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David Roodman's Microfinance Open Book Blog


I blogged a year ago about installing solar panels on my roof. The system passed its one-year activation anniversary on November 3. Here's the production history. The vertical axis is in watt-hours:

Roodman solar panel power production, first year

As you can see along the bottom, the system produced 7.54 megawatt-hours in its first revolution about the sun. It would have been closer to 7.60 if one panel had not failed in August, taking 2.5 months to get replaced. That is pretty close to the 7.80 MWh predicted by the National Renewable Energy Lab's PVWatts solar calculator. Production was below PVWatts predictions in the fall and spring because of rain and clouds. At the end of January, an ice storm shut the system down for a few days. (The revival was nonlinear: as panels became re-exposed, they warmed up, melting the remaining ice faster.) But from mid-May to mid-August---up until Hurricane Irene initiated this grey autumn---the weather was clear and daily production rarely dipped below 20 kWh. This made up for the shortfalls in the shoulder seasons.

You can browse the production data here.

Data on our electricity consumption are much sparser, partly because the electric utility, PEPCO, was literally unable to process data from an electrical meter going backwards, and issued some bills with made-up readings. (They have slowly gotten their act together and righted the wrongs.) For the year, we gave PEPCO 1.23 more MWh of electricity than we took, meaning that we consumed 7.54 -1.23 = 6.31 MWh on-site. The Pham-Roodman household is now a net power producer. Maybe I will install The Energy Detective to generate high-resolution consumption data and educate my family.

As I explained before, our system has no storage batteries. When we produce more than we consume, such as on sunny spring days, we push that energy into the neighborhood grid. When we consume more than we produce, such as at night and on the hottest afternoons, we pull from the grid.

This has essentially wiped out our $1,000/year electric bills. If I understand right, PEPCO will soon be buying our surpluses. (I'll spare you the details behind that uncertain future-tense statement.) And, as I also explained last time, early next year, we should get our first payment from PEPCO for the right to the first 8 MWh we will have produced towards its mandated solar production target. (The target will climb to 0.4% of PEPCO's power in 2020.)

One strange thing about being on the grid is that having 6.45 kW of panels on your roof doesn't change the moral implications of flipping a light switch. With or without panels, each extra kilowatt-hour we consume means that the conventional power system must generate that much more, presumably from a mix dominated by coal, gas, and nuclear. Next on our personal eco-agenda, therefore, are a few more panels for making electricity or perhaps hot water. We're also planning a light renovation of the second floor that will include insulation. And I hope to replace our 30-year-old air conditioner with one much more efficient. That should drive up our electricity surplus...I wonder how many miles a plug-in Prius could go on that.

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