To answer this question, let’s look at the data for the solar year 2020 when our electrical use included space conditioning.
Our annual use totaled 13,428 kWh that year, while our annual production amounted to 11,390 kWh. The solar array produced enough electricity to cover 85% of our annual consumption.
To reach net-zero, we would need to be at 100% or above. So we are around 15% short of net zero and had some more homework ahead of us.
The moving goal post…
When we embarked on the project in 2009, all-electric homes were not a thing yet, heat pumps were hard to find, and solar arrays were uncommon.
At the time my focus was on using a solar hot water system to heat the building and for domestic hot water, and a photovoltaic array to cover our electrical needs. But I always found myself on thin ice when attempting to cover space heating and domestic hot water with a solar hot water system alone. In other words, getting away without a natural gas connection seemed impossible, which made the net zero goal hard to reach.
I pivoted my focus into significantly reducing the overall energy load of our building. If I had to use natural gas as an energy source, I wanted to use as little as possible. That put us on the path of our deep energy retrofit.
And it paid off.
Interim results in 2012 showed that our improvements reduced our electrical consumption by an estimated 57% and preliminary results from 2016 showed that we reduced our heating needs by an estimated 80%.
A lot has happened since 2009. Green building technologies that once were only known from excotic places like Europe or Asia suddenly made an appearance in the U.S. market, such as heat pumps. And with that, my focus on solar hot water fell away, because heat pumps emerged as a more economic option that I still could use, even if the sun was not shining.
Reducing the general electrical load of a building also has become easier since 2009 with increasingly efficient energy star appliances, LED lighting, etc.
What is standing in the way of net zero?
Yet we are not net zero, to my chagrin. What is standing in the way are two key factors:
- That we still rely on natural gas for cooking, domestic hot water, and occasional heating. And we still have a gas dryer.
- That our solar array is not large enough to cover 100% of our energy needs should we go all electric.
The second point should be reasonably easy to solve. Because we have reduced the energy load of our building significantly, we have enough room to expand our solar array to cover 100% of our energy use. And we plan on doing so – eventually – once technology catches up.
Regarding the first point – our natural gas connection – it helps to know how much natural gas goes towards what source in our building.
Analyzing our utility bills over the past seven years revealed that about 1,000 therms (77%) went towards space heating, with only 300 therms (23%) going towards domestic hot water, ranges, and the dryer.
The 300 therms seemed to be easy to solve. We can replace our gas dryer with a condensing dryer. The gas ranges can be replaced with induction stoves. And the heat pump water heater technologies have improved to the point where we could say goodbye to a gas fired water heater too.
As for the 1,000 therms going into space heating, one could argue that we solved that problem already with the addition of our minisplits. We used them to heat our building during the solar year 2020, and it worked.
But there is a problem: we drank the kool aid.
We originally, and occasionally still rely on our hydronic heating system, powered by a high efficiency boiler. The steel baseboard radiators and radiant floors deliver a comfort during the heating season that is unmatched.
The good news is that we potentially could replace our boiler with an air-to-water heat pump that used CO2 (R744) as a refrigerant. These units are slowly making an appearance in the U.S. market and are able to deliver 130F water even at very low exterior temperatures. 130F would be a suitable temperature for our hydronic heating system and domestic hot water.
Not only that, but an air-to–water heat pump would be two to three times more efficient than our high efficiency boiler. In other words, it would only require half or one third of the energy input to produce the equivalent of 1,000 therms heating output.
I am hopeful to eventually replace our boiler with an air-to-water heat pump and solve the 1,000 therms that were needed for space heating. We subsequently could cut our natural gas supply to the building, and yet still enjoy the comfort of our hydronic heating system.
That must be expensive!
In the big picture, what is the cost of doing nothing?
And on a project basis, if it is expensive depends on one’s mindset.
Most of our system decisions, such as the heating system, were not solely based on the economics of the day, or “what is the cheapest system I can get.” We were comfortable investing in systems with a longer payback period as long as they came with:
- a high level of energy efficiency,
- some level of resiliency and longevity,
- improved indoor comfort and health without an energy penalty, and
- systems that were somewhat future-proof so that they could adapt to technology upgrades.
This required a lot of research and careful planning at the onset of our project. And it required a lot of luck, as we were gazing into the future trying to predict the path green building technologies would take.
And in practical terms?
It appears that our utility room layout could accommodate the switch from boiler to air-to-water heat pump without revamping the whole hydronic heating or domestic hot water layout.
And because we were mindful when we installed an all new electrical metallic tubing (EMT) based electrical system, providing 240V for the induction stoves and potentially the condensing dryer should just be a matter of simple rewiring.
The one item that wasn’t even remotely on the radar in 2009, and that I still have to wrap my head around, is how best to integrate and accommodate EV charging stations.
In summary: We are not net-zero yet. We are fairly close, and we know the path that will take us there.