In the previous two posts of our photovoltaic system, we reviewed the amount of kWh we used and produced, and our electrical cost. But how does our electrical consumption and cost compare to other homes in Illinois?
To find out, I took the data per solar year for our 4,500 sf building (kWh used and cost) and divided it by the three apartments to have a per-household baseline, which I compared against the residential average monthly bill data for Illinois published by the U.S. Energy Information Administration (EIA).
Since we activated our solar array on December 24, 2019, I show data starting with solar year 2020. The latest EIA data available is for 2023. 2024 data has not yet been published, so I will skip that year.
Electricity consumption
Broadly speaking, our electricity consumption per household/apartment is about half of that of the average Illinois household, or less.
What these numbers show is that our investment in the building envelope improvements paid off by significantly reducing our overall energy load. I am talking about good insulation, good air sealing, good quality triple glazed windows, good exterior doors, a high quality and highly efficient ventilation system, etc.
What also helped was us being picky about what appliances we bought. They were almost exclusively Energy Star rated appliances. And not just any Energy Star appliances. We picked the ones that had the best (or one of the better) efficiency ratings.
Cost and bills
If I use solar year 2020 as a baseline, the Illinois average annual electricity cost per household was 8.4 times our cost, thanks to our solar roof array. And 2020 is the best year to focus on, since the only heating source was our air source heat pumps.
This significant cost reduction reflects that our investment in the building envelope improvements reduced our overall energy load, plus the electrical production from our 8.58 kWh solar roof array reduced the amount of electricity we had to purchase.
Let me say this again:
I started two posts ago with bragging rights because we generated a surplus two years in a row. That is an encouraging data point. The real bragging rights, however, are less in the surplus and more in the relatively low electrical load that we have been able to achieve and maintain.
We were not distracted by the low hanging fruit: “Let’s switch to all LED lighting…”. I credit our motivation to eliminate unnecessary energy hogs (such as the proverbial old refrigerator in the garage), our selectiveness in picking right-sized and efficient appliances, AND most importantly, our laser sharp focus on the building envelope improvements during the deep-energy-retrofit. The latter gave us, without a doubt, the biggest bang for our buck.
In the next post I’ll explain, at least partially, why the building envelope improvements were such an important investment by using our air source heat pumps as an example.
To continue the five year review of our solar array, our annual electricity bills reliably follow the trends outlined in the previous post, with the exception of the solar year 2024. To explain this outlier, we need to take a look at how deficits and surpluses as well as fixed costs affect the billing.
This analysis is mostly relevant to ComEd customers in northeastern Illinois with a net-metering agreement. Customers with other electrical suppliers and in other service territories are likely subject to different tariffs, charges, and agreements.
We find three billing categories on our ComEd bills:
Supply costs are the cost for the electricity (kWh) we pulled from the grid (supplied by ComEd). If our solar production per given month is larger than our electrical consumption, this zeros out.
Delivery costs are also often referred to as fixed costs. These are the costs for having an electrical meter and a connection to the grid (grid tied system).
Taxes & fees are a percentage applied to the electricity (kWh) we pulled from the grid (supplied by ComEd). If our solar production per given month is larger than our electrical consumption, this zeros out.
The chart above shows the cost per solar year for each of the three billing categories, plus the total annual cost, since we activated our solar array on December 24, 2019.
For the three apartments in our 4,500 sf building, our annual electrical cost started at a “whopping” $403.71 for solar year 2020. Not that surprising, considering that this was also the solar year where we ran the biggest deficit.
Equally unsurprising is that our lowest annual cost was $141.44 during Solar year 2023 with the 995 kWh surplus. Because of that surplus, we incurred no supply cost and taxes & fees. What stands out is that our annual cost rose to $170.55 in 2024, despite the 715 kWh surplus.
That increase is due to the rise in the delivery cost (fixed fees) during solar year 2024 by a total of $29.10. And as long as we have an electric meter and a connection to the grid, we will be subject to the delivery cost.
