as short as possible, not creating extra finish work for us by designing around living areas, and complying with all regulations.
The total cost of the project was around 60k$ including the installation of an EV charger and some fixes in the existing main panel. Almost half of the price was the battery itself.
The federal and state incentives paid back 20k$ as tax credits. Our grid company charges around 0.22$ per KWh which makes about 2640$ for the year, but this saving is actually a little bit less due to the difference between buy and sale prices which the grid company uses for generated electricity. We also get about 100$ a year from
RECs. This puts the return of investment time to somewhere around fifteen years.
Conclusion
We enjoy having less dependency on the grid, our greatly reduced environmental impact, not paying electric bills, and taking advantage of the energy already available around us. It is a huge upfront investment and the incentives are not great unfortunately.
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