The assumptions I'm making are the following:
- The reinsulation reduces gas usage by 30% below current average (around 350 therms per year for heating). Several years ago, I constructed a spreadsheet model of the house and calculated what the reduction in energy use would be for changing the insulation to closed cell foam. It came out to about 30%. This ignored the floor, which we will have done, and included the kitchen and master suite, which we won't, though the kitchen ceiling is now at R-30 using fiberglass batt.
- The on-demand electric hot water heater provides backup hot water heating in winter so that total hot water gas reduction including solar thermal and backup on-demand electric is 20%. The electric hot water heater will be offset from the solar PV.
- The heat recovery ventilation system is around 800 kwh per year (running in winter only) and is offset by solar PV.
- We swap out our current, 6 year old polycrystalline 165 watt per panel 18 module solar PV system for a new system consisting of 20 Sunpower 315 watt per panel modules.
- The Nissan Leaf which we are hoping to purchase next year uses 0.25 kilowatt-hours/mile and we drive it around 8000 miles per year, which is about what we usually drive our "commuting car" (as opposed to our long distance car which is more like 10,0000 miles per year).
The chart shows an almost 90% reduction in carbon emissions from the estimated pre-2002 baseline. This includes extra capacity in the new solar PV (around 1131 kg/yr) which the house will not use and will therefore offset some of the natural gas emissions from heating.
The contributions from each component (electricity, gas, and cars) to the total household footprint can be seen in this graph:
Notice that the electricity footprint is now negative because the house is generating more carbon-free energy that it consumes. This offsets the natural gas and gasoline usage. One car will be completely carbon free, since it will run off electricity generated by the sun, but our current plug-in Prius will still require gasoline.
Finally, the bottom line: the cost. The following graph shows the cost per kg carbon eliminated:
The cost takes into account the federal and state subsidy for the Nissan Leaf, which brings the price below $20,000.
The efficiency measures here are the most expensive. Reinsulating the house requires taking off the drywall, blowing in closed cell foam insulation and otherwise sealing up any air leaks, putting back the drywall, and installing a ventilation system to ensure that the house gets enough fresh air in the winter when the windows are all closed. This is expensive and disruptive work, but, unfortunately, there is no way around it if we want to reduce the energy use due to heating.
So by the time we finish next year, we should have achieved almost 90% reduction beyond the estimated pre-2002 baseline. Environmental groups are calling for an 80% reduction by 2050, and this is the aspirational target for governments, though, the steps they've been taking are laughable so far. We will have beat the deadline by almost 40 years. The house and cars will not be total net zero energy, but they will be as close as we can get them, given:
- We need some way to get out of town with the car, and gasoline or diesel are the only realistic fuel choices right now. We could do an under the table conversion of the plug-in Prius to E85, but unfortunately there is very little E85 available in California.
- We have little choice with the heating but to continue using natural gas. The complexity of installing a geothermal heat pump doesn't match the architectural configuration of this house. The house simply does not have enough room for the complex machinery. And the cost of a geothermal heat pump is far and away above the value it provides (something like $700 per kg of carbon eliminated). The amount of money we'll have put into the house for the upgrades even without the geothermal heat pump probably exceeds the resale value, not that we are thinking of selling it of course. Green remodeling does not yet pay in terms of resale value, at least according to a friend who is in real estate. As for cooking, continuing to use a gas stove is a choice: we both prefer gas to electric, and the existing gas stove is still in pretty good condition so we don't see any point in replacing it.
Now, I wish the contractors would finally get started on our system remodel! We are still waiting on a second bid for the drywall removal.