Tag Archives: photovoltaic

Solar posts

With the roofing membranes and flashing installed, we were finally able to get to the first component of our solar PV array: The solar posts.

PV modules are installed in rows, and mounted on rails, which are in turn attached to the solar posts. A shorter solar post is used along the south side of the row and a longer post on the north side, to give the module a tilt towards the sun.

The solar post assembly consisted of four components:

  1. A base
  2. The post
  3. A curb
  4. Sealant

The base was seated in a layer of sealant and anchored into the solar blocking, which we had previously installed. The post was subsequently screwed tight into the base. The curb was also seated in a layer of sealant over the base and post, and then filled with a sealant composite. This way the roof penetration for the base anchor should be completely waterproof.

I had taken scrupulous measurements for the location of the solar blocking, which was concealed since the installation of the roofing membranes. The big question was, would we be able to accurately trace the location of the blocking with the measurements?

It turned out that we did not need the measurements at all. The morning dew pattern that collected on the roof showed us exactly where the solar blocking was, due to the thermal difference between the lumber used in the blocking and the roof insulation on either side. If you look carefully at the time lapse, you may be able to spot that pattern.

Sometimes a little luck goes a long way.

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Solar blocking

Nope, I am not trying to block the sun, but this is a good reminder that the whole roofing project was happening because we were getting ready to install a photovoltaic array on our roof. And one question to resolve was: How should we attach the solar panels to the roof?

Some systems are weight based, meaning they are not physically attached to the roof structure, but instead weighed down by concrete blocks, for example. This method has the advantage that there is no hardware that penetrates the roofing membrane. The disadvantages are that the roof needs to be able to accommodate the extra weight, and more importantly that the City of Chicago would not permit weight based systems, according to my solar installer.

We needed to come up with a solution to anchor the solar array to the roof joists.

The solar panels are mounted onto rails in rows. There is one rail towards the bottom and one towards the top of the panels. The rails in turn are mounted onto posts, which are anchored to the roof structure. There is a short post towards the front of the panel and a longer post toward the back, so that the panel faces the sun at an angle. (If you struggle with some of the terminology, go to: Solar lingo)

If you look at the plan sheet above, you can see that almost none of the posts (red dots) line up with the roof joists (dotted line going from left to right). So, anchoring the posts directly into the roof joists was not an option. Instead, we planned to indirectly anchor them. We installed and anchored three rows of blocking that was running perpendicular to the roof joists (the dotted line running from top to bottom), to which we could anchor the posts.

The blocking consisted of two two-by-fours laid long side down, stacked on top of each other, and anchored into the roof joists.

This solution had to fit into our new roofing system. That is why we used two layers of 1 ½” polyiso boards, so that the insulation would be flush with the blocking for the solar posts and ease the installation of the roofing membrane.

I had marked the layout for the solar blocking right after the roof tear off and kept track of it during the insulation installation, including where to anchor it to the roof joists. I cut and removed insulation and laid out the two-by-fours. I made sure to stagger the joints between the bottom and top row. I also made sure that no joint was over a roof joist, or at an anchoring point for the posts.

After I had the two-by-fours anchored down I foamed around the edges to keep the insulation assembly sealed.

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The roof project

We were ready to get the solar PV array installed on our roof. We did our research on when a renewable energy installation makes sense and got ourselves familiar with the solar lingo. We went through the basics on how to size a solar PV system, and how it gets connected. And the timing was perfect to maximize the rebates available for a solar PV system installation.

That sounds pretty ready, right?

Yes, this was a trick question, and no, we were not ready at all.

You see, you don’t want to put a roof mounted solar system onto an old roof. You want to put it onto a new roof (or at least fairly new roof), so that the solar system has about the same life expectancy as the roof.

Not only did we have to address the old roof issue, we also had to deal with the desperately needed repairs to our front parapet and the attached cornice. Plus we had to raise the elevation of the side parapets, because we had planned to add insulation on top of the roof deck.

So what did our path to roof solar look like?:

  • Cornice bracing
  • Front parapet demo
  • Cornice repair
  • Rebuilding front parapet
  • Raising elevation of side parapets
  • Roof tear off
  • Insulation installation
  • Integration of solar system attachments
  • New roofing system installation
  • Flashing
  • And (drum roll) – installation of the solar array

Well, it is good to get all these items out the way and fixed up once and for all. If we were diligent about it, we knew we wouldn’t have to touch any of those items again for a few decades to come.

