One year energy usage

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.

The stats:

- 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!

Comments

[Eric]

So avg. 317kWh/month? I'm trying to remember, is all of your heating via the mini-splits? We come in at about 366kWh/month avg, but that's with gas heat - and a very old house. :)

[David White]

Hooray Marc!

[Marc Rosenbaum]

Yes, 317 kWh/month average, and it's an all-electric house now, so monthly usage has ranged from about 200 kWh to 500 kWh, and in a colder winter I expect the monthly usage could go to 600 kWh or more.

[Bob Lemaire]

So if I read this correctly, your achievement of both the THC and PH standards (existing and proposed) is exclusive of the PV component (which gets you to net sub-zero). That is amazing given that the house was not anything real special to begin with and the envelope improvements were relatively minor.

I think further discounting SE by the immediately useable PV energy as you suggested in the regional PH proposal is as legitimate as the baked in effect of any other solar mechanism like increased glazing, daylighting, or solar thermal.

You approach is scalable for a lot of homes in the northeast.

[Marc Rosenbaum]

Bob
Yes, this energy usage is not counting the net zero aspect of the PVs. I don't want to brag about this - we're a conserving household of two, and this was an unusually mild winter. The scalability has to do with choices to use less, and machines that out the energy right where you need it, and efficient appliances and lighting. But the big nut for most houses is the heating load, and I started with a better than average house.

[Eric]

Very impressive Marc. Here I thought we were a conserving family of 4, but you've put me to shame. Now I'm wondering, can I live without the ultra-efficient beer fridge? :)

(Aside: do you ever need dehumidification in the basement? Or does your geyser system take care of that?)

[Marc Rosenbaum]

Eric - I think we're conserving partially by inclination, partially by being at work too much :-), partially because for relaxation we read instead of watch TV (we don't have one), and certainly because we're two rather than four people. We haven't been using a dehumidifier in the basement - the foam on the walls warm up the surfaces enough, and stop moisture transport through the walls. I was looking at some data from last summer and we only had really high RH down there (over 90%) leading up to Hurricane Irene.