Sunday, January 30, 2011

More Accurate Solar Cost Accounting

My previous post updating the results of our ten year struggle to reduce our carbon footprint mentioned that I was not satisfied with the cost accounting for our previous and current solar PV system. The percent reduction chart showed a 33% reduction in our carbon footprint between 2003 and 2004 due to the 2.5 kw solar PV system, and a per kilogram lifetime carbon reduction cost of $0.43 over the projected 30 year lifetime of the system (though we only used the system for 6 years). For the 7.05 kw system we are currently installing, the projected reduction is shown as only 0.05% and the lifetime per kilogram cost is shown as $2.10 over the projected 30 year lifetime of the system. The calculations were based on measured or projected reduction in electricity use from the year before to after the system was installed. These results made little sense to me, because the cost of solar PV has come down dramatically in the last 10 years, even though the subsidy has dropped even more.

The problem with this calculation is that the original system was installed along with other improvements, including removing the pool, replacing many of the lights with CFLs, and replacing the washer and dryer with more energy efficient models. The reduction due to these measures was not broken out. For the new system, as mentioned, we still are projected to draw around 2000 kwh/year from the grid, but the actual amount is slightly less than we draw now. That accounts for the 0.05% reduction between before and after the system is installed, but the lifetime reduction cost did not account for the fact that the new system will also be offseting the same electricity use as the old. 

Unfortunately, I don't have actual figures for the amount of electricity generated by the old solar PV system, since I did not write down the production amount recorded on the SonnyBoy inverter before it was decommissioned, and the PG&E bills (upon which most of my measurements are based) do not break out total usage from usage net of solar production. But based on the carbon calculator at, I calculated the expected energy production for the 2.5 kw system at 3780 kwh/year, which amounts to 1.12 metric tons / year of carbon using the carbon calculator at Here, again, there are some inaccuracies. The  calculator might be too optimistic, and the calculation is based on the current mix of energy sources in the California grid. Since the grid has become greener over the last 10 years, the actual amount of carbon eliminated could be more. The other measures we took in  2004 - removing  the pool, CFLs, and  appliances - then account for 1.193 metric tons of eliminated carbon, slightly more than the solar PV.

The effect of this change on the lifetime cost of our current PV system is then rather dramatic (and more likely to be correct) as the following graph shows:

This graph shows the cost of the 2004 system, $0.54/kilo,  as slightly greater than  the cost of the 2007 system, $0.46/kilo.  The cost of the other measures is around $0.34/kilo. For the other measures, I've estimated the lifetime as 30 years, even though the appliances and CFLs will have a shorter lifetime. The pool removal probably contributed the largest amount to the other measures, and its lifetime is essentially the lifetime of the house (actually probably more, it will be permanent) which is around 30 years.

Note that in the graph, the reduction in backup hot water carbon emissions is attributed to the "Reinsulate + on demand electric hw" rather than to the solar, even though the solar will be contributing the renewable power to offset the electricity used for backup hot water.  But apportioning the costs between  the two would be difficult, and since the backup hot water reduction would not be possible without the electric on-demand  heater, the breakdown seems fair.

Anyway, the projected results from our current system remodel are mostly theoretical. It will be interesting to see over the next couple years how the improvements we've made actually impact our carbon footprint. The Tigo energy maximizer we'll be installing on the solar PV system has included a real time measurement package that records the amount of power generated on a web site, so we should now have much more accurate data in an easy to access form. The gas usage data will unfortunately be not much changed than before,  since it will come from the PG&E bill, though we now have a SmartMeter for our gas meter which gives accurate, daily gas usage information on PG&E's web site.  Perhaps if I get ambitious, I might try to do some Ajax hacking to consolidate this information in one place so I can display real time information about our energy usage and carbon footprint.

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