We moved in on November 1st, 2013, and the PV system had been activated on October 20th. The NSTAR net meter read zero when installed, read 99980 (it goes down from zero if the energy is net exported) on Nov 8th 2013, and read 98852 on Nov 10th 2014, for a net energy export of 1,128 kWh in that year. The PV system made 6,198 kWh November through October, so we used 5,058 kWH for the year (these numbers are slightly off because of the non-simultaneity of the utility meter "year" and the fact that I read the PV meter on the 1st of each month). The monthly energy flow to and from the grid are shown in the graph below, assuming for simplicity that the NSTAR meter read zero on November 1st 2013.
Usage was a tad bit higher than I projected (somewhere around 4,700 kWh/year). One reason is the minisplit heat pump - it took me until early March to change the settings at the deeper levels of the controls that were enabling the unit to operate by cycling on at full speed then shutting down, repeatedly, working at its least efficient operating point. The subject of another post to be sure...
The eMonitor wasn't up and running until late January, so I don't have a full year's breakdown of where the energy is going, but I'd say the heat pump used almost 35% of the energy, the heat pump water heater just over 10%, and the fridge and chest freezer about 15%. The remaining 40% is cooking, water pumping (we planted several dozen trees and shrubs which we irrigated), lights, the HRV, and plug loads.
The net exported energy would power an electric vehicle for over 3,000 miles/year.
Tom DeMarco wrote, "you can't control what you don't measure" (widely misattributed to Peter Drucker BTW). Well hell, there's lots you can't control no mater what. A less ambitious implementation of measuring is to aid in understanding how a system operates. The eM is good for this.
One useful feature is the setting of Alerts. The eM software offers a lot of Alert choices. It can tell you when your electric cost for the month passes a certain level. I like the ones such as:
- rawing little or no power for an extended period
- or or freezer door was left open
- ppliance seems to have been on for an extended period
- rigeration equipment is short-cycling, a condition that is likely to require a repair.
- (s) of equipment uses at least % more energy than it did before
On one of our SMC projects, we got an alert that the water heater was using more energy than typical, and we traced it to a hot water leak. One aspect of Alerts is that you need to be prepared for false alarms. My eM tells me almost daily
"It looks like your 'Refrigerator' door may be open; you might want to check it.If the door is closed, check the seals. This alert is triggered by a compressor that is staying on longer than usual, so if it is not a door or seal problem it could indicate another problem with the unit."
It doesn't seem to be used to modern refrigerators, which draw less power but run for longer when they are on. Also, I get an Alert that tells me
"Your SiteSage service has detected that your Minisplit heat pump circuit is not drawing any power or the expected level of power."
That's because I have a superinsulated house with good solar gains on sunny days, so the heat pump turns off most days by mid-morning, and doesn't come back on until the evening some time.
Using the eM I can see where our energy goes. I can tell if we left the basement lights on all day. I can see how much energy the preheater uses in the HRV to defrost the core. It's been most useful in diagnosing an issue with the Fujitsu heat pump.
At House 5, the outdoor compressor unit was right outside the living/dining room, about five feet from where we sat for reading or eating. We virtually never heard it run. In the new place, the compressor would periodically whine so loud we could hear it all over the house. Using the eM, this is what a day's worth of energy usage looked like:
Here I selected a day where there were minimal solar gains, so the unit ran all day. The outdoor temperature was around freezing. The pattern showed that roughly once an hour the heat pump would ramp up over seven minutes to absolute full output, in fact, the input power measured of about 2,400W was about 1/3 higher than the unit is rated for. I verified that the eM was measuring correctly by putting a ammeter on the line - it measured just under 10A, correlating nicely with the power the eM measured.
The nifty aspect of inverter-driven minisplit heat pumps is that they modulate their output depending on the building load. And running at a reduced output, using the same heat exchangers, is more efficient. And running the indoor and outdoor fans, and the compressor, at reduced speeds is much more efficient. So my unit was heating the house fine, but using more power than necessary and making a racket. How that got fixed is a whole separate story, but it was the power traces from the eM that convinced Fujitsu tech support that there was something worth looking at here. Here's what the energy use looks like in the unit running properly:
Yesterday also had little solar gain, and was about 45F outdoors. The heat pump's peak power draw here is 400W, and it averaged about 200W over the twenty-four hours. It is likely running at a COP of about five! And it is quiet, too.