And what happened to our surplus? Think of it as a charitable donation. With our net metering agreement, at the end of every solar year, our account is reset. If we run a deficit, we need to pay up for the electricity we pulled from the grid. If we run a surplus, it will be erased and we start at zero for the next solar year.
With solar vs. without solar
We just established the bill total for the whole building per solar year with our solar array. But what would we have paid if we did not have the solar array on our roof? Well, there is a spreadsheet for that.
Broadly speaking, we would have paid around $1,500 for electricity per solar year for the three apartments in our 4,500 sf building. That translates into roughly $1,200 to $1,300 in savings per solar year.
These are real “feel good numbers”, which are in line with the savings prediction we got from Lisa Albrecht at All Bright Solar (our solar installer), during our consultations with her when we went down the solar road.
Two years have passed since my last review of our photovoltaic roof array, which we activated December 24, 2019. And since we produced our first surplus during the solar year 2023, followed by another surplus for 2024, I feel it’s time to brag – a little.
Consumption
In the five years since we had our 26 module, 8.58 kW photovoltaic roof array installed, the electrical consumption for the three apartments in our 4,500 sf building ranged from 13,428 kWh at the high end per SY to 10,574 kWh at the low end.
I attribute the rather high electrical use of 13,428 kWh during solar year 2020 to the fact that all heating was provided by our air source heat pumps, and a cold spell that swept through Chicago with temperature in the single digits during the month of February.
The lower annual consumption during solar year 2022 and 2023 was partially due to user behavior (using the natural gas powered radiators for heating rather than the air source heat pumps), and milder weather conditions during the heating season.
Production
The electrical production varied over the past five solar years due to a number of factors. It is, however, difficult to exactly quantify what factors may have had what impact.
The solar modules lose some efficiency over the years. Our modules, the Panasonic N330/N325, have a specified annual degradation rate of 0.26%. This would be hard to notice or measure year over year.
It is much more likely that weather and/or shading conditions as well as soiling of the panels are the driving factor for the variation in production. That said, it does not explain the decline from 11,390 kWh during solar year 2020 down to 11,052 kWh for solar year 2021, considering that the percent of possible sunshine increased from 56% to 58%.
The drop in percent of possible sunshine between solar year 2021 and solar year 2022 from 58% down to 50% may, however, explain the production drop from 11,052 kWh to 10,319 kWh.
The uncharacteristic jump in production to 11,633 kWh for solar year 2023 and 11,801 kWh for 2024, can be partially attributed to the rise of the percent of possible sunshine to 57% and 58% respectively. However, production was still higher than solar year 2020 and 2021, which had almost identical amounts of percent of possible sunshine.
What may account for the difference was the removal of a dead Ash tree [LINK – Photovoltaic roof array shade study] on the adjacent property. Prior to the removal (solar year 2020, 2021, and 2022) that Ash tree partially shaded our array in late fall, winter, and early spring (see also video below).
Surplus
For the first three solar years our electrical consumption outpaced the electrical production, and we ran small deficits.
That changed during the solar year 2023 with a production surplus of 995 kWh, followed by a surplus of 715 kWh in 2024. This surplus does coincide with the above mentioned jump in electrical production. But it probably also needs to be attributed, at least partially, to user behavior and weather conditions.
For example: The smaller surplus for solar year 2024 is due to the increased electrical consumption, which I attributed to a slightly higher air conditioning load compared to solar year 2023.
Our January cold spell along with new data from the 2020 Residential Energy Consumption Survey inspired me to further dissect the issue of space conditioning. When it comes to energy use in a building, space conditioning is the 900 pound gorilla in the room. We reduced our space conditioning load through three steps:
First step: The deep energy retrofit, which significantly reduced our overall energy needs through building envelope improvements among other things. This blog is packed with information on insulation, air sealing, window selection, etc. And you can find a summary post here on steps to reduce your overall energy needs.
Second step: Adding a photovoltaic array to our roof top to cover our remaining energy needs. You can search this blog for “solar” or “photovoltaic” to find detailed information on this step.
