Archive for the 'Energy Calculations' Category

Why we bought a Plug-in Hybrid

photo of Prius

If you remember one of our early solar posts when building the house, we initially assumed that solar was going to be too expensive for us. Then we ran the numbers and found that the rate of return was actually quite attractive over the log run.

Same goes for our new plug-in hybrid. We had been a one car family since Jill and I met. Our four-wheel drive Ford Escape was both our commuter car and utility vehicle. We made multiple moves in that car. We carted goats and chickens and guineas in that car. We transported a year’s worth of hay. We hoped the Escape would carry us a few more years, but at over 200,000 miles our trusty steed was starting to cost us more in gas and repairs than buying a new car.

We now have a truck to haul goats and other farm trappings when needed, so Jill started researching commute-friendly cars. She drove three hybrids, Honda Insight, Toyota Prius and Ford C-Max. The C-Max had the nicest interior and was more similar feeling to the Escape, but was the most expensive and the mpg reviews were mixed. The Insight had a great price point, but had the worst mpg estimate and interior feel. So we focused on the middle-priced Prius which had the best mpg estimates and a good track record.

With our current net metering plan, it’s better for us to use the excess than sell it back to the utility which pays us very little. It initially looked like a plug-in hybrid was out of our price range (even with federal credits) but when I added up the miles: 19,500, the gas: 970 gallons, and gas dollars: about $3,600 a year, I realized the extra savings in gas might justify the higher cost.

Here’s the breakdown…

The Escape was averaging about 20 mpg. The regular Hybrid Prius averages 50 mpg and would save us 580 gallons of gas and $2,150 per year. Toyota says the Prius plug-in gets roughly 95 mpge (that’s miles per gallon plus electric). We test drove the plug-in for a few days on back country roads and found it was closer to 85 mpge. The plug-in component gives you an extra 13-14 miles before the hybrid engine kicks in.

We estimated conservatively that 70 mpge would save us 690 gallons and $2,560 per year, but use approximately 1,400 kWh per year. That would cost us about $210 in electricity charges if we were paying for electricity, which means we would only be saving $2,350 per year in gas.

The regular Prius is $24,995. Toyota had a special offering interest free loans up to 60 months. So $25k/5 = $5,000/year plus $1,450 in gas equals $6,450/year. Now remember, we were paying about $3,600/year for gas, and $2,000 or more per year in maintenance. So we’d be paying an extra $830/year for a new car.

The plug-in version is $8k more. $33k/5 = $6,600/year. But in addition to 0% interest for 60 months, they were also throwing in a $4,000 rebate for plug-ins. There’s also a $2,500 federal tax credit. That brings the price down to $26,500, only $1,500 more than the regular Prius. That works out to $5,300/year plus $1,030 in gas, plus $210 in electricity equals $6,540. That’s about $90 extra per year for the plug-in, assuming we paid for electricity. Take out the electricity (because we produce excess electricity per year) and the plug-in is cheaper than the regular Prius.

Of course we didn’t start at the $33k price, Jill bargained them down. After rebates and tax credits we’re paying about $100 less a year for the plug-in over the regular Prius. So we are basically paying about $520 more per year (not counting electricity, and assuming gas prices stay the same) for a new car that is easier on the environment. If we get closer to 85 mpge then we will only be paying $330 extra per year. If/when the price of gas goes up, we save more.

The only down side. The Prius is definitely not going to make it up our driveway some days in the winter.

It’s official, we earned our 5+ Energy Star rating. But the result was not what we expected.

Our contractor used our house as an opportunity to apply for status as an Energy Star builder. It worked. He’s now an Energy Star approved builder, and we have a 5+ star Energy Star home. In the process we discovered a big difference in the projected energy usage by our Energy Star certified rater using REM/Rate and our certified PassivHaus consultants who used the PassivHaus spreadsheet.

