We looked at the monitoring data from our inverter for our 26 module, 8.58 kW array in the previous post. To understand how the solar array production offsets our energy used, I compared it to our monthly usage data from our electrical bill for the solar year 2020 (April 1st, 2020 till March 31, 2021).
| Solar year 2020 | Our building (kWh) | One household in our building (kWh) | Illinois household average (kWh) Based on 2021 EIA data |
| Electricity used | 13,428 | 4,476 | 8,736 |
| Electricity produced | 11,390 | 3,797 | |
| Annual deficit | 2,038 | 679 |
The table above shows the energy use data for our 4,500 sf building as a whole, and for each of the three households (apartments).
The solar array produced enough electricity to cover 85% of our total annual electricity consumption during the solar year 2020. It effectively reduced our electricity use to 679 kilowatt hours (kWh) per year per household, which was less than the average monthly Illinois household use of 728 kWh (Based on 2021 EIA data).
Our monthly bill per household for the solar year 2020 averaged $11.24 compared to the Illinois average of $95.56 for 2021 (Based on 2021 EIA data).
A key takeaway from these data is that even prior to factoring in any solar production, our deep energy retrofit has resulted in a 49% reduction in energy use when compared to the average Illinois energy consumption per household.
Once factoring in the photovoltaic array production our energy consumption was reduced by 92% compared to the Illinois average.
2020 solar year review
The gray column in the chart above represents the amount of kilowatt hours the 4,500 sf building with its three apartments/households used any given month. September was a low use month with only 633 kWh, while February was a high use month with 2,139 kWh.
The blue column is the energy we produced with our photovoltaic roof array for any given month. We discussed these data in the previous post.
The green (and orange) column reflects the kilowatt hour rollover month by month, which is a product of the net-metering agreement with our utility. It is the difference between kilowatt hours used and kilowatt hours produced and carried over to the next month.
Take April for example: The difference between the 825 kWh used and 1,094 kWh produced is 269 kWh (rollover). In May the difference is 463 kWh. Add the rollover of 269 kWh from April, and we end up with 732 kWh rollover for May, and so on.
From April to September, our production was exceeding our consumption, and we were building our rollover nest egg for the winter. Starting with October, our consumption exceeded our production, and we slowly began to eat into our rollover credits. By January, we had used up all rollover credits and started to run a deficit (-299 kWh), meaning that for the first time since April 1st 2020, we actually pulled energy from the grid for which we would be invoiced.
At the end of the solar year, the building had used a total of 13,428 kWh, which was offset by 11,390 kWh production from our photovoltaic roof array. That left us with a deficit of 2,038 kWh, for which we would be invoiced.
How does this compare?
Data published for 2021 by the U.S. Energy Information Administration (EIA) lists 10,632 kWh as the average annual electricity consumption for a U.S. residential utility customer. 2021 EIA data for Illinois lists an average annual consumption of 8,736 kWh per residential utility customer, or household.
| kWh use per year | kWh use per month | |
| U.S. household average | 10,632 | 886 |
| Illinois household average | 8,736 | 728 |
| One household in our building w/o solar | 4,476 (actual) | 373 (actual) |
| One household in our building household w/ solar | 679 (actual) | 57 (actual) |
When adjusting the numbers for our building to a household basis for the solar year 2020, we get 4,476 kWh use per apartment per year. If we factor in our electrical production, we are down to 679 kWh per apartment per year. The numbers for our building include space conditioning (heating and cooling).
2020 solar year billing
Because the cost per kilowatt hour, service charges and net metering agreements vary by energy provider and service area, this section may be mostly useful to our Chicago readers.
Our electrical bills for the building for the solar year 2020 added up to $404.47.
As mentioned above, we did not pay for any electricity until our rollover credit ran out in January. We were still responsible for delivery charges on our bill (i.e. customer and meter charges). They averaged $12.83/month from April through December, leading to a total of $115.47. Delivery charges don’t go away because we are still connected to and benefit from the electrical grid.
For January, February, and March, we paid a total of $289 for the 2,038 kWh deficit we accumulated for the building over those three months.
The table below compares our average bills on a apartment/household basis to those for residential customers in Illinois, based on 2021 EIA data.
| Average month bill | Average Electrical cost per year | |
| Illinois household | $95.86 | $1,150.32 |
| One household in our building | $11.24 | $134.82 |
More about our actual savings and ROI on our photovoltaic array in the next post.