Third step: Installing heat pumps (also known as minisplits) for space conditioning. You can find more information by searching this blog for “minisplit” and “heat pump”.
The first step was the heavy lifting, and got us the biggest bang for the buck. In fact, our building’s energy consumption for space conditioning ended up below the national average.
And in monetary terms, cooling our building in 2020 was “free” because of the second step: our photovoltaic roof array which provided the needed electricity. Heating our building was almost free. It cost us $173.40 to heat our 4,500 sf building in 2020.
If you would like to know about the nuts and bolts behind those numbers, keep on reading!
Parsing out space conditioning
I used the solar year 2020 (April 1st, 2020 till March 31, 2021), because it was a twelve month stretch where all our space conditioning needs were covered by our heat pumps (single head minisplits). I separated out the general electrical consumption from the energy used for space conditioning by looking at our electrical use on a monthly basis, plus factoring in data from our home energy monitors. The building’s average monthly electrical consumption for everything but space conditioning was 700 kWh.
Our building’s energy use for the solar year 2020 totaled 13,428 kWh. Assuming the average use of 700 kWh per month, we used an estimated 8,400 kWh during the solar year 2020 without accounting for heating and cooling, which took an estimated 5,028 kWh.
Our deep energy retrofit allowed us to reduce that number from 46% to 37.5%, an estimated 8.5% decrease during the solar year 2020.
We are not talking about how much energy is used here, but how that energy use is distributed across various categories, from space heating to refrigeration and all other.
When comparing the 2020 data to that of 2015, we see that these numbers are fairly constant. They are actually hard to change, particularly in existing buildings, because of long established construction types, materials, and methods.
The fact that we were still able to shrink the percentage of energy going towards space heating and air conditioning by a whopping 8.5% for the solar year 2020 is a testament to the success of step number one: reduction of our overall energy load through building envelope improvements. And it pays off:
In terms of heating cost…
…how did I get to $173.40 to heat our 4,500 sf building for the solar year 2020?
From April through to December we only paid for fixed costs ($12.83/month for customer and meter charges) because our photovoltaic array combined with our net-metering agreement covered our electrical needs. For the last three months of the solar year (January, February and March) we had to purchase electricity and paid a total of $289 for the 2,038 kWh we used.
Total
kWh w/o space conditioning
Space conditioning
Jan 2021
1,839 kWh minus
700 kWh =
1,139 kWh
Feb 2021
2,139 kWh minus
700 kWh =
1,439 kWh
Mar 2021
1,254 kW minus
700 kWh =
554 kWh
Total
5,232 kWh (or 100%)
3,132 kWh (or 60%)
Total cost
$289 (or 100%)
$173.40 (or 60%)
Looking at the total kWh consumed and the breakdown between kWh for space conditioning and kWh for everything else, an estimated 60% ($173.40) of that energy went towards space conditioning (heating) our 4,500 sf building with the minisplits for the three months we ran a deficit.
There is nothing mysterious about this, as long as you don’t fall into the trap by starting your project with a heat pump.
Follow the three steps, and numbers like this (or better) can become a reality:
Address thermal deficits in the building envelope first to significantly reduce the overall energy load of the building.
Combine those improvements with a renewable energy project, such as a photovoltaic array, that now has the potential to cover 100% or close to 100% of your energy needs.
Install an efficient heat pump system that is small and compact due to the reduced overall energy load of your building, and subsequently is largely or entirely powered by your renewable energy system.
But there was something magical about this: We ended up with a very comfortable home!
Our annual use totaled 13,428 kWh that year, while our annual production amounted to 11,390 kWh. The solar array produced enough electricity to cover 85% of our annual consumption.
To reach net-zero, we would need to be at 100% or above. So we are around 15% short of net zero and had some more homework ahead of us.
The moving goal post…
When we embarked on the project in 2009, all-electric homes were not a thing yet, heat pumps were hard to find, and solar arrays were uncommon.