First a bit of background. At the beginning of this project we briefly considered certifying the house under one or more of the prevailing certification programs, PassivHaus, The Thousand Home Challenge (THC), LEED and Energy Star. (At the time there was no Net Zero certification.) In the end we decided it was not really worth the cost or effort for PassivHaus or LEED. Most of these certifications have little to offer the homeowner besides bragging rights. We felt that a verifiable record of the performance of the house over time would be enough bragging rights for us. (We’re still considering trying for the THC, but the spreadsheet leaves us a bit bewildered).

We did hire a certified PassivHaus energy consultant (DEAP Energy Group) to run the numbers on our proposed design to estimate our required heating loads, energy usage and required PV array to get to net zero. After all our research, I felt the PassivHaus spreadsheet offered the most accurate projection of energy usage and the associated training for certification promised that there was some intelligence applied to using the spreadsheet. We were very happy with the analysis and feel confident that the results will be confirmed after our first year in operation. We’ve purchased energy and temperature monitors to provide some data for analysis at the end of the year.

Although we had opted out of the certification programs, our contractor, Warren, decided that this project was an excellent opportunity to apply for status as an Energy Star builder, which would grant the house an Energy Star rating if it met all the conditions. (For a house to be certified as an Energy Star home, it has to be built by an Energy Star certified builder. The general process is described on the Energy Star site, How New Homes Earn the Energy Star.) Energy Star represents the lowest hurdle out of the other certification programs, but is also the most widely used. There are several versions of certification available, from 2.0 (now unavailable in 2012) to 2.5 and 3.0. Each version is more stringent than the previous. These versions are timed to replace the prior versions over time. After reading through the high-level requirements, we thought we’d be able to easily qualify under the 3.0 version.

However, because we were already many months into the build before we engaged the energy star certified rater, Newport Ventures, we missed the opportunity to be certified under the newer 2.5 or 3.0 standard. Our choice of HVAC contractor also contributed to this problem, as they were not certified under Energy Star requirements. If we had done our homework earlier, we might have been able to certify under one of the higher standards. So although the house is more efficient and built to more stringent standards than even the 3.0 version, we were only able to qualify under the 2.0 version.

One of the steps in the Energy Start certification process is to rate the efficiency of the house. The result is a HERS (Home Energy Rating System) index value. It is an index value (percentage) because it compares the efficiency of a specific house (ours in this case) with a reference model home that is based on the 2006 International Energy Conservation Code (new house efficiency / model house efficiency = HERS index). The lower the value the better the efficiency. For an excellent review of HERS, see Martin Holladay’s article on GreenBuildingAdvisor, How Is a Home’s HERS Index Calculated? He also describes the various pros and cons of using HERS.

To qualify as an Energy Star home, the house must earn a HERS index value of 85 or better. The lower the value the more efficient the home.

Warren got the final report (Home Energy Rating Certificate) a few days ago. Our HERS index, as rated by Newport Ventures, is 22 which equates to a 5 1/2 star Energy Star rating. We should have been happy, right? We were, except for one thing, we were expecting it to be zero or negative. One of the hallmarks of HERS is that a Net Zero house should get a zero index value or lower.

Unfortunately the certificate itself is quite vague. It doesn’t make it clear whether the power generation capability was included in the score or not, although it appears to be included. It projects our heat load to be half what the PassivHaus spreadsheet indicated (6.9  vs 12.4 MBTU/yr.) and it projects our lighting and appliance load to be much higher. HERS projects our total energy usage to be 42.4 MBTU/yr., DEAP projected 20.4 MBTU/yr. The HERS heating estimate alone makes me suspicious.

We’re going to try to get a bit more clarification from Newport Ventures in the coming weeks. We didn’t expect the numbers to match up exactly, but we did expect them to be in the same ballpark. We will report back when we learn more.

Thousand Home Challenge

I was reading The Right Target, a post on Marc Rosenbaum’s blog a few weeks ago and realized that we too might be able to qualify for the Thousand Home Challenge.

The goal of the Thousand Home Challenge is to reduce the energy usage of 1,000 homes by 70 to 90%. It outlines two paths to meet the challenge. Option A is a specific reduction from current usage. We’re going to ignore that since we’re building a new house.

