After 2 years of living in our cold-climate Passive House, how is its real performance measuring up to the energy model used during the building's design phase?
An energy model is a digital replica of a building that simulates heat gains and losses based on the building's characteristics and climate. It allows us to optimize the design by testing out different options and quantifying their impact on energy consumption. As with any tool, it is important to understand the energy model's potential and its limitations. As the saying goes, "All models are wrong, but some are useful." An energy model cannot perfectly predict energy use because there are too many variables at play, including the behaviour of the humans who occupy buildings, but it is a powerful tool for making informed decisions during design.
Let's begin by defining some metrics and describing how data was collected, then we will compare the house's real-world energy consumption with the model.
Performance Metrics
Thermal Energy Demand Intensity (TEDI) is the annual energy demand for space heating. It represents the heating required to balance the energy gains and losses to maintain a stable interior temperature. TEDI is expressed in kWh of heating demand per m2 of floor area, making it easy to compare the performance of different buildings. Heating accounts for the lion's share of energy consumption, which is why it gets its own metric.
Statistics from Natural Resources Canada's Energy Data Use Handbook (see "References" below)
The average TEDI for residential buildings in Canada is 111 kWh/m2 per year.
The international Passive House Standard limits TEDI to 15 kWh/m2 per year.
TEDI is also a good indicator of a building's ability to maintain liveable temperatures in the event of an extended power outage, which is referred to as its thermal resilience. This is an important consideration in the face of increasingly frequent extreme weather events due to climate change.
Total Energy Use Intensity (TEUI) is the sum of all energy used in the building, divided by the floor area (once again, to simplify comparisons between different buildings). For the pie charts above, the TEUI would be the sum of all of the slices of the pie, in kWh/m2.
The average TEDI for residential buildings in Canada is 181 kWh/m2 per year.
For this project, the PHPP* energy model estimated TEUI to be 65 kWh/m2 per year if we used a combination of the wood stove + direct electricity for heating, or 49 kWh/m2 if we relied only on the electric baseboards for heating.
*PHPP is the Passive House Planning Package, the energy modelling tool used for PHI Passive House certification.
What is the difference between TEUI, Primary Energy (PE) and Primary Energy Renewable (PER)?
Data Collection
Our house is fully electric, with the exception of a small wood stove that is rarely used. Once in a while we'll light a fire during a festive dinner or a power outage, but this is infrequent enough that I think it's reasonable to exclude the wood stove from our calculations to keep things simple. For PHI certification, our certifier asked us to model the space heating as 50% wood stove + 50% electric baseboards, because this is more conservative due to the wood stove being less efficient. Since our real use is almost 100% electric, I created a duplicate energy model that is fully electric for an apples-to-apples comparison with our real consumption.
TEDI data
Collecting data on our energy consumption for heating was easy thanks to smart thermostats from Sinope. We also have a Sinope "smart switch" for our hot water tank, which makes it possible to measure its energy consumption.
We are participating in Hydro-Quebec's dynamic rate program which means that electricity is cheaper than the typical base rate in winter, except during peak demand events (colder weather), when the rate increases to encourage people to consume less and put less strain on the grid. An added perk of these "smart" thermostats and switches is that they can automatically turn off the heat during peak pricing events. With a Passivhaus and automated thermostats, we don't even notice the peak events, even when it's -31C outside and the heat has been off for 2.5 hours as shown in the image below.
A great example of our climate extremes: -31C in winter and +34C in summer!
TEUI data
Normally, it is possible to add up your utility bills, divide by floor area and, hey presto, you've got a real-world TEUI value! In our case, things were more complicated because we live on a farm, so there are several non-residential energy uses that I had to subtract from our utility bills to try to isolate the house's energy consumption from the rest, namely:
Charging the electric car;
The barn & workshop, which are on the same utility bill as the house;
The water trough de-icer, a heating element that ensures that the horses and sheep have a source of drinking water all winter;
The block heater for our old tractor, a soviet-era behemoth that is used to move 500lb hay bales once a week, and for the occasional odd job.
Detailed Methodology for Estimating Non-Standard Consumptio
Non-house-related energy consumption: the EV, the barn, the trough de-icer and Bella the Tractor's block heater.
Reality vs Model
TEDI comparison
When comparing the annual energy consumption for space heating, the real measured performance tracks quite closely to the all-electric PHPP model. There is a slight offset, which is unsurprising since weather varies from one year to the next, but we're within 3 kWh/m2 of the modelled result. For comparison, I've included the TEDI from the "code minimum" version of our house in the alternate version of PHPP that I created for my comparison of operational and embodied carbon. This result should be taken with a grain of salt because PHPP is calibrated for high-performance projects more than conventional construction, nevertheless it is clear that the measured results are much, much closer to the "as-built" PHPP.
Things get even more interesting when we look at monthly energy consumption for space heating (below). Our heating season is three months shorter than the "code minimum" model, which corresponds with anecdotal evidence from when we lived in our old leaky house. In the passive house, we only need heating from November to March, which is an unusually short season for our climate.
The monthly consumption between 2022 and 2023 matches up quite closely, with the biggest discrepancy occurring in November 2023. It has been an unusually mild winter, but it feels like we haven't seen the sun in a long while. When the sun is out, it provides all of the heat we need, even when it is very cold out (-20C or colder), but in the absence of sun, the baseboards run more frequently.
Bay, the small elderly horse, demonstrating how grey, foggy and mild it has been this winter and regretting having grown such a woolly winter coat.
TEUI Comparison
The overall annual energy consumption also tracks quite closely to the modelled value, as shown below.
I was able to break the TEUI down into three main categories:
Space heating, with real consumption data from the "smart" thermostats;
Domestic hot water, with real consumption data from a "smart" plug;
"Other" for everything else (TEUI - space heating - domestic hot water = "other").
The "other" category is where we observe the greatest discrepancy between the real and modelled consumption. We're still trying to figure out why this might be the case, and have the following (unproven) hypotheses for where that energy might be going:
The ERV preheater;
The pump for the well that provides us with drinking water;
The pump for the septic leach field.
The higher-than-anticipated "other" loads are partially compensated by a reduction in energy use for domestic hot water, so the overall energy consumption remains is close to the modelled estimate.
Conclusion
Although there are slight differences between the modelled and measured consumption, with variations from year to year depending on weather patterns, overall the house is performing much as expected. The thermal resilience has been the most surprising aspect of the design: even though we knew what to expect in theory, it's still extraordinary to be in a cozy warm house where the heat doesn't need to turn on when it's bitterly cold outside, as long as the sun is shining.x
The cats are 100% in favour of this sunbeam-powered heating system, too!
Acknowledgements
Many thanks to my partner in life and in energy nerd sleuthing, Jeff Turner, who helped me hunt for unmeasured loads and develop reasonable estimates for the weird farm loads. He also graciously provided editing feedback on the article.
References
The energy consumption statistics for Canadian buildings come from Natural Resources Canada's Energy Data Use Handbook. I used data from 2020 since it was the most recently available at the time of publication.