Over the past couple of months, I’ve been tying up loose ends on our project. In addition to upping my accounting skills by sorting out the GST and capital gains tax owing for the sale of the other half, I (finally) finished up our Passive House certification documentation, which was tedious if not difficult. As I await what will most surely be a request for more documentation from the certifier, I thought I’d have a look at how our actual energy consumption compares to both the modeled consumption and our project goals.
Here’s our latest 12 months of BC Hydro electricity bills (May 2020 to April 2021, for our “half” of the duplex, which includes our place and the small rental suite):
As a crude estimate of heating energy, we can take the lowest monthly consumption (in this case, last July or this April, at about 560 kWh) and assume that is the base – or non-heating – consumption for each month, and everything above that is for heating.
Here’s how this compares to the per unit area consumption in the model:
| Measured | Modeled in PHPP | |
|---|---|---|
| Annual total site energy, kWh/m2 | 47 | 31.9 |
| Annual total heating energy, kWh/m2 | 12.8 | 14.5 |
So we’re at 12% below the predicted heating energy consumption, but 47% higher than the predicted total site energy. I have a few thoughts on this:
- The Passive House model notoriously underestimates base load consumption, partly because the system was developed by Germans who use less energy than North Americans. Not that aspiring to be more like our German friends by taking two minute showers and turning off the water while shampooing isn’t worthwhile, but even 47 kWh/m2 per year is very low when you consider that the North American average is more like 220. Even the German average is over 100.
- May 2020 looks anomalous. My notes tell me that last spring we had the upstairs in-floor heating in the bathroom set a bit too high (it is an electric resistance mat under the tiles).
- We also had a tenant in the suite through August who controlled his own electric resistance wall heater and tended to prefer relatively warm temperatures. I also note that we have not had a tenant in the suite since September, so these two considerations might cancel each other out, but we’re likely to see some continued variation as we rent out the suite.
- The model does not include electricity consumed in Matt’s workshop – another reason why some sub-metering would be useful.
- Working From Home and a bit of Schooling From Home. That would be me, for most of the last year, Matt for half the year, and our daughter for a few months. More lights on throughout the day, plus one or two computers and various power tools in use likely bumped up our total.
Even given these variations from the model, I think these results are amazing. They show that our enclosure first approach has achieved its intent, i.e. to dramatically decrease the heating demand of our home. I also feel very confident about our choices to invest in a very high performing heat recovery ventilator and to spend a bit more for a very efficient CO2 heat pump hot water heater.
Now let’s have a look back to our original project goals as they relate to energy:
- To follow passive design principles with the potential for net zero energy consumption and zero operating carbon emissions
- To build three units of housing that consume less energy than the original single family house
Given that BC’s electricity grid is mostly clean, we are allowed to claim zero operating carbon by virtue of being all-electric. In truth, there are still carbon emissions associated with electricity generation in the province – most significantly when we import from carbon intensive sources – but it’s still dramatically lower than using natural gas or other combustion fuels on-site.
Regarding the original house’s energy consumption, recall that our pre-project consumption was ~123 kWh/m2, which is 2.6 times higher than where we’re at now on a per unit area basis. And that was in a home that was unacceptably heated to anything close to today’s comfort standards. Our new neighbours have only been in the other half of the duplex since October and we’ll have to wait until they’ve been in for at least a year to get a total for the whole site. Beyond this, we will be looking to add solar PV and see how close we can get to net zero energy consumption.
Was there a sense of sacrifice to achieve this level of energy consumption? That’s the beauty of building like this. We have way less energy consumption and better comfort and air quality. Yes, I do enjoy finding ways to reduce waste of all kinds. Yes, I did decide that having very warm feet when walking into the upstairs bathroom was not worth the electricity cost and dialed that thermostat back. But, to the horror of my German friends and the water starved nations of the world, I still take long hot showers. What can I say? It’s where I do my best thinking and it’s a tough habit to break. All in all, we feel pretty spoiled.
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