Option B seems to blend aspects of both passivhaus and net-zero. It is based on a low energy target and verified using actual energy usage data over a year. Renewables like our solar PV array, count as credits toward meeting our target. (I think the net-zero house movement would benefit from having a low energy target to avoid the ‘6,000 sf house with a 20 kW array’ problem.)

The target is based on a combination of climate, size, number of occupants, heat source and whether it is attached or detached. They have a handy spreadsheet you can download at the thousand home challenge website. Just punch in your numbers and it will tell you your target.

Here’s the run down on our inputs.

  • Climate – The closest weather station data was determined to be North Adams, Massachusetts, but I chose Glen Falls, New York, since this is what we used in the original energy calculations.
  • The home’s finished floor area is 1,200 sf.
  • Two occupants, not counting critters.
  • Detached single home.

Based on that scant information, the threshold calculator produced a target value of 5,619 kWh/year (site energy) if using electricity for heating. This works out to roughly 19.2 million BTUs/year.

If you remember back to our earlier energy posts, our energy consultants estimated our yearly energy use at 5,995 kWh/year, or 20.4 million BTUs/year.

So if we ignore the solar PV array, we would be roughly 376 kWh short of meeting the Thousand Home Challenge.

I like a challenge, so I’ve signed us up for more information and to start the application process. I’ll post more info as we progress through the process.

The Energy Nerd that Kicked the Hornet’s Nest

Martin Holladay is stirring the pot over at GBA. He has posted a piece today entitled, Are Passivhaus Requirements Logical or Arbitrary? While I tend to agree with his points in general, I also agree with many of Mike Eliason’s points in his response, A Passivhaus Rebuttal: In Defense of the Standard.

There are clearly lots of ways to design and build a house with very low energy needs. Of all the houses built to the Passivhaus standard thus far, there are bound to be those that are smart (and economical) about how they meet the standard, and those that are forced to rely on one or two methods alone. There will also be examples where the press or marketing is a bit over the top.

Although we are not shooting for Passivhaus certification in our own home, we chose to use as many methods as we could up front to minimize energy loses through the envelope. Factors like orientation, square footage, volume, number and size and type of windows on each side of the house, choice of appliances, etc. all contribute toward this goal.

In the game of trade-offs, if we had decided to build on the north side of the hill rather than the south, or we wanted a ‘great room’ with cathedral ceiling, or big windows on the north side of the house for a great view, it would have required us to beef up on other factors of the envelope in order to get to where we are now.

Check out the articles. I think this is the level of conversation we need in order to craft a better building standard.

Solar Power Planning

To get to net zero energy use, we must produce enough energy to offset our energy usage over a year. We originally planned to live in the house a few years before installing photovoltaic (PV) panels on the roof, but after we ran the numbers we decided it made sense to do it now rather than wait.

First a little context. How much energy should we be trying to offset? Our energy consultants estimated our total site energy usage at 5,995 kW hours per year (kWh/y). Based on this, they recommended a 6 kW array which they estimated would produce roughly 6,000 kWh/y. At $0.14 kWh that’s roughly $840 per year total energy costs (not counting the Basic Service Charge of $16.21/month and misc. charges). That includes heating, cooling, hot water, well pump and plug load (it’s an all electric house).

It is important to note that we’re using site energy numbers rather than source. Source energy is the energy that the power company must generate in order to supply power to the site. Roughly 2/3 of this power is lost on it’s way to the site. Some net zero homes try to offset their usage based on source energy rather than site energy. Our estimated source energy usage is approximately 16,187 kWh/y. This would require a much larger array, more space and, of course, a much bigger wallet!

The next step was to determine if we have enough space on our roof to generate that much power. We already made sure that the roof was facing due south and pitched the the correct angle to maximize production over the year. The rule of thumb is to pitch the panels at the same angle as your latitude. Our latitude is 43 degrees and our roof pitch is 45 degrees. We also have no shading during the primary energy production hours (unless you count snow as shade.)

