Update On Projected Carbon Footprint

Since my last post on the estimated carbon footprint after our current system remodel and projected purchase of a Nissan Leaf, a few things have changed:

• The design for the solar PV system was finalized,
• Figures for yearly comparative gas usage with and without solar thermal hot water are available,
• A more accurate cost estimate is possible.

So this post provides an update on the projected carbon footprint and cost per kilo carbon eliminated for our direct carbon emissions (except for flying) after the current system remodel is complete and after the Nissan Leaf.


Perhaps the biggest impact on our projected carbon footprint - upward unfortunately - came from finalizing the solar PV design. We now anticipate that the solar PV system will generate around 7,024 kwh per year. This is around 2000 kwh per year less than our anticipated utilization, considering the new HRV system, the new electric hot water backup, 8000 mi/year on the Nissan Leaf, and our existing utilization, including the plug-in Prius. Interestingly enough, 2000 kwh per year is almost exactly where we fall short of net zero today, though we will be offsetting considerably more carbon sources through  the new system than we currently do. Since our solar resource will be maxed out with the new system,  we have no more room to expand, short of new technology that generates more power per square foot.

On the positive side, the impact of solar thermal hot water on our hot water usage was considerably better than anticipated. With the current gas-fired tank as a backup, the solar thermal hot water system eliminated 23% of our yearly gas usage, rather than 15% as previously assumed. With the installation of the new electric on-demand hot water backup, the additional amount of gas consumed to heat water during the winter will be eliminated, reducing total gas consumption by 31%.

The assumptions in the projections are about the same as before. The result modifies our per cent carbon reduction graph as follows (please click on figures to get full picture):

Our projected carbon footprint reduction has decreased from 90% to 78%. This is primarily due to the reduction in expected solar power from the PV system. Previously, we had expected to completely eliminate our grid draw,  and have a a 15% surplus. Now, we expect to draw around 2000 kwh per year from the grid.

The contributions from each component to our annual carbon footprint can be seen in this graph:

Since we are projected to use all the solar PV we generate, we no longer have a negative footprint due to generating more solar PV than we use. The expected size of our carbon footprint is around 2.8 metric tons per year. This is over twice the size of the carbon footprint (around 1 metric ton) that this Swedish family is shooting for. In our case the deciding factor is our plug-in Prius, which gets around 80 mpg but still generates around 1.1 metric tons of carbon per year. The Swedish family has a plug-in Volvo but no second car, since in Sweden, the public transport system is adequate for transportation around town if you don't need to haul anything. In addition, the Swedish electrical grid is 52% carbon free, due to hydropower and nuclear energy, whereas in California, it is considerably less (but better than many other states which generate electricity using coal), maybe around 30% including large hydro and nuclear.

Finally, the cost per kg carbon eliminated over the life cycle of each technology employed:

I've included more technologies here than in the original post, in fact, every technology we've employed since we moved into the house in 2003. Note, however, that I still do not have an accurate partitioning for how much the reduction in 2003 was due to the original solar PV system and how much was due to eliminating the pool. I'm working on estimating that. For this reason, the original solar PV system looks considerably cheaper than the current system, but it is probably about the same price or somewhat cheaper, considering that the cost per watt has decreased and we are installing more capacity, and that the grid is getting greener. Another point to note is that the price of the 200 amp upgrade has been  distributed between the Nissan Leaf and the on-demand electric hot water heater, both of which require 200 amp service. I've also included the price of carbon offsets to make up for the GHG emissions during closed cell foam installation. Otherwise, the assumptions are the same as in the previous post.

So we're projected to fall somewhat shorter of our net zero goal than originally planned, but we are projected to achieve 78% carbon reduction, from the 2001 baseline. This is just 2% short of the general policy goal expressed by many analysts of reducing carbon by 80%. Time will tell whether we can do better. Through efficiency, like bicycling in summer instead of taking the car to work, perhaps we can reduce even more, and some of the technologies, like the closed cell foam insulation and HRV, may turn out to be more effective than originally thought. I'll be monitoring our gas and electricity usage to see how much reduction actually occurs.