If you live in California and worry about your home’s carbon footprint, chances are that getting rid of all your gas appliances has the biggest impact. Then, maybe worry about rooftop solar if you want to save money.
In late November 2021 we moved to our new home in the North Bay. For various reasons we ended up with a fairly large house, with over 3500 sqft of living space. On the bright side, the home is fairly new, from 2005.
Our new digs came with all original appliances. So we are currently rocking two 17 year old gas furnaces, paired with equally old air conditioners and an equally old gas water heater. Homes built in 2005 have reasonably good insulation. — Unfortunately the under floor insulation needed to be removed and replaced, due to it having been improperly installed, so we spent the first winter without underfloor insulation. Luckily heat rises, so not having underfloor insulation is not as bad as it sounds.
During the winter months it became painfully obvious that we were burning a lot of natural gas, in a grossly oversized furnace, to keep the upstairs warm. The downstairs we barely heated due to ducting problems and the furnace being deafeningly loud.
In late spring of 2022 I tried to disaggreate our natural gas consumption by usage, estimating year-around numbers based on actual data for the months of December 2021 till June 2022, extrapolating out using degree days for our location for heating and cooling estimates.
It appears that we use around 900 therms (or about 22 MWh) of energy to heat our home over the course of the year. The natural gas water heater adds another 145 therms (or about 5.4 MWh). Given the amount of clothes washing and drying we do, the natural gas clothes dryer is probably around 32 therms or around 0.95 MWh. Finally, since my husband is an avid cook, our gas cooktop seems to add about 24 therms, or about 0.7 MWh per year.
Given that burning one therm of natural gas releases 11.7 lbs of CO2 into the atmosphere (and given that there are about 2205 lbs in a metric ton), we get the following figures for energy usage and CO2 emissions caused by the natural gas appliances in our home:
|CO2 (metric tons)
I also attempted to disaggregate our current electricity consumption in a similar way. Using data from the California Independent System Operator for 2020, the average kWh of electricity in California causes the emission of about 0.495 lbs of CO2. While we are subscribed to the “100% renewable” tier of our local CCA, let’s take this number, since one cannot track the flow of electrons directly:
|CO2 (metric tons)
Note that the EV charging number is an “allowance”, covering about 5000 miles for an electric car with an average efficiency of 3 miles per kWh, and not yet based on actual numbers. — Electricity consumption for heating is caused by running the pretty powerful blower motor that moves hot air through the ducts.
Comparing emissions from electricity use and the burning of natural gas in our home shows an interesting result:
Only about 25% of CO2 emissions caused by our household come from the consumption of electricity from the grid, while 75% can be traced to the on-site burning of natural gas.
Here is an aggregate view:
Fix The Problem: Stop Burning Gas!
One problem with natural gas appliances is the concept of locked in emissions: Burning one therm of natural gas creates 11.7 lbs of CO2. This number will not change over the entire 20 year lifespan of a new gas appliance. On the other hand, the amount of CO2 emitted per kWh from the power grid has been trending downwards for several years, and will continue to do so. — If you buy any electric appliance now, as the grid gets cleaner, so will the appliance.
Eliminating emissions from natural gas comes down to two things: First, be more efficient, since the most environmentally friendly kWh is the one that you don’t need to produce. — Second, go electric, to avoid the lock-in.
It turns out that these two things go hand in hand, especially for producing building heat, which is by far the largest cause for CO2 emissions in our home. — Being more efficient firstly means improving the building envelope:
- Air-sealing the building: Avoid that hot air simply escapes through various ceiling and wall penetrations.
- Improving the attic insulation. — Add enough blow-in insulation to the already existing one to ensure that the joists are covered in insulation. Wood conducts heat.
- Restore the under-floor insulation.
- Install honeycomb shades, to reduce overnight heat losses through windows and to be able to shade the building during hot and sunny days.
Secondly, getting rid of oversized forced air/natural gas heating and replacing the existing heating system that is approaching the end of its life with a ductless mini-split heat pump allows for a dramatic reduction in the electric energy needed to heat a building.