Stay tuned as I will keep posting about each of the steps and the decisions involved.

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How to connect a solar system?

You have a solar PV system, but what now? Where does the electricity you generate go?

A certain percentage will power the electrical loads in your building. Say the summer sun is shining and you sit at your computer in your home office while you have the AC running. Your solar PV system is likely to provide the electricity for your computer and your AC.

It is also likely that at times your system will produce more electricity than you use. Where does that electricity go?

There are two principle options:

  1. You can have an off-grid system, meaning you are not connected to the electrical grid and any excess electricity you produce would need to be stored in a battery system for later and night time use. Off-grid systems are used in remote areas where access to the electrical grid is difficult and/or costly.
  2. You can have a grid-tied system, meaning you are connected to the electrical grid and any excess electricity you produce is fed into the grid. A commonly used analogy is that the electrical grid acts like your battery.

Deep energy retrofits are likely to be in urban or suburban areas where access to the electrical grid is readily available and as such I will focus on grid-tied systems.


The process of connecting a solar PV system to the grid is called “interconnection.” It is typically regulated on a state basis. Here in Chicago we are in the ComEd service territory and ComEd explains interconnection as follows:

“An interconnection is an electric connection between a utility’s grid and a private generation system (PGS). A PGS, also known as Distributed Generation (DG), has the capability to send energy to the utility’s energy grid.”

Source: ComEd – Connecting to the Grid

The interconnection process with ComEd involves an application and a fee. ComEd asks for customer and account information, solar contractor information, and technical details about the solar PV system. The good news is that you probably will not have to deal with the application as it is typical for your solar installer to submit the interconnection application on your behalf.

Once the application is processed, ComEd issues an approval letter, which typically comes in the form of an email. After that, the solar PV system can be turned on, which is literally flipping a switch at the inverter.

Depending on your location and the size of your solar PV system, ComEd may need to conduct upgrades to the grid to get it ready for interconnection, which potentially could add cost to your solar project. Any upgrades would happen prior to you receiving the approval letter.

Lisa Albrecht, from All Bright Solar had the following comment when I contacted her about interconnection in Illinois:

“Currently, most ComEd interconnection applications are approved but as solar continues to grow, it is possible that the local grid may not be able to accommodate additional solar generators backfeeding into the grid. Early adopters will have the advantage of easier approvals. If you experience issues, be sure to have your installer reach out to the Illinois Solar Energy Association for input as they track these types of issues and may be able to assist if this becomes a growing problem.”

If you are not in Illinois, or have an electricity provider other than ComEd, check your local  information about interconnection, as it may differ from what has been described above.

So, are we done with applications and paperwork? Not quite yet. Because we still have to figure out how you get credited for any excess electricity you feed back into the grid.

Net metering

Let me build on the example from above: During the months when the summer sun is shining, it is possible that your solar PV system will generate more electricity than you use. And during the winter months, you probably will use more electricity than your solar system generates.

The process of net metering provides the framework on how you and your electricity provider (ComEd in our case) account for the summer surplus and with winter deficits. net metering also involves an application, but again, it is likely that your solar installer will submit the paperwork on your behalf.

Before I go any further, let me note that net metering varies state by state. If you are outside of Illinois, check the net metering agreements for your location or with your local utility, because it may differ from what I describe here.

In Illinois, we have a deregulated energy market. I asked Lisa Albrecht about what impact that has on net metering. She offered the following:

“In a deregulated energy market, net metering falls to the supplier – the company you contract with monthly. So that could be any number of companies – MC2, Green Mountain Power, Constellation, ComEd, etc.  Although all Illinois suppliers are required by law to offer net metering programs, many do not comply. It may be easiest to go with ComEd whose policies are clearly outlined. Any time a solar owner changes suppliers they will need to apply for net metering with the new company and will forego any accumulated credits.”

I will explain those accumulated credits below.

The smart meter

For net metering to work, we need to measure two variables:

  1. Energy flow from the electrical grid into your home
  2. Energy flow from your solar PV system into the grid

This required two electrical meters in the old days. But with the dawn of smart meters, things got simpler. A smart meter has two channels. Channel A measures the energy flow FROM the grid. Channel B measures the flow of energy TO the grid.