After we got these control settings fixed, the heat pump used less than two-thirds of the energy per heating degree day than it did when it was cycling to full output then off. The eM helped us in the process of understanding this and ultimately solving it.
Another useful application of the eM is to be able to see how much power appliances draw when they are nominally off. I deliberately try to avoid appliances with electronics and clocks, because they draw parasitic power all the time. We own a microwave, toaster oven, refrigerator, chest freezer, dishwasher, clothes washer, clothes dryer, and heat pump water heater that all have no displays or clocks. According to the eM, none of them draw any power when off. In contrast to that, here's the March 2014 power trace for our induction range:
On thirteen of the days we didn't use the range, and because it draws about 6W continuously, on these days it uses 0.13-0.14 kWh. So the monthly baseload is about 4 kWh. This month, that will be 28% of the usage. If the eight appliances I listed above all drew 6W continuously, then we'd consume about 450 kWh/year in parasitic power (or what some people call phantom loads, or vampire loads). That would approach ten percent of the total anticipated annual energy usage here. I'm told some cable boxes draw 40W continuously!
I have a new toy. It's a totally geeky toy - almost everyone else would rather have a large plasma screen TV. It's called eMonitor and it is made by Powerhouse Dynamics. You can buy one from folks at Powerwise Systems or Energy Circle. It's a device that measures energy usage in your house circuit by circuit, and uploads that info to a web site where it can be viewed in a number of ways. Any circuit being measured has a current transformer (CT) (or sometimes two, for some 240V loads), and you can get versions with 14 CTs, or ultimately as many as you want. I chose one that does 24 CTs. That doesn't get every circuit - I have some left over in what is called Unmonitored Power, maybe 3% of the usage. If I'm really curious I can always move some CTs from one circuit to another.
It's a costly doodad. Mine lists for $695. Plus, you pay for access to the web site to see the data. In my case, I paid $240 for two years of monitoring access, plus an additional $108 to include my solar electric system (PV) in the mix. I installed the system myself, but they (or their lawyers) recommend that it be installed by a licensed electrician. It took a couple of hours to put it in and then enter the circuits on the web site, so it's clear what each circuit being monitored is feeding. If your electric panel has been poorly or inaccurately labeled (the norm in my experience) count on a couple of hours, preferably with another person, to track down what is on each circuit breaker.
When you're done with the installation, the panel is pretty darn full of wires. The eMonitor has a component that stays inside the panel, that the CTs connect to, and it communicates wirelessly to the unit outside of the panel that connects to the modem and thereby the outside world.
When I go to the Sitesage web site (recent, very unfortunate name change :-), this is what I see:
The top of the page shows how much power the house is using, how much is being produced by the PVs, and what the net power draw or export is. It also shows the largest power users. In the snapshot above, you can see that the heat pump water heater (HPWH) is running, the Fujitsu minisplit heat pump is on at a very low level (outdoor temperature of 48F is shown - they must get that from another web site, there aren't temperature sensors connected to the eMonitor, although Powerwise can fix you up with all kinds of sensors too), the unmonitored power is 65W (my Macbook Pro, actually, as I write), and the heat recovery ventilator (HRV). There are other tabs for more information, or to set alerts, etc.
Next on the page is a donut chart showing energy usage over the past 30 days. It's been wintry, so the minisplit has been the dominant user. I'm not sure what the horizontal bar graph in the upper right corner is measuring.
Here I can see total usage vs. production for various periods - either by month for a year, or by day for a month. Average outdoor temperature, and heating and cooling degree days are shown as well. On the right, the actual energy usage pattern can be viewed for various durations.
The button entitled Export Data allows you to to export data by minute, day, or month into an Excel spreadsheet - very nifty.