At the time my focus was on using a solar hot water system to heat the building and for domestic hot water, and a photovoltaic array to cover our electrical needs. But I always found myself on thin ice when attempting to cover space heating and domestic hot water with a solar hot water system alone. In other words, getting away without a natural gas connection seemed impossible, which made the net zero goal hard to reach.
I pivoted my focus into significantly reducing the overall energy load of our building. If I had to use natural gas as an energy source, I wanted to use as little as possible. That put us on the path of our deep energy retrofit.
And it paid off.
Interim results in 2012 showed that our improvements reduced our electrical consumption by an estimated 57% and preliminary results from 2016 showed that we reduced our heating needs by an estimated 80%.
A lot has happened since 2009. Green building technologies that once were only known from excotic places like Europe or Asia suddenly made an appearance in the U.S. market, such as heat pumps. And with that, my focus on solar hot water fell away, because heat pumps emerged as a more economic option that I still could use, even if the sun was not shining.
Reducing the general electrical load of a building also has become easier since 2009 with increasingly efficient energy star appliances, LED lighting, etc.
What is standing in the way of net zero?
Yet we are not net zero, to my chagrin. What is standing in the way are two key factors:
That we still rely on natural gas for cooking, domestic hot water, and occasional heating. And we still have a gas dryer.
That our solar array is not large enough to cover 100% of our energy needs should we go all electric.
The second point should be reasonably easy to solve. Because we have reduced the energy load of our building significantly, we have enough room to expand our solar array to cover 100% of our energy use. And we plan on doing so – eventually – once technology catches up.
Regarding the first point – our natural gas connection – it helps to know how much natural gas goes towards what source in our building.
Analyzing our utility bills over the past seven years revealed that about 700 therms (70%) went towards space heating, with only 300 therms (30%) going towards domestic hot water, ranges, and the dryer.
The 300 therms seemed to be easy to solve. We can replace our gas dryer with a condensing dryer. The gas ranges can be replaced with induction stoves. And the heat pump water heater technologies have improved to the point where we could say goodbye to a gas fired water heater too.
As for the 700 therms going into space heating, one could argue that we solved that problem already with the addition of our minisplits. We used them to heat our building during the solar year 2020, and it worked.
The good news is that we potentially could replace our boiler with an air-to-water heat pump that used CO2 (R744) as a refrigerant. These units are slowly making an appearance in the U.S. market and are able to deliver 130F water even at very low exterior temperatures. 130F would be a suitable temperature for our hydronic heating system and domestic hot water.
Not only that, but an air-to–water heat pump would be two to three times more efficient than our high efficiency boiler. In other words, it would only require half or one third of the energy input to produce the equivalent of 700 therms heating output.
I am hopeful to eventually replace our boiler with an air-to-water heat pump and solve the 700 therms that were needed for space heating. We subsequently could cut our natural gas supply to the building, and yet still enjoy the comfort of our hydronic heating system.
That must be expensive!
In the big picture, what is the cost of doing nothing?
And on a project basis, if it is expensive depends on one’s mindset.
Most of our system decisions, such as the heating system, were not solely based on the economics of the day, or “what is the cheapest system I can get.” We were comfortable investing in systems with a longer payback period as long as they came with:
a high level of energy efficiency,
some level of resiliency and longevity,
improved indoor comfort and health without an energy penalty, and
systems that were somewhat future-proof so that they could adapt to technology upgrades.
This required a lot of research and careful planning at the onset of our project. And it required a lot of luck, as we were gazing into the future trying to predict the path green building technologies would take.
And in practical terms?
It appears that our utility room layout could accommodate the switch from boiler to air-to-water heat pump without revamping the whole hydronic heating or domestic hot water layout.
And because we were mindful when we installed an all new electrical metallic tubing (EMT) based electrical system, providing 240V for the induction stoves and potentially the condensing dryer should just be a matter of simple rewiring.
The one item that wasn’t even remotely on the radar in 2009, and that I still have to wrap my head around, is how best to integrate and accommodate EV charging stations.
In summary: We are not net-zero yet. We are fairly close, and we know the path that will take us there.
