Did I make the right decision when I installed a solar+storage system at my house three years ago?

In 2015, my combined electric and gas bills had hit $500 a month, once in the summer and once in the winter. That jolted me. We were empty nesters living in a four-bedroom, two-story house built in 1979. The square footage is 2,400 square feet and we moved into the house in 1988.  

In February 2016, after much research, I went ahead with a whole house energy upgrade. The energy upgrade involved replacing the 4-ton central a/c with a SEER 18 model (the previous one was 10 years old with SEER 11) and the 80,000 BTU gas furnace with a 97% efficient model (the previous one was 10 years old and 805 efficient), doubling attic insulation, and reducing duct leakage from 58% to 8%. I also gave away our 8-person electrically heated spa.

Even then, my power bills averaged $202 a month despite our substantial investment in energy efficiency.

In June 2019, as part of my commitment to clean energy, and to lower my driving costs, I replaced my 2005 Porsche Boxster with a Tesla Model 3. I bought that car after talking it over with a dozen friends who owned that car.

In December 2019, after talking to several friends who had solar on the roof, I installed a solar and storage system at my house in December 2019 to lower my bills. None of my friends had a battery but I had one put in to enhance my resilience during power outages, and contribute to a cleaner environment. The payback period for the system was estimated at 9 years (it would have been just 7 if I had not installed a battery).

The system has been in operation for three years now. Its performance is summarized in the table below at the annual level.

Table 1: Energy produced and consumed by year (kWh)

If I had not installed solar, my consumption numbers would have ranged from 9,680 kWh to 11,420 kWh across the three years. By installing the system, my consumption numbers fell by more than 95%, and ranged from 400 to 590 kWh across the three years.

I get an annual “true-up” bill at the end of each billing year. It’s computed using the Net Energy Metering 2.0 formulas but the presentation could be simplified. Its multiple pages long. Even then, some basic information is missing. It does not include a monthly summary of net usage by pricing period and cost by pricing period. Even more surprisingly, it does not show the price by period.

Summary financial information across all pricing periods by month is contained on page 2. Table 2 shows the details for 2022.

Table 2: Annual True-Up Bill for 2022

For the year 2022, the key numbers are in columns labeled “Total Charges,” and “Non-Bypassable Charges.” They are, respectively, $475.50 and $167.44, for an annual total of $642.94, yielding a monthly value of $53.58.

Across the three billing years, my bills have averaged $58.5 a month, down from the $202 a month before the system was installed. Since electric rates have risen by 41% during this time period, had we not installed the solar+storage system, the bills would have risen to $285 and kept on rising in the years to come.

It’s also worth noting that the average monthly bill of $58.5 is much higher than the number that’s often put forward by those who think rooftop solar creates a cost shift from lower income customers to higher income customers. They often assume that the number is $20 a month with some going so far as to say that its zero.  

The role of the battery

The battery allows me to arbitrage against the three-period TOU rate (EV2-A) that I have chosen. The sun charges the battery in the morning while the house is powered by the grid at the off-peak rate (currently 24.1 cents/kWh). Once the battery is fully charged, it goes into standby mode. From that point on, the sun powers the house and the surplus power is exported to the grid.

At 4 pm, the battery begins to discharge if solar power is insufficient to power the house on its own. This is the time when peak prices are in effect (55.4 cents/kWh as of this writing). They remain in effect until 9 pm. From 3 to 4 pm and from 9 pm to midnight, the mid peak price is in effect (44.4 cents/kWh).

When the power in the battery is down to 30%, it goes into standby mode. If there is a power outage, the battery begins to power five essential circuits in the house (the refrigerators and freezers, the modem, several plugs and lights).

This feature has come in very handy during the 8 power outages we have had since June 2021. Most of them have been a few hours long. The outages are associated with the underground distribution system and are not due to the PSPS events or generated-related load shedding.

In December, there was a planned outage, so that PG&E could work a grid reliability project. It lasted for eight hours. In a few hours, the house became really cold since the gas furnace was not running without power. We left the house and went to an outdoor shopping mall.

Just two days later, there was an unanticipated outage which lasted 12 hours. The house became really cold once again and we came up with errands to run and people to visit during the outage. All our neighbors kept each other informed about the outage via a group text. We returned once power had been restored. The cause of the outage was not disclosed.

Variation in power production across months and years

The following chart shows the monthly and seasonal variation in solar generation across the three years.

The amount of power generated depends on the orientation of the roof. It varies across the seasons based on the length of the days. It also varies based on cloud cover. Production is highest in the months of May, June, July and August.

The production numbers in the second half of 2022 were higher than in the prior years because two giant Redwood trees had to be taken out in the backyard since their roots were threatening the foundations of the house. Because of the drought, the roots were much closer to the surface of the lawn than they would normally have been.

Monthly Consumption and Production Data by Month for 2022

These two variables are shown by month for the year 2022 in the following graph.

In April through September, production exceeded consumption. The relationship was reversed from October through April. There was a surplus in half of the year and a deficit in the other half. Overall, consumption exceeded consumption.

Typical days in each of the four seasons

All these data are from the SolarEdge monitor which is connected to the inverter. It provides data at varying levels of granularity, ranging from daily to monthly to annual. Within a day, it shows information at the 15-minute level. The updates occur every 15 minutes.

I have picked a typical day for each month to illustrate the variation in usage and production across the 15-minute time intervals. The charts are self-explanatory. System production includes generation coming directly from the sun and generation coming from the battery as it begins to discharge at 4 pm.

The Winter Season

In the winter months, consumption exceeds production. Anytime I charge the Model 3, consumption shoots up, as can be seen in the graph for March 29, 2022.

The Spring Season

The equation begins to reverse in April. Production usually exceeds consumption, often by a considerable amount.

The same is true in May. Usually, the weather is mild, days are getting longer, and the air conditioning system has not yet turned on.

Consumption begins to rise as the temperature warms up in June through the summer. But solar production also rises unless the sky is cloudy, in which case production plummets.

The Summer Season

In August, the relationship could go either way.

The Fall Season

As fall begins, the consumption-production relationship can go either way.

Conclusion

Overall, I am very satisfied with the performance of the solar+storage system. It has delivered the results I was hoping it would deliver.

It has substantially lowered my bills and provided resilience during a number of power outages.

Yes, I made the right decision in installing it.