That covers the hardware requirements. Now let’s take a closer look at how net metering is administered in Illinois under the umbrella of ComEd:

“Net metering will provide customers credits for the excess generated energy that flows onto the distribution system according to the terms of ComEd’s net metering tariff…”

“When you have a solar system and are being billed for net metering, you are charged only for the net amount of energy you use during each billing period (i.e., the amount of energy we deliver to you minus the excess amount you send back to the smart grid). And, if you send more energy to the grid than we deliver, you can receive credits on your bill.”

Source: ComEd – Net metering

The electricity bill

Let’s take a look on how this would manifest itself on your electricity bill.

If your solar PV system generated more electricity than you consumed, you end up paying $0 in supply charges (the actual electricity).

But you are still responsible for Customer and Meter Charges, the two fixed charges under Delivery. And these don’t go away because you are still connected to and benefit from the electrical grid.

The remaining variable charges under Delivery and Taxes & Fees are based on kWh. If your solar PV system generated more electricity than you consumed, your net metering credits cancel out those variable charges.

The solar year

Let’s assume your system is sized to cover your cumulative electrical needs for twelve months as described in the previous post.

When your system generates more power in the summer months than you need, your bill will reflect a “rollover” credit in kWh that is carried forward to the next month. You can continue to accumulate rollovers each month for your excess generation.

These accrued rollover credits become very useful during the winter months, when your electrical consumption exceeds your solar production. At that point you start drawing down on your rollover credits. And ideally, the accumulated rollover credits will last you all winter.

Even if they don’t, i.e. your solar PV system size is not big enough to cover your annual electricity needs, you still end up paying less than without a solar PV system.

Energy credits can be carried over from month to month but not from year to year. ComEd’s policy has a reset in either April or October and most select April to start their “solar year.” The selection to whether the solar year is reset in April or October is made on the net metering application.

For more information on net metering from ComEd, see also:


A 110%

Before we get too excited, let me point out that there is a limit to the size of your solar PV system if you intend to qualify for net metering and receive a one-to-one retail rate credit for the electricity you feed back into the grid:

“Solar energy systems of customers who want to participate in net metering are limited in capacity size so that they generate no more than 110% of the customer’s prior 12 months of electricity usage.”

Source: Source: ComEd – Net metering

For example, if your electrical usage for the past twelve months was 10,000 kWh, your system can generate up to 11,000 kWh of electricity, and you would qualify for net metering. If your system is above 110%, (i.e. 11,001 kWh) you will not qualify for net metering, but instead put on a different tariff (see also below).

I understand from talking with Lisa Albrecht that calculating the amount of kWh your system could generate is still a rather basic process on ComEd’s side, meaning that certain variables that may impact your solar production, such as shading, are not accounted for. Lisa points out that using more robust calculation tools that are also used for financial analysis should yield a more accurate performance forecast. But ideally, you would not have to worry about these details as your solar installer should make sure that you end up with a right-sized system, and be approved for net metering.

One little snag

Net metering works beautifully if you have a track record for your electrical consumption over the past twelve months. But what if you build a new home or you would like to install a solar system that also should charge the electrical vehicle you plan to buy? In other words, you don’t have that track record yet. You will be above the 110% production threshold and as such won’t be eligible for net metering.

You can still proceed with your dream of a solar PV system. Size your solar PV system to your needs and instead of net metering, ComEd will put you on a tariff that is called a Rider POG, which unfortunately pays quite a bit less than the retail rate and won’t offer credits on distribution and transmission service charges.

Once your electrical consumption is at or below 110% (which will probably take up to twelve months), you can apply for net metering and benefit from the one-to-one retail rate credit and rollovers.

I call this a snag because this is not encouraging electricity conservation.

Closing thought

The solar policy landscape is constantly evolving, and not always in a positive direction. There have been challenges and changes to net metering in some states, rolling back programs, which led to court battles. When you read this, make sure to check the current net metering rules for your area and the time of your installation.

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How to size a solar PV system?

Let’s first check to see if your roof is suitable for a solar PV system. Look it up on a recent satellite image. If you can see your roof, so can the sun. If you want to conduct a more sophisticated analysis you can get in touch with a solar professional that has a Solar Pathfinder. This neat gadget will tell you what objects, such as adjacent trees and structures, will shade your solar system and for how long across the seasons.

