One year ago today NSTAR installed a commercial meter here at my request so I could track energy delivered to my house and energy exported to the grid, in addition to knowing what the overall net energy was. When the meter was installed, it read 000,000 kWh. On a meter without a grid-tied power source, this number would only go up. Here, it can go either way, depending on whether we use more than the PV system produces, or not.
After one year, the meter reads 97,409. Either we've used a hell of a lot of energy, or we've sent more energy to the grid than was consumed on site. I think it's the latter. The PV system didn't go operational until the afternoon of June 9th, so this net export of 2,591 kWh includes about two weeks where the energy flow was only incoming.
- 3,813 kWh consumed on site
- 2,752 kWh imported
- 5,343 kWh generated and exported
- 6,403 kWh generated
- 1,060 kWh generated and consumed on site
This year's usage can be compared with program benchmarks such as Passivehouse (PH) and Thousand Home Challenge. My allotment of site energy usage according to the Thousand Home Challenge (see post Finding the Right Target, Part 2) is 5,375 kWh, so we used 71% of our allotment. Of course, it has been a very warm winter, so it's not quite as rosy as that looks. But we've met the THC quite handily.
The most meaningful Passivehouse criterion is Primary Energy (PE, see post Primary Energy). Primary energy is the energy consumed to get the energy to the site as well as the site energy. For electrical grids it's about three times higher than the site energy (the Passivehouse software used 2.7 as the primary energy factor in Germany - various sources say it's over 3 here in the US). The PH PE criterion is in kWh per square meter of Treated Floor Area (TFA, following the German convention for calculating usable floor area). This house would have about 150 square meters of TFA. The criterion is 120 kWh/square meter of TFA, so the limit here would be 18,000 kWh/year of PE, or, using a PE factor of 3, 6,000 kWh of site energy usage. We managed to be comfortably below that limit this past year.
I have proposed an amendment to the PH standard for New England (see post New England PH amendment), where I propose that the PE limit be set according according to number of bedrooms rather than according to floor area. How did we do according to the amendment? A three bedroom house is permitted 13,600 kWh/year, a good bit lower than permitted under the standard as is. With a PE factor of 3, the site energy limit would be 4,533 kWh/year. We squeaked under with 3,813 kWh this past year, but a really cold year might yield a different result!