Tag Archives: photovoltaic

Dissecting space conditioning

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.

solar year 2020Building (kWh)One household in our building (kWh)
Total energy use13,428 (100%)4,476
General energy use (excluding space conditioning)8,400 (62.5%)2,800
Energy use for space conditioning5,028 (37.5%)1,676

2020 data from the U.S. Energy Information Administration shows that space conditioning consumes 46% of the building’s energy use in 2-4 unit apartment buildings like ours (or 52% on average per U.S. household).

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.

TotalkWh w/o space conditioningSpace conditioning
Jan 20211,839 kWh minus700 kWh =1,139 kWh
Feb 20212,139 kWh minus700 kWh =1,439 kWh
Mar 20211,254 kW minus700 kWh =554 kWh
Total5,232 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:

  1. Address thermal deficits in the building envelope first to significantly reduce the overall energy load of the building.
  2. 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. 
  3. 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!

Related posts:

How close to net-zero are we?

To answer this question, let’s look at the data for the solar year 2020 when our electrical use included space conditioning.

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:

  1. That we still rely on natural gas for cooking, domestic hot water, and occasional heating. And we still have a gas dryer.
  2. 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.

But there is a problem: we drank the kool aid.

We originally, and occasionally still rely on our hydronic heating system, powered by a high efficiency boiler. The steel baseboard radiators and radiant floors deliver a comfort during the heating season that is unmatched.

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:

  1. a high level of energy efficiency,
  2. some level of resiliency and longevity,
  3. improved indoor comfort and health without an energy penalty, and
  4. 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.

Related posts:

Photovoltaic ROI

In the previous two posts, we studied the electrical production from our 26 module, 8.58 kW array from 2020 to 2023 and compared it against our electrical consumption for the 2020 solar year.

The actual monetary savings of our photovoltaic array over the first three years averaged $1,257 per year, which is slightly more than we expected. At this rate, we are on track to make our $10,047 investment on the array back by year eight.

Looking to the future, if this savings rate continues, the array will have also paid for most of the $31,971.04 roof project investment (the cornice repair, parapet repairs, and reroofing) by year 25.

Slicing and dicing the savings

Our solar installer, Lisa Albrecht from All Bright Solar, shared an Excel spreadsheet with me that allowed me to take a detailed look at our savings. I input the various charges from our electrical bill plus the electricity we pulled from the grid, the electricity we fed back into the grid, and our monthly rollover credits, and it shows me what I would have paid without our solar array versus what we actually paid.

Our savings per solar year (April 1st through to March 31) were as follows:

2020 solar year$1,175.76
2021 solar year$1,232.94
2022 solar year$1,361.86
Three year total$3,770.56

Our savings follow closely the Return on Investment (ROI) calculations that Lisa shared with us when planning the installation of our solar array.

Our out of pocket investment into the solar array was $10,047 after the various rebates. Subtracting the first year’s savings reduced our liability from $10,047 to $8,888 (prediction), or $8,871 (actual). In other words, our ROI for the first year exceeded the prediction by $17.

Subtracting the second year’s savings from the net gain/loss of the first year reduced our liability to $7,697 (prediction), or $7,638 (actual), with the ROI exceeding the prediction by $59 – and so on…

As I mentioned in a previous post, the state incentive we received is based on how much kilowatt hours our 8.58 kW photovoltaic array is predicted to produce over the first 15 years.

10% of that incentive is withheld until year 15 and released as long as we meet the predicted production target. Because our production is exceeding the prediction, it would be safe to assume that we can add another $1,160 incentive payout to year 15 in our ROI calculation.

So far we are on track to make our money back on our $10,047 investment by year eight, giving us an ROI of 12.5%. After that, we can pocket all the savings from our 8.58 kW photovoltaic array. If Lisa’s predictions hold true, we would also have saved around $31,838 on our electrical bills for our 4,500 sf building with its three apartments by year 25.

That $31,838 would cover 99.5% of the cost for our roof project (the reroofing, cornice repair, and parapet repairs).

That all said, I don’t expect to exactly meet these ROI targets, because the numbers don’t account for maintenance and repair costs we may face over the next 25 years. But we hope to get close to them.

At this point I feel I need to clarify again that the savings are not just because we bought into a renewable energy system and slapped a photovoltaic array onto our roof. These savings were made possible because our deep energy retrofit significantly reduced the overall energy load of our building first, which then was followed by a renewable energy investment.