Next, we can answer how to size your solar system. To calculate the size required to cover most if not all of your electricity needs, you need to know the following three variables:

  1. Your actual (or predicted) electricity consumption
  2. Average sun hours for your location
  3. System inefficiencies, or derate factor

Electrical consumption

Gather your electric bills for the last twelve months. Look up the total kWh you have used each month and add them up to get the total for the year. If you have a multi unit building like we have (i.e. you have more than one electrical meter), do the same for each electrical meter to reflect the total annual kWh consumption for the whole building.

For example, the consumption between January to December 2019 for all three units in our building was 10,648 kWh. We rounded it up to 11,000 kWh to be on the safe side.

If your solar system is meant for a new building, you have to come up with a predicted annual electricity consumption. This will depend on your habits and behaviors, which you may be able to extrapolate from your old electric bills, and the electrical appliances and their efficiency in the new building.

Average sun hours

It would be nice if this information would also be included on your electrical bill. Until then, you can look up it up online on the National Renewable Energy Laboratory’s (NREL) website:

Geospatial Data Science – Solar Resource Data, Tools, and Maps

You’ll find two options to determine the sun hours for your location:

  1. U.S. Annual Solar Global Horizontal Irradiance (GHI)
  2. U.S. Annual Solar Direct Normal Irradiance (DNI)

For a more cautious result, use the Direct Normal Irradiance, as it only takes into account solar radiation from the direction of the sun. The Global Horizontal Irradiance is the sum of Direct Normal Irradiance and Diffuse Horizontal Irradiance (DHI), i.e. reflection of sunlight from objects.

On the web page, scroll down to the Direct Normal Irradiance section and click on the U.S. Annual Solar DNI link. Find your approximate location on the map and determine your sun hours based on the color coding and the map key.

The sun hours for our location were a little tricky to read as we are right between two zones. It could either be 4.0 to 4.4 or 4.5 to 4.9. We ended up picking 4.0 sun hours to be on the safe side.

Side note: Don’t get intimidated by the lingo on the website. If there is a term you don’t know, it is probably explained in NREL’s glossary section:

Solar Resource Glossary:

System inefficiencies

Each solar system will have some losses due to shading, wiring, dust on modules, converting direct current (DC) into alternating current (AC), etc.

The recommended factor to account for system inefficiencies varies from 1.38 in a June 2019 paper published by the University of Arizona, Cooperative Extension (Calculations for a Grid-Connected Solar Energy System), to 1.2 according to a NREL publication in August 2002. The item that drives the inefficiency factor the most is probably the amount of shading.

We went with the default value of 1.14 (or 14%) from the NREL PVWatts Calculator.

Calculating the DC system size

Let’s quickly recap on what we have determined so far:

  1. Annual electricity consumption: 11,000 kWh
  2. Sun hours: 4.0 for our location
  3. System inefficiencies: 14%

We can now plug these values into the following equation to determine the DC size of the array:

(Yearly kWh Usage / 365 days / average sun hours) x inefficiency factor = DC solar array size required (kW)

(11,000 kWh / 365 days / 4.0 average sun hours) x 1.14 inefficiency factor = 8.59 kW DC

To cover all of our electricity needs we probably need a system sized at 8.59 kW DC. To double check the result and to adjust and play with various variables, I recommend to take this number to the NREL PVWatts Calculator. Here you can begin to fine tune the system based on type of modules (panels), tilt, orientation, shading, etc.

The PVWatts Calculator also has a function in which you can draw an area on your roof that you would like to set aside for a solar system, and it calculates the probable system size in kW DC for you.

How many modules do we need for a 8.59 kW DC system size?

Current modules (panels) produce between 310 to 350 Watts. If you read this and this post is a couple of years old, double check these numbers against current module product information.

If we assume a module production of 330 Watt (0.33 kW), we simply divide this by the system size.

8.59 kW DC / 0.33 kW = 26 modules

Actual system size

The above should give you the tools to calculate the probable solar system size and number of panels you may need. Look at this process as a feasibility study, but not a final determination.

For the latter, I recommend working with a solar professional whose analysis will be more thorough and grounded.

In our case, the numbers were very close. The professional analysis recommended a system size of 8.58 kW and an array with 26 modules.

Other resources:

Solar Radiation Data Manual for Flat-Plate and Concentrating Collectors

NSRDB: National Solar Radiation Database

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