Here is how I estimate for all of these improvements to stack up. Note that the “15% Ducting Losses” figure is the actual amount of heat energy currently delivered to the conditioned space over the course of a year, after taking the current system’s losses into account:
Stacking all these efficiency gains, we should be able to reduce the primary energy need to heat the building from the current 22.5MWh to about 2.8MWh, a reduction by a factor of eight!
Replacing a gas water heater with a heat pump water heater results in a similar reduction of primary energy usage:
Converting from a 17 year old gas water heater to a heat pump water heater should drop the energy usage from about 5.4 MWh to around 1.1 MWh, or a reduction by almost a factor of 5! — Bonus points for cooling down a South-facing garage that gets quite warm…
Does it Compute?
How does converting your heating and water heating system (and replacing your gas clothes dryer with a new electric one and a washer) stack up? — Actually, quite well, even with today’s power grid emissions…
Estimates show that the total CO2 footprint caused by the energy consumption in our household should go from about 7.8 metric tons per year to about 2.9, or a reduction by over 60%: Converting to all-electric reduces the household CO2 emissions by about 4.9 metric tons, while rooftop solar alone only has an offset potential of 1.95 tons.
It is interesting to note that the CO2 emissions reductions achievable by converting to an all electric building are more than twice as high as what could be achieved by just adding rooftop solar.
As a side note, CO2 emissions from clothes drying go up when switching from gas to electric, since we went with a non heat-pump drier. — But there are other ways to offset this:
What If You Add Solar?
Net-Zero Is a Possibility
Doing the all-electric conversion should bump the total electricity consumption of our home to about 11.5 MWh per year. It appears that the yield from a solar array is about 1.5MWh per year per 1kW DC in our region. Therefore, if you wanted to fully offset the home’s energy consumption you would need an 8kW solar array, or about 20 (residential) panels. — While this is on the larger end for a residential system (but so is the squarefootage of the house…), this is achievable.
Here is a sample layout from OpenSolar on the building in question using 21 400W panels , estimated to produce about 13.5 MWh anually. More than enough to offset the total energy consumption of the home, with room to grow.
What About Financials?
Gas is Cheap, Yet You Eke Out Savings
One therm of gas currently costs between USD 2.165 and 2.633 for residential customers, depending on what tier your consumption is in. However, there is quite a lot of volatility in natural gas prices — Unfortunately this means that the per kWh price of energy from natural gas is fairly low, estimating about 8.2 cents per kWh, averaging current gas costs and using the fact that there are 29.3 kWh in a therm.
Buying 1100 therms of natural gas will cost around USD 2600 at this rate. — Eliminating natural gas and going all-electric causes an increase of about 3.2 MWh in electricity consumption. — PG&E’s standard residential tariff is both a tiered and a time of use system, resulting in energy prices between 34 and 49 cents per kWh.
Tricks can be played by taking advantage of time of use and switching to a different tariff, but for a back of the envelope calculation, 3.2MWh of electricty will cost around USD 1300 when bought from the grid.
Despite the much higher cost per kWh for electricity when compared to gas, the all electric conversion will still result in around USD 1.300 of annual savings.
There are more complex questions to be answered, such as the higher price of the equipment that is only partially offset by incentives, but at least we are not making things worse from a utility-bill point of view.
Solar For The Savings!
Throwing solar into the mix makes the financials more enticing:
- Buy 1100 therms of natural gas for USD 2600
- Buy 8.2 MWh of electricity for USD 3300
- Total Utility Bill: USD 5900
All/Mostly Electric – No Solar
- Buy 24 therms of natural gas for USD 60
- Buy 11.5 MWh of electricity for USD 4600
- Total Utility Bill: USD 4660
All Electric + Solar
- Get 11.5MWh per year from solar, for USD 0.1 per kWh.
- Total “Utility” Bill: USD 1150
Future net-metering shenegans aside, the all electric + solar option saves about USD 4750 a year in utility bills in the first year.
As a back of the envelope calculation, assuming a 2% cost of energy increase per year over a 20 year lifespan, the all electric conversion will save about 115000 USD, easily paying for both the all electric conversion of the building, plus the solar array.