Energy Performance - Plan and Reality
As I wrote earlier in this blog post, one of our key requirements for this home was to build an energy efficient house without a significant increase in building cost. We wanted this house to generate more energy (through solar panels) than what was needed to live in the house. Ideally, the generated excess energy should be enough to charge our electric car.
The energy efficiency of new homes in BC is guided by the BC Energy Step Code. The Step Code provides performance requirements for new construction with the goal that all new homes constructed from 2032 on are "net zero ready". The Step Code defines 5 steps and currently offers municipalities a choice of when they want to mandate what level of energy efficiency. Step 1 is slightly improved over the current minimum Building Code requirements, and Step 5 represents the highest energy efficiency - what they call "Net Zero Ready". This is roughly equivalent to the European Passivehouse standard. Starting in 2032, all new buildings are supposed to conform to Step 5.
My initial goal was to achieve Step 3 or Step 4 of this Code, and have a building which uses 20%-40% less energy than a standard BC Building Code conform new building. The main reason for this "low goal" was cost and knowledge of sub-trades: the higher you want to go on the scale, the more you have to invest. A house using less energy means more insulation, better air tightness (and knowledge how to achieve this) and overall higher material cost. Modeling and predicting energy efficiency contains a lot of variables. Some are resolved at the end of construction (at the air tightness test). The real result will only be known after 1-2 years of living in the house, by monitoring in detail your energy production and use.
Once the main construction was complete, our Energy Advisor performed an air tightness test and checked all energy relevant components of the home. This is part of the "official" evaluation of the house. The result made us very happy, as we found out that we achieved Step 5 of the Energy Step Code and were rewarded with an incentive from our municipality for voluntarily achieving this energy efficiency. According to our official EnerGuide Rating from National Resources Canada, our home is supposed to use less than 70% energy of the "Reference House" (a house which would be built to current BC Building Code standard).
Planning of a Netzero home requires the following two factors to balance out - and to estimate these values as accurately as possible during the design phase:
- Energy use of the home for heating, cooling, hot water and all lights and appliances.
- Energy generated by all sources (in our case, only the solar panels on the roof).
If the energy demand of your house is much higher than what the solar system can generate, you will not achieve Netzero. On the other hand, if generated energy is much higher than what's needed, you are wasting money for building an oversized energy generation system.
In our case, I started with the design of the house and calculated the energy demand using the Hot2000 software provided by National Resources Canada. The final energy demand numbers, after tweeking the design many times were:
- Heating, cooling and hot water: 4,680 KWh
- Lights and appliances: 7,120 KWh
- Electric vehicle charging for 18,000 Km/year: 2,500 KWh
These numbers add up to 14,300 KWh, which was the basis to start our solar system design. Our south facing roof pane could accomodate a maximum of 35 standard sized solar panels. At the time we were shopping around for solar panels, the "sweet spot" between performance and price were panels with 390 Watt, which would have resulted in a total generation capacity of 13.65 KW. A commonly used solar panel prediction calculator is pvwatts from the US Department of Energy. For our location and roof angle, these solar panels would generate about 15,500 KWh energy per year. This is in ideal conditions without any tree shading. While we have hardly any tree shading on the roof in the middle of summer, we get quite a bit of shade in the winter. So this number came really close to our what-we-need calculation.
After many discussions, we decided to scale our system down a little bit, start with 28 solar panels (10.9 KW) which would generate a maximum of about 11,400 KWh per year. We designed the system in a way that it would be very easy to add another 7 panels without changing any major components. The reason was that the estimate use numbers have a lot of variables, and we were hoping that we could use even less - and didn't want to spend the money for an over-sized solar system.
Real World Monitoring
We moved into the house in February 2021 and from beginning of March 2021, I started monitoring our energy use in great detail using a system called iotawatt. This system clamps onto different circuits in the breaker panel and can accurately monitor the energy use of up to 14 different circuits. The most interesting circuits for me were:
- solar generation
- import/export to/from the grid
- energy use of the heatpump
- energy use of the electric vehicle charge
At the time of writing this article (last update March 2022), I have 12 months of monitoring data. The graph to the right shows the results of our actual energy use versus the prediction on a monthly basis. The graph shows that our monthly energy use (excluding the EV) is much lower (41% on average) than the energy use predicted by the Hot2000 sofware.
Some possible reasons for this difference is the following:
- We didn't install any cooling system before we moved into the house, as we thought such a well insulated house won't heat up very much in the summer. But this was very wrong and the super heat wave in 2021 made us change our mind on that. We are right now installing 2 fan-coil-units, which will be mounted to the wall in the living room and in the master bedroom. These units can heat or cool using water conditioned by our heatpump.
- We love having a fire in our wood stove quite often. In mid of November, with outside temperatures as low as 0-5 Celsius at night, we had the wood stove burning for 2-3 hours in the evening - and that provided plenty of heat. We don't even turn on the in-floor heating at all.
- In December the temperature plummeted some days to -10 Celsius, raising to only -2 during the day. Even in these times we didn't have to turn on our in-floor heating system. We burned 6-8 pieces of wood in the stove every afternoon/evening. That brought the temperature in the livingroom up to 25 degrees and more. During the night the temperature slowly fell to about 21/22 Celsius in the morning. I think one reason that the house "stores" so much heat is our thermal mass: The timberframe structure in the livingroom is made of many solid timbers, which are all inside of the SIP panel walls without any thermal breaks. In addition the 2" concrete floor slab and 1/2" hardwood floor provide thermal mass as well. All this mass heats up while the stove is running, and then slowly releases the stored heat during the night.
- The in-floor heating in the basement was turned off in the winter as well. The ICF walls insulate very well, and even at -10 Celsius outside the temperature in the basement was still 14 to 15 Celsius. As we currently don't spend much time in the basement except doing some work in my workshop, these temperatures are just fine.
And how did the solar panels perform in our first summer? The graph on the left side shows monthly solar generation versus the predicted generation by the pvwatts calculator. We had an unusually hot and sunny summer, and therefore, our solar generation was higher than the prediction. Since September, our generation is lower than the prediction. This, I was expecting, as the prediction doesn't know about the shading of our panels when the sun is lower. We also had a record rainy October and November, and snow for many days in December.
Our "BC Hydro anniversary date" is March 1st. This is the date when BC Hydro will calculate how much energy we have used or fed back into the grid for 1 year. If our balance is negative, BC Hydro will pay us about $0.04 per KWh back. For our first year we have generated about 1800 KWh more then what we have used (I haven't got the March invoice yet).
For our second year the energy use is expected to go up, as we now have the ability to use our Fan Coil Units for cooling the house in the summer.