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:

About Marcus de la fleur

Marcus is a Registered Landscape Architect with a horticultural degree from the School of Horticulture at the Royal Botanic Gardens, Kew, and a Masters in Landscape Architecture from the University of Sheffield, UK. He developed a landscape based sustainable pilot project at 168 Elm Ave. in 2002, and has expanded his skill set to building science. Starting in 2009, Marcus applied the newly acquired expertise to the deep energy retrofit of his 100+ year old home in Chicago.

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