Tag Archives: performance

Five years of solar – Comparing (Part 3 of 3)

renewable energy, ,

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.

Related posts:

Five years of solar – Cost (Part 2 of 3)

renewable energy, ,

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:

  1. 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.
  2. 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).
  3. 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.

Related posts:

Five years of solar – kWh (Part 1 of 3)

renewable energy, ,

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.

Related posts:

Photovoltaic ROI

renewable energy, , ,

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

renewable energy, , ,

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: