The Thousand Home Challenge is an initiative of Affordable Comfort. Its goal is the transformation of America's housing stock via a demonstration project of one thousand homes that reduce site energy use by 70 - 90%. What differentiates the THC from other programs is that it is based on twelve months of actual, not modeled, energy usage, and that it allows, indeed encourages, multiple paths to success. It is the only initiative that recognizes that the principal barrier to deep energy reductions is not a lack of technology or insurmountable cost, but rather the choices that people make in how they use energy.
There are two options available to THC candidates. If your house has a recent full year's worth of energy use data, Option A is a 75% reduction from that baseline. This is a good option for an existing energy hog! Option B is an energy budget calculated by the THC Calculator, using as inputs your zip code, square footage of finished floor area, number of occupants, and whether the house is free-standing or attached. The Calculator spits out a site energy budget (see the previous post on Primary Energy for a clearer discussion of site vs. source of primary energy) for heating, cooling, DHW, and electrical loads.
There are some aspects of the THC that aren't optimum, in my opinion. I'd like to see primary energy be the metric, but this would confuse people and conflicts with other programs (no initiative besides Passive House that I'm aware of even calculates primary energy.) This means that one BTU of electricity used to heat water is equal to one BTU of natural gas to heat water, even though in primary energy terms they are different by about 2-1/2 to 1. It means a kWh of exported solar electricity is counted as offsetting an imported one, which I support, although people differ about this. THC does allow twice as many BTU for heating if a combustion fuel is used than is allowed if electricity is used - this is a compromise between a COP of one for electric resistance heat, and a COP of three that might result with a really good minisplit heat pump.
The THC budgets are low. Ours for House 5, with two occupants, is 5,375 kWh/year if the house uses electricity for heat. If a thermal fuel is used for heat, the budget limit is 7,644 kWh/year, with the difference coming from doubling the allowable heating budget from 2,269 to 4,539 kWh/year. Here's how THC calculates the House 5 allowance for Option B:
The percentages allocated to each end use change depending on what type of energy is used for heating, but the cooling, DHW, and all else (electricity not used for the other three) remain at the same absolute levels. The blended space heat is 50% electricity and 50% thermal fuel.
How tough is this budget? As is when we moved in, the house would use 300 - 350 gallons of oil annually for heating, and perhaps another 100 gallons for DHW. Call it 425 gallons. That's 17,277 kWh/year by itself, or about three times the THC allowance for heating with a thermal fuel plus DHW. The allowance leaves 1,893 kWh/year for cooling and all other loads. Average annual electrical usage per househould in MA is about 7,300 kWh/year (but this is undifferentiated as to end use and undoubtably includes all of the above mentioned four categories of energy end use.) Yet I think that it's clear this is a rigorous target.
How might people achieve the THC standard? The three principal components available to a household are energy retrofits to reduce load and equipment efficiency upgrades; on-site renewable energy (but wood is not counted as a renewable, so this really means solar in most cases); and occupant behavior. If a community gathers together to reduce energy usage, community solutions beyond the scale of a house are eligible. Where it gets interesting to me is that occupant behavior can be huge. The range of variation of occupant choice of heating setpoint; cooling setpoint and indeed usage of cooling at all; DHW usage in gallons per day per person (gpd/person); and electricity usage for lighting and plug loads, is surprisingly large. In one group of eight homes built as a neighborhood for which I have been compiling monthly submetered energy usage data, we see DHW gpd/person varying three to one, and lighting and plug load usage similarly. It's easier for households with more people to achieve lower usage per person (many energy loads don't go up in proportion to number of people in the household) yet overall, the usage of energy in this country is in most cases based on occupant choices. And choice is the one input that can change in an instant, with no cost. It's climate change action by epiphany :-)
Once I got my energy threshold from the THC Calculator, I looked at how much we were using and how far we were from the allowable amount. Surprisingly, we're close. Here's an estimate of annual usage:
I think we could qualify for the THC where we are now, without added renewable energy. Here's what's different from the house we moved into a year ago:
- The big move is the replacement of the oil boiler with the Fujitsu heat pump and a very efficient electric water heater (in progress). We take advantage of a number of things here. The first is the excellent COP of the Fujitsu heat pump and the THC 2:1 thermal fuel/electricity ratio. The second is the fact that the oil forced hot water system used two or more kWh/day in electricity to run the burner, the pump and zone valves, and the controls. The third is that in seasons when no heat is called for, the boiler made DHW at a very low efficiency (see Out with the old, in with the new).
- We tightened the shell up, dropping the blower door number from 900 to 650 CFM50 (see What's a renter to do?), thereby lowering the heating load.
- We ran the main level at 66F - 70F, depending on whether we actively occupied the space, and accepted 2 - 4F lower temperatures in the upstairs, depending on how cold it was outdoors. The upstairs was only heated passively by the heat pump on the main level. We chose to close off the first floor bedroom and bath and run them without heat, except what they got by conduction and leakage through interior walls, so these spaces ran close to basement temperature (which dropped below 50F briefly in January, once we turned to the heat pump for heat, and the energy dumped into the basement by the oil heating system was greatly reduced.)
- We swapped out the refrigerator and the range/oven for much more efficient models (in a future post I'll discuss energy used for cooking, but suffice it to say that electric cooktops are roughly double the efficiency of gas cooktops...)
- We use a lot less electricity than most Americans - no TV for instance. Yet we use the lights, listen to tunes, use power tools and laptops, etc. - we haven't felt particularly abstemious or deprived.
So, it looks as though a year from now we could qualify for the THC. This feels like an admirable goal, yet, in a way, with this house, it feels too easy. We're going to add in other strategies, with a variety of benefits. They include:
- We're going to insulate the basement and rim joist (subject of future posts). This is primarily to warm the basement up, and reduce ground moisture transport in, with the goal of making it less dank in the humid MV climate. Almost everyone here runs a dehumidifier continuously in the warmer months. The rim joist work will reduce air infiltration and puts basement insulation where it does the most good, above grade where it's coldest.
- We're going to do something to upgrade the windows, without replacing them (also a future post or two here...)
- There's a bit of work to do in the main house attic, where the cellulose has settled some in the sloped roof bays.
- We intend to install a 4.76 kW Sunpower solar electric array sometime in the next month. The combination of the falling prices of PVs with the MA SREC program, the MA Clean Energy Center rebates still available though diminished, and the federal 30% tax credit for renewables makes the installation of PVs a better investment than any other available to people like us, who don't have supercomputers to manipulate the stock market.
- We also will install a small solar DHW system. I did some hand-wringing about this, especially vs. a heat pump water heater, but I think I'm going with what I know.
- We're going to put in a woodstove. I think it's a good complement to the heat pump, because keeping a stove going in mild weather is more challenging, and heat pumps are more efficient as the outdoor temps rise, so these could be ideal together. There's a lot of oak here on MV, enough to glean a bit of cord wood each year. Plus, it's nice to have heat, and in an extended emergency the potential for DHW and cooking, when the power is out. Finally, this past winter was the first time in thirty years I didn't have a wood heater, and when you're cold there's nothing like sidling your butt up to one!
Lots of fodder here for more posts!
Anyway, back to the target question. The PV array here will have a bit of shading, and I don't know exactly how many kWh it will make annually. Unshaded it would be 5,700 kWh or more in a typical year. This means we could likely be net zero on an annual basis, even perhaps without the solar DHW system. I think we'll net at least 5,000 kWh/year from the system. Looking at our projected usage more closely by month, I think it is possible that if we primarily use the wood stove to heat for the coldest and cloudiest months - say December through February - we might be able to hit net zero on a month by month basis. It's dicey in December, when the shading on the solar systems will be greatest, but that makes it a worthy target.
To summarize our target:
We will shoot for monthly net zero electrical energy - that is, to use no more electricity than the PV system generates on a monthly basis - and supplement with no more than a cord of firewood. We'll aim to qualify for the THC along the way. Will we get to our goal? Don't change that dial....