We have approximately 646 sf of south facing roof area.This is enough room for roughly 30 panels, 3 rows of 10. One of the most popular panels being used right now is a 230 watt panel. 30 panels x 230 watts = 6,900 watts or 6.9 kW. This is DC power. Factoring in some losses converting DC to AC, and the solar potential for our geographic location, this equals roughly 7,000 to 8,000 kWh/y AC.

This is more energy than we require, but it makes good use of our available area and gives us some extra capacity for a future barn and electric UTV. I must note that over-producing is not necessarily economically beneficial for a homeowner. Any excess we produce one month is subtracted from our next bill, dollar for dollar. This is good for offsetting usage, but if we produce more than we use over a period of a year, the power company is only obliged to offer us payment at avoided cost. Defining avoided cost is for lawyers, but it’s safe to assume it means pennies on the dollar. (If you know how to calculate the avoided cost and where to get the data, please let me know in the comments!)

I originally estimated the system would cost in the range of $20k just for the panels and inverter, not counting all the other wiring, roof clips and rails, inspections and labor. I figured design and installation costs could double that number. I knew there were state and federal tax credits but I was surprised to find out how much federal stimulus dollars are available. In New York, NYSERDA administers this money, and rebates roughly a third of the cost of a solar PV system (including labor). I pay taxes, so it’s nice to get a little something extra back.

Th rebate brought the total cost of ownership down to a level that made sense to go forward with the installation now rather than wait. I called a few solar installers and settled on GroSolar. (GroSolar recently sold the residential part of their business to SolarCity.) Once the rebate and tax credits are figured in, the entire 6.9 kW system is less than $15k. Assuming we just count the $840 calculated above as saving per year, then the system would pay for itself in roughly 18 years, assuming electricity stays at $0.14 kWh.

We are scheduled to get our system installed in April. I’ll post again when I have more details on all the components of the system.

The cost of infiltration

There are several ways to approximate the cost of a leaky house. Before I proceed, however, I just want to mention there are other much larger longterm monetary benefits to building a tight house, like a longer lifecycle and less maintenance due to keeping critters and moisture out of the building enclosure. I’m sure there’s a way to calculate the value of these additional benefits, but it’s beyond my abilities (and you’d have to use a lot of assumptions to get there).

Having gone through the numbers, there is some value to understanding the factors that are used to estimate the cost of infiltration, as well as the proportion of infiltration cost to the overall energy costs required to heat a house.

The primary method to estimate infiltration is to calculate the energy required to condition the air replaced by infiltrated air. First we must convert the ACH50 value derived from the blower door test to a ‘natural’ value (ACHnat) that might be experienced when the house is not being pressurized by artificial means. Then calculate the energy required to heat the naturally infiltrating air on a yearly basis. Then calculate the cost of the energy used.

Converting ACH50 to ACHnat is based on many assumptions including the climate zone, height of the house and the degree to which the house is shielded from the wind by surrounding vegetation and other structures. Energy Star recommended a value of 17.8 for ‘well shielded’, 14.8 for ‘normal’ and 13.3 for ‘exposed’ locations in Zone 2. Let’s use the normal value for now.

Dividing 1 ACH50 by 14.8 equals 0.067 ACHnat. This means approximately 6-7% of the air volume in the house (992 ft3) will be replaced by infiltration each hour. [This is not entirely accurate since the volume of the basement is included as conditioned space, yet it is not heated directly.] Let’s assume the inside temperature is 65°F and the outside temperature is 10°F. It takes 0.018 BTUs per hour to heat 1 cubic foot of air 1°F. 992 * 0.018 * 55°F = 982 BTU/hr to heat the infiltrated air.

1 BTU equals 0.293 Watts, so 992 * 0.293 = 288 Watts or 0.288 kW. At $0.14/kWh it would cost $0.04/hr while the temperature outside is 10°F. (We’ll take into account that an air source heat pump (ASHP) can be 2 to 3 times more efficient at converting a kW to a BTU later.)

We can use heating degree days (HDD) to estimate the cost over a typical heating season. In our climate area we have 7100 HDD in a heating season. Substituting 7100 for 55 in the above calculations and multiplying times 24 to get a daily value means you would spend roughly $125 for one heating season just to heat infiltrated air.

But as I mentioned earlier, an air-source heat pump can be 2 to 3 times more efficient at converting electricity to heat than simply using electrical resistance. There are other factors that also temper that number. I found the following formula in several blower door manuals (see here, section 6.3.d). The formula introduces two correction factors, one for the efficiency of the heat source and one for everything else.

Annual Heating Cost = (26 x HDD x Fuel Price x CFM50 / N x Seasonal Efficiency) x 0.6

  • 26 is the result of multiplying the heat capacity of air (0.018) x 60 minutes x 24 hours.
  • HDD is the Heating Degree Days (7100).
  • Fuel Price is $/BTU. 1 BTU = 0.000293071 kW. Multiplying times the price of electricity in our area $0.14 kWh gives us 0.0000410299.
  • CFM50 is 245 for 1 ACH50.
  • N is the Energy Climate Factor (14.8).
  • Seasonal Efficiency is the efficiency of the heat source. ASHP range in values from 1-3. Let’s use 2.5. This is one of the correction factors I described earlier.
  • 0.6 is the second correction factor. I found a brief description of what this value represents here.

Using these inputs, we will spend about $30 per heating season just to heat infiltrated air.

If you set Seasonal Efficiency and 0.6 each to 1, then you get roughly the same value I described earlier, $125.

$30 represents the annual cost for air infiltration and it’s less than 6% of the estimated $510 we will spend annually to heat the entire house (space heating only). We can now compare the cost of heating the infiltrated air per heating season for a number of different ACH50 and CFM50 values.

ACH50 / CFM50 0.6 / 137 1 / 245 2 / 469 3 / 704 4 / 939 5 / 1174
Annual cost ($) to heat infiltrated air 17 30 60 90 120 150

In fact, these numbers should be even less because I’ve included the basement volume in these calculations. The basement is not a heated space but it tempers the temperature in the house.

If we can hit a target of roughly 1 ACH50 we will pay 20% of the cost of an EnergyStar house with 5 ACH50.

You can also compare the different values based on Energy Climate Factor.

Another way to think about it, our cost is roughly proportional to the amount of air leaked in a blower door test, 6% leakage equals 6% cost of our projected annual heating bill. Saving 6% per year is a lot better than the interest I’m earning on my other investments, and those savings increase as the price of electricity goes up.

Just remember, 6% in this example is based on the expected performance of our house (insulation values, % of window area, orientation, climate, exposure, etc.). A house with lower insulation values, and a warmer climate will find a different % of savings by building a tighter house.

How tight is tight enough?

As the snow piles up outside, I have turned my thoughts to Spring. Not only because it’s warmer and nature is waking from it’s long slumber, but also because I hope we will be conducting our first blower door test by then. Specifically I’ve been considering what type of results I should be expecting from the blower door tests, and how to interpret them.

We made the decision early on to build an air tight house. This guided our choice of building materials, Zip system and tape for the exterior sheathing, gaskets and acoustical sealant to seal all other connections, and foam to seal gaps at the rims, windows and doors. The blower door test will help us seal any gaps we missed. But what target should we aim for? How do we know when the house it tight enough?

There are a few standards that define acceptable infiltration levels in a way that can be accurately verified by a blower door test. The PassivHaus standard requires <= 0.6 ACH50. Energy Star 2.0 requires <= 5 ACH50, and Energy Star 2.5 and 3 require <= 4 ACH50. The building code defines prescriptive methods to control infiltration rather than specify testable targets.

So why the big range from 0.6 to 5? And what is ACH50?

The range is due to the different goals of the organizations behind these standards. PassivHaus is a non-governmental organization started in Germany. They set the infiltration standard at the lowest level they thought was easily attainable in residential construction as part of a strategy to reduce overall energy use of the home to very low levels and protect the enclosure from moisture that is transported by the infiltrated air. EnergyStar is a governmental organization that works with the construction industry to set standards. They set targets they think can be achieved by builders over a period of time without upsetting a large population of their constituency. The code is the least common denominator.

ACH50 is Air Changes per Hour at 50 pascals. Basically air is blown into (or out of) the house up to a specific pressure, 50 pascals (very low pressure, 0.00725 PSI), and then see how hard the fan has to work to keep it at the same pressure. From this they can calculate the cubic feet per minute at 50 pascals, or CFM50, of air that is replacing the air that was sucked out at this standard pressure.

To find ACH50 which is in hours, multiply CFM50 * 60 then divide by the House Volume. ‘House volume’ is rarely defined and sounds rather vague and open to creative interpretation. Is it the exterior dimensions and the building height? Or is it the interior dimensions? Do you include the basement?

Because house volume is the divisor, the bigger the value the better your resulting ACH. Which leads me to believe that people often fudge their volume by using the outside dimensions instead of the interior volume which can be quite a difference when your walls are 12 inches thick. PassivHaus even subtracts the interior floor and wall volumes (gotta love German precision).

As for the basement, if the air barrier separates the house from the basement then don’t count it. In our case, the air barrier continues down the foundation wall and under the slab, so I have including the basement volume. This decision has several consequences. Our ACH value will be lower than if we just use the house volume area, and it will cause inaccuracies later when calculating the cost of infiltration since the basement is not heated directly. I think this is why PassivHaus generally recommends building on slabs, not basements.

Let’s look at an example. If our blower door test in the Spring reveals a value of 240 CFM50, then we could end up with the following ACH values depending on how you define ‘house volume’.

240 CFM50 Exterior Interior Interior minus
interior walls
and floors
Dimensions 32’ x 22’ x 23.5’ 30’ x 20’ x 17’ +
28.5’ x 18.5’ x 8.5
trust me
Volume (ft3) 16,544 14,682 13,697
ACH50 0.87 0.98 1.05

Listing the different standards we can see the range of allowable CFM50 values.

Standard Max ACH50 allowable Max CFM50 allowable
PassivHaus* 0.6 137
Uphill House Target** 1-2 245-489
Energy Star 2.5 and 3** 4 979
Energy Star 2.0** 5 1,224

* 13,697 ft3 = Interior volume (including basement), minus interior wall and floor volumes.
** 14,682 ft3 = Interior volume (including basement).

Not only is volume vague, why use volume at all? What does the volume of air in your house have to do with the air tightness of your envelope? Another option is to use surface area of the envelope or building shell instead of volume. See EarthCraft House Guidelines to calculate surface area. EarthCraft recommends a target of <= 0.5.

Marc Rosenbaum also likes specifying air leakage using the surface area because it is not biased towards larger volume homes. He refers to this measurement as CFM50/ssf or shell square footage, and sets a minimum target of <= 0.05 CFM50/ssf for super-insulated homes. (See his presentation on the Efficiency Vermont site.)

Using Rosenbaum’s recommendations the resulting CFM50/ssf are:

4,189 ssf* Super-insulated Conventional
CFM50 200 400
CFM50/ssf 0.05 0.10

* Our shell square footage is 4,189. Slab and ceiling (32 x 22 x 2) + N/S walls (32 x 25.75 x 2) + E/W walls (22 x 25.75 x 2).

Considering the vagaries of ACH, the air leakage ratio (CFM50 / ssf) should be open to less creative interpretation. But there is a similar standard that only counts surface area above ground. See the MLR – Minneapolis Leakage Ratio definition. They also state that a value < 0.5 is good and > 1 is bad, same as EarthCraft, yet they count substantially less surface area, pushing up the allowable air infiltration.

Comparing the PassivHaus and Energy Star standards with Rosenbaum’s recommendations, I’m inclined to shoot for a target range of 200 to 300 CFM50.

So far we have only considered the various standards for air tightness, not the resulting costs of infiltration at various rates nor the additional costs of constructing a tight house.

Next week I’ll post some of the numbers I have been working on to quantify the cost of infiltration. Toward the end of the project I will try to quantify our cost of building a tight house.

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