There's a lot more to say, about looking circuit by circuit, setting alerts, and how this is useful. More in another post.
One thing I learned many years ago following the energy usage of buildings I designed was that as the energy needs of the building are reduced, and the fraction of those needs supplied by solar energy increases, the variation in back-up energy from year to year increases. Let's look at House 5 this winter and last winter.
First, the variation in heating load - this winter (Nov-Feb) is 20% colder:
We've used 23% more energy over these months, and here's how it breaks down between the heat pump and all other uses:
Heat pump energy used is up 32%:
As it gets colder outside, the heat pump efficiency drops, so it makes sense that heat pump energy goes up faster than heating demand. However, another difference in what we use is that Jill now works 3/4 time and is home Mondays and Fridays, so I'm guessing the average thermostat setting has gone up a tad, and there are more kWh going to lights, computer, music, etc. All other uses are up 15%, with the biggest bump in January:
Meanwhile...it's been noticeably cloudier this winter, so PV production over the four month period is down 22% over last year:
Consequently, our net imported energy last year over this period was 18 kWh, and this year is 45 times higher at 804 kWh!
If you design or build zero net energy buildings, it's important to communicate to the owners that these year to year variations are significant. Manage expectations! How much they use for plug loads/appliances/lighting is under their control beyond a certain baseline. How much they use for hot water is similar. At a given thermostat setting, how much they use for heating and cooling is dependent on the weather and the amount of sun available, and how much a solar electric or solar thermal system generates depends on solar availability and how much of the time the collectors are covered with snow. If you want to be pretty certain that a building is net zero every year, the solar electric system probably needs to be oversized by close to 30% (a SWAG). Driving the building's heating load down will make this variation smaller, as it takes one weather-dependent factor and makes it smaller.
I tried an experiment this week during our cold snap. We've kept the door closed to the first floor ell (bedroom and bath) and let it run cold, because the Fujitsu wasn't sized to heat that space too. I opened the door early in the cold snap, and let the heat pump go, leaving it set on 70F. What I found was that overnight the main space went to 66F, and the upstairs and back bedroom were 3-4F lower.
My calculated heat loss in these conditions is about 24,000 BTU/hour, and the heat pump is rated at about 17,000 BTU/hour at about 10F. You'd think it would not be able to keep up. My heat loss number may well be too high, and the rated output of the unit may be quite conservative.
One other thing I wonder about is that even though the room was not at the setpoint it seemed that the unit didn't run on full output much. My system has the temperature sensing built into the wall cassette, so it may be sensing a higher temperature than out in the room. It may make sense in severe weather to set the thermostat up to 72F instead of our normal 70F. Unlike a boiler, these variable speed units taper off the output as the space approaches the setpoint instead of always running at full bore, so that may be a disadvantage of a smart unit - it's trying to stay at a more efficient operating point instead of making me as comfortable as possible.
Anyway, comfort trumped further experimentation and we closed the door again to the ell.
Habitat for Humanity of Martha's Vineyard is really getting the superinsulated, energy efficient home down pat. Last Saturday, accompanied by job super extraordinaire Lee Taberner, I did a final blower door test on the second house they've built at the end of Bailey Park Road. The result - 203 CFM50. Real tight, well under 1 ACH50. Along with this, the HVAC system is a minisplit heat pump, a heat pump water heater, and a heat recovery ventilator.
HfHMV has made the connection between energy efficiency and permanent affordability. So has the Island Housing Trust (South Mountain is currently building two new homes for IHT, also in West Tisbury). It's heartening to see the two most significant affordable housing providers on MV so committed to high performance homes.
I am reluctant to have anything in the house that uses energy 24/7. With 8,760 hours per year, small usages add up. One such is the exhaust fan on the composting toilet - at about 20W it uses 175 kWh/year, or 4-5% of our energy usage. Another is the cable modem for internet access we have from Comcast, and the Apple Airport Express wireless router. I've been measuring both. The router uses just over 3-1/2W, the modem 6-1/2W. Together they are 88 kWh/year. I imagine that if we were off-grid we'd unplug them in the winter when solar power is short. Being grid-tied, we're just lazy about it.
Last year, during November and December 2011, our net energy with the grid was 98 kWh - we imported 98 kWh more than we used. This year during the same period that figure was three times higher, 299 kWh net import. Why so different?
For one thing, it was cloudier. The PV system made 68 kWh more last year during this time. The larger difference is that we used more - 947 kWh this year, vs. 814 kWh last year. How come? I can think of three things:
- It was colder this year - 1,580 heating degree days (base 68F) vs 1,296 in the same period last year, mostly in a colder-than-average (yes, we still have those occasionally!) November. So the heat pump ran more - 98.5 kWh more, in fact. That's the big one.
- Jill is working three long days a week instead of going to work five
days/week, so she's home more, using lighting, a higher thermostat
setting, the computer. A small impact, but it's likely real.
- Jill's Christmas cookie marathon, in which she made 16 different types of cookies. Lotta oven usage, plus kitchen appliances and lighting, plus dishwasher, plus more hot water cleaning up. This extra usage of energy is very well appreciated by many cookie recipients!
You can see that the big difference is weather-related - colder and cloudier. The lower energy use a house is, the larger these year to year variations will be, on a percentage basis. Yet the benefit of solar-driven, energy efficient building is that the percentage variations may be high, but the absolute difference in dollars is small. So the difference in our net energy flow over those two months year to year is about $37 total.
With the exception of one week in February 2011 where I switched back to the oil boiler to take some data before it went away, the Fujitsu 12RLS has now been heating the house for two years. The meter reads 2,584 kWh. So, about $250/year to heat House 5, in mostly milder-than-normal weather. This is about 1/4 the cost of operating the oil heating system.
Most houses in the northeast have a boiler and forced hot water heating, and most of the rest have a forced air furnace - both are central heat systems. Without some energy retrofit work, most houses can't be converted over to a single zone minisplit and have adequate heat throughout the house. In cases where the central heating system is due for replacement, a multizone minisplit may be worth considering. We've done just that at SMC, for a client with a 30 year old boiler and a poorly designed distribution system. That system cost over $20K installed, though.
A single zone minisplit costs about $4K installed. In cases where the entire house doesn't need to be fully heated, or houses in which a point source heater can carry the load of the house in mild winter weather, a minisplit can be a great retrofit. In the Pacific Northwest a major study has been conducted using a single zone minisplit as a retrofit to the many electrically homes there (http://www.bpa.gov/energy/n/emerging_technology/DHP.cfm). On average they have shown a 40% reduction in heating energy, with some homeowners experiencing much higher savings (the ones most likely that kept the doors to the bedrooms open!) The electric resistance heat is still in place, to be used as needed. It's very possible to consider a similar approach in fossil fuel heated homes. The best candidates are houses with open plans, so the heat pump can heat a good portion of the kitchen/dining/living space, and houses where the other rooms are located where natural convection (warm air rising) can transport heat to them. It would be best for the existing heating system to be one that has more than one zone, so that the zone(s) not well heated by the heat pump can still be heated by the existing system.
Best suited might be houses where a number of the rooms are not occupied - the large house with a single occupant, who needs a bedroom, bath, and the public areas heated, not the other four bedrooms and two baths. In essence, it's going back to the days when a central hearth kept the public spaces warm and the peripheral spaces were much cooler.
These changes will likely be driven by fuel prices, so they are more appropriate where there isn't natural gas - houses where oil, propane, and electric resistance are the primary heating fuels. As of the 2009 EIA energy use surveys, there were almost 9 million households in the northeast (New England and mid-Atlantic states) using those fuels as the main heating source. That's a significant opportunity.
I've been AWOL for a while, sorry about that. I'm putting together my first online course, on Zero Net Energy Homes, for the NESEA Building Energy Masters Series. You can read about it here:
Meanwhile, the House 5 PV system just passed 8,000 kWh generated! And we have a surplus credit of over 3,700 kWh, which we can allocate to another meter.
I have plans to post more about the hot water question, especially on heat pump water heaters, and also on the insulating blinds. Stay tuned!