Minisplit cooling pause

A typical summer in Chicago comes with heat and humidity that is every now and then interrupted by cooler spells with lower dew points. Those spells can be pleasant enough for us to stop running the minisplit in cooling mode and instead open the windows.

Once the heat and humidity roars back into town, we shut the windows in a hurry and power up the minisplit for that pleasant cool breeze. Except, there isn’t much pleasantness in that breeze, unless you enjoy a musty and mildew-drenched flavor.

If you abruptly stop the minisplit in cooling mode, the fins on the evaporator/condenser will still be drenched in condensate droplets. It is not easy to see in the above pictures, but believe me, the droplets are hiding in there.

And they will be sitting there for several days like a bunched up, wet towel in the corner of someone’s bathroom. If, after a few days, you dare to pick up that towel and give a sniff, you experience a similar flavor to that of the minisplit after it had been paused for a few hours or days. It is a death knell to indoor air quality (IAQ).

The good news is that this is an easy to solve problem. Rather than abruptly stopping the minisplit in cooling mode, switch it to low speed fan mode, and let it run for half a day or overnight. The fan keeps drawing air across the fins and will slowly dry them out.

It’s like taking your wet towel and hanging it up to dry. That towel definitely will smell a lot better – and so will your minisplit once you start it up again in cooling mode.

If you would also like to dry out the condensation collection pan at the bottom of the indoor unit, keep the minisplit in fan mode for a good day. This is definitely recommended at the end of the cooling season (end of summer).

And if you turn off cooling mode for a week or longer before starting it up again, you may want to consider cleaning the condensate drain line, as described in the previous post, just to be on the safe side.

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GreenBuilt Home Tour 2020

July is GreenBuilt Home Tour month in Chicago! This year the whole event shifted to a virtual format so that you can enjoy the tour from your favorite office chair, recliner or couch.

The tour stretches across four Wednesdays in July at 3:30 PM for a web series highlighting green homes throughout Illinois and NW Indiana. Each date during the 2020 GreenBuilt Home Series will focus on one of four themes:

  • July 8th – All-Electric Homes
  • July 15th – Deep Energy Retrofits
  • July 22nd – Passive House Showcase
  • July 29th – Wellness + High Performance Homes

Each web event will allow you to hear from the building team behind the featured green homes and learn about the latest sustainable home technologies in action.

And yes, we will be participating again with our deep energy retrofit on July 15th!

Learn more and register here!

Minisplit cooling startup

Cooling season has started. Our living space has been comfortable in terms of temperature and humidity since we turned off the heating mode on our minisplit back in March. Now it is time to bring the temperature and humidity down a notch so that we can sleep comfortably at night.

The last time the minisplit ran in cooling mode was about eight or nine months ago. Since that time, dust may have accumulated in the condensate collection pan. Once that dust mixes with the first condensate from the heat exchanger coils, it may cake up and block the drain line that is supposed to safely evacuate the water to the outside.

If that is the case, you will notice water droplets on the luvers and a water puddle on the floor under the minisplit.

It’s time to turn the minisplit off and clean that condensate drain line. Or, even better, as a routine maintenance item, preemptively clean the condensate drain line at the beginning of each cooling season.

To do so, find the discharge point of your drain line, which typically would be outside the building. Take a wet/dry shop vacuum with a narrow nozzle. Fit the nozzle over the drain line and proceed to evacuate any water, dust and crud that may have accumulated in the drain line since it last ran in cooling mode. Once the vacuum doesn’t pull any more water or crud out of the drain, start up the minisplit in cooling mode and monitor whether you get any more spillover from the condensate collection plan on the indoor unit. If you do, repeat the cleaning process. If you don’t, great job, and enjoy your cool building interior!

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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 end 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

U.S. Solar Radiation Resource Maps: Atlas of the Solar Radiation Data Manual for Flat-Plate
and Concentrating Collectors

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