Related posts:

Photovoltaic production vs. electricity used

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 2020Our building
(kWh)
One household in our building
(kWh)
Illinois household
average (kWh)
Based on 2021 EIA data
Electricity used13,4284,4768,736
Electricity produced11,3903,797
Annual deficit2,038679

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 yearkWh use per month
U.S. household average10,632886
Illinois household average8,736728
One household in our building w/o solar4,476 (actual)373 (actual)
One household in our building  household w/ solar679 (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 billAverage 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.

Related posts:

Photovoltaic array monitoring

A previous post covered how the inverter is an integral part of our 26 module, 8.58 kW photovoltaic array. Both, the inverter and the array, service our 4,500 sf building with its three apartments/households.

The inverter allows us to monitor our kilowatt hour (kWh) production in real time, either through an android/iOS or web app. The app interface also gives us access to historical production data, such as the past three years (2020 – 2022).

During those three years, the lowest monthly production on record was for December 2022 with a meager 273 kWh. Our worst production day was January 31, 2021 with 1 Wh due to a foot of snow on the modules.

June 2022 gave us the highest monthly production with 1,495 kWh, and the best daily yield so far fell on May 29, 2021 with 68.7 kWh.

Our array averaged 11,000 kWh in annual production from 2020 through to 2022, which is within 4% of what we were aiming for (11,459 kWh).

The 11,000 kWh for our 4,500 sf building breaks down into 3,667 kWh of self-produced electricity per household per year, or 305.6 kWh per household per month.

Annual production overview

Even though some months are sunnier than others, and despite the solar irradiation varying each year for any given month, the production across the year is remarkably consistent with an average of 11,000 kWh per year.

The overall small production decline since 2020 reflects the estimated annual system decrease, which is specified to average 0.26% per year for our Panasonic 330 watt modules.

While planning the system with our solar installer Lisa Albrecht from All Bright Solar, we targeted an annual production of 11,459 kWh. 11,000 kWh per year brought us within 4% of that goal. Some shading during the winter months from an adjacent tree was likely responsible for the 4% shortfall.

The “Estimated energy” is the minimum amount of energy we need to produce to fulfill our contractual obligations for the state solar incentive we received. The state incentive is based on how much kilowatt hours our 8.58 kW photovoltaic array is predicted to produce over the first 15 years. This number was calculated by our solar installer, and determined our total state incentive amount.

Monthly production overview

The data for the past three years on a monthly basis shows the expected seasonality in our production. We can also see the varying yield each year for any given month, a product of a given month being more overcast or sunny in one year than in another.

We are not always meeting our production goals between the months from October through to January. We have yet to meet the production goal for November, and January isn’t far behind. We suspect that the shading of the adjacent tree during these months is partially responsible for the lower than predicted production.

However, Lisa Albrecht mentioned that this is a trend across her portfolio and she assumes the region. Current forecasting models use 30 years of historic weather data to predict future performance. However, climate models suggest we may experience heavier cloud cover in Q1 & Q4 in our region as our winters warm.

The good news is that based on our net metering agreement the production goals are tallied by solar year, and the excess production during summer makes up for the underproduction in winter.

Worst production

Not necessarily a meaningful data point, but an interesting one. The absence of solar irradiation this past December (2022) manifested itself in a meager output of 273 kWh. Did snow cover have a negative impact? No. We only had two days that produced measurable snow, and I quickly cleared the snow from the array.

Out of the 31 days in December 2022, we had five sunny days (>20 kWh), two mostly sunny days (>15 kWh), and we had 15 gloomy days (<5 kWh). We had to turn lights on during the day for half of that month because it was so overcast.

The worst production day in the past three years was on January 31st, 2021 with a meager 1 Wh. We had a big snow storm moving through and there was no point clearing the snow off the modules until the storm system had left the area the next day and the wind had subsided.

Best production

No need to look any further than the height of summer. June 2022 with 1,495 kWh was the month with the highest yield so far, closely followed by July 2020 with 1,487 kWh.

The high solar irradiance in June of 2020 resulted in 19 mostly sunny days (>50 kWh) and only four mostly overcast days (< 30 kWh). Another 5 kWh in production would have given us a monthly total of 1.5 MWh.

Our highest yielding day in the past three years was on May 29th, 2021 at 68.7 kWh. That came as a surprise, because it was still a good three weeks away from the summer solstice. Humidity levels and the associated haziness around mid to late June must have kept yields below that of May 29th, 2021 for the past three years.

The above production data will set the scene for the next post, where we will compare it against our electrical use.

Related posts: