Eight months of data - On the way to net zero

I've been traveling a lot for the past several weeks - teaching three days in the Passive House Consultant training, and about Deep Energy Retrofits and Zero Energy Houses in VT, IL, and WI. Next week is Building Energy 2012 in Boston (you ARE attending, right?) then I get to stay home for a while!

We now have eight full months of measured data, beginning in July 2011. The solar electric system came on line the afternoon of June 9th, and the commercial meter from NSTAR was installed May 25th, so I have our net energy usage from May 25th (we've exported a net of 1,590 kWh since then - I estimate we had 137 kWh on the meter when the PVs came on line) but I don't have sub-metered usage until some time in June,

It's been an astonishingly warm winter so far. We've had 73% of typical heating degree days. It has been slightly sunnier than last year (using PV production at the Eliakim houses as a surrogate). So this first year is unlikely to be typical in the long run (although I could get used to it!) As an example, in January last year we used 418 kWh in the heat pump, and this year it was 277 kWh. From July 1st to March 1st, we've used 2,639 kWh. the PVs have made 4,107 kWh, and the net exported is 1,468 kWh. I think we'll use less than 4,000 kWh this first 12 months, and I'm guessing the PV system will make at least 6,000 kWh. The surplus would power an electric car for 6,000 miles, which is more than our on-Island miles (Jill is driving 3-4,000 miles annually - her car stays here - and I'm in the 8-9,000 miles/year range, but almost all is off-Island).

Here's the Used/Produced/Net data by month:



It's clear that shading from trees hasn't affected us as much as I expected. It's most evident in December, as expected. Some people say that the PVWatts software we use to project production is quite conservative. Sunpower, the manufacturer of the PV panels South Mountain Company installs, says that there are important aspects of their panel technology that lead to higher production per watt than other products, and this may be part of what we're seeing. In the chart below, the predicted production is before any shading. Even with modest shading (one large tree to the south) the system has produced 13% more so far than the predicted unshaded output. Woohoo!

And here is the energy usage by end use (R/F is the refrigerator and the 15 ft3 chest freezer in the basement):

The heat pump is the most varying end use, peaking in January, the coldest month. There are other interesting trends. The energy used by the refrigerator and freezer drops from the summer to the winter, as the spaces they are located in get cooler. This both increases heat gain through the cabinet and also makes the compressor work harder. Conversely, the energy going into heating DHW increases, as the losses from the tank increase as the basement cools off, and the incoming water temperature drops. Cooking energy peaks in the holiday season - much more baking - and drops the last two months, partially due to all my traveling. Jill doesn't cook when I'm gone.

Here's a closer look at DHW usage and energy:

Again, usage dropped off when I was traveling, and energy in per gallon increases from the summer.

Comments

[Bob Lemaire]

Marc,

What is the net metering policy with respect to your surplus? Is it use it or lose it?

Also, in the discussion of the R/F do you mean to say the heat gain and compressor energy decreases in the winter? It's subtle but is there a technical reason that you can cite for the apparent increase in efficiency of the hot water heat-pump from August to September?

I'll be curious to see if you go with an electric vehicle. Island life seems like a natural for the range of EVs that are on the market today.

[Jan Juran]

Hi Marc: interesting data, low usage in Feb. looks like approx 280 gallons DHW used and 100 KWHr to produce. 100 KWHr=341,230 BTUs. At a delta T=60F (50F water supply, 110F DHW) and 8.34 BTU to raise 1 gallon 1F: 140,112 BTU heats 280 gallons (albeit at 100% efficiency). In perfect hindsight, would you consider using several modulating Rheem or Steibel tankless electric HWHtrs (low $200s internet price, plus install cost, <40 Amp typical electricity draw) in the bathrooms and kitchen instead of the Geyser/Marathon in the basement? No wait for HW, 70F water contained within any pipes located in conditioned space=lower delta T, adding a possible DWHR "GFX" plumbed to the shower drains could cut 30-40% off the KWHrs and the Amp draw used for showers.

[Marc Rosenbaum]

Hi Jan
We have a somewhat higher delta T in the winter, about 70-75F. I don't have the amperage available in my 100A panel for any instantaneous electric. Maybe in new construction...