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Using Energy Modeling to Get to Zero

Computerized modeling allows designers to predict performance based on the specifics of design

Computerized energy modeling gives building designers a chance to adjust performance by tweaking structural assemblies, mechanicals, and renewables and arrive at the most cost-effective plan.
Image Credit: Zero Energy Project

A computerized energy model is an essential element of building design for high-performance homes, because it allows the designer to predict the energy performance of a building based on specific site characteristics, structural assemblies, mechanical efficiencies, and renewable technologies. By adjusting these elements of building performance, the designer can choose the most cost-effective combination of features. While models are just predictions — and therefore will never be completely accurate — skillful use of a modeling tool should lead to more affordable, energy-efficient buildings.

That leaves two questions. Just how important is energy modeling for cost effectively designing zero energy homes? And how accurate is energy modeling?

Now more than ever

For several decades, the mantra of high-performance building design was to slash the amount of energy used for the two largest building loads: space heating and water heating. If these efforts are successful, less energy needs to be purchased or generated on-site.

While load reduction is still an essential design goal, the dramatic decline in the price of rooftop solar electric systems creates a new design question: What is the optimal balance among shell measures (insulating and sealing walls, windows, ceilings, etc.), high-efficiency mechanical equipment, and on-site renewable systems? However, there are four factors that come into play in different projects and regions of the country that make answering the question more complicated:

  • A wide variety of interwoven energy measures can be taken to reach net zero;
  • The cost of renewable, especially solar, energy systems is dropping rapidly;
  • Government and utility financial incentives vary in different parts of the country;
  • Local equipment and construction costs change rapidly with local market conditions.

Instead of making a judgment call based on intuition, energy models offer designers quantitative, project-specific feedback and guidance.

The majority of residential buildings fail to take advantage of this tool and rely, instead, on building codes as the sole benchmark of energy performance. Even most third-party certification programs, such as Energy Star, Zero Energy Ready Homes, Living Building Challenge, and Earth Advantage, use energy modeling only for program compliance rather than as an iterative design tool to find the most cost-effective set of energy saving and renewable energy measures for a given project.

Designers and energy consultants can unlock the value of energy modeling as a design tool by modeling different combinations of energy-related features to see which combinations yield the best performance for the least cost. If your goal is to build an affordable zero energy home, then an energy model is an essential planning tool for making the smart decisions needed to design a a cost-effective building.

Modeling programs

There are a variety energy modeling programs to choose from. Since computerized predictions utilizing differing assumptions can never be precisely right, I decided to use two different modeling programs and compare their results with the actual energy usage of my own home.

I selected REM/Rate, software commonly used in Energy Star and many utility programs, and a free online calculator created by Build Equinox called Zero. Making this comparison gave me an opportunity to explore options for designing zero energy homes more cost effectively based on the information from these two models.

My zero energy home has 939 square feet of living space and was built in 2015. It’s superinsulated, very airtight, and uses a minisplit heat pump for heating and cooling. Hot water is supplied by a heat-pump water heater. There is a 4.3 kW photovoltaic system on the roof. My home is in Climate Zone 5 with about 7,000 heating degree days. In order to get the most consistent results from the two programs, I used the same inputs for both models.

The first chart shows results from each energy model compared to the actual energy use of my home.

How does modeling compare to reality?

Neither program closely predicted the performance of the home over these two years. Build Equinox Zero predicted energy use that was 37% above the actual, while REM/Rate’s number was even higher. That’s not surprising. Each year presented a widely different weather pattern. For example, 2016 was relatively mild, while the early months of 2017 were marked by sub-zero temperatures and about 30 inches of snowfall that completely covered the solar panels for several weeks.

Computer models work with long-term weather averages, and so they can’t be expected to match any two-year snapshot. To complicate matters, the climate may be changing faster than predicted.

Furthermore, neither computer model can predict residents’ actual energy-use patterns. In the case of my family, we are probably more frugal in our use of energy that the “average” family assumed in these models, which could in part account for both models overestimating our actual energy use. As in the models, a designer or consultant would certainly want to take a conservative approach. Better-than-expected performance would mean that extra energy could help power an electric vehicle or electric yard equipment.

Of the two programs, Build Equinox Zero came closer to predicting actual overall performance in our case. Its estimate fell into the positive energy category, matching the reality of our home energy use, while REM/Rate was more conservative and predicted that the home would not reach a zero energy level of performance.

Why was neither program very close to reality? To answer that question, it helps to dig into the more detailed reports.

Net energy is the amount generated through solar panels minus the total energy use. So, a negative number means the scenario does not reach zero energy, while a positive number means that the home would generate more energy than it uses.

What was learned

While each energy model produced slightly different results, both models reveal a significant fact about residential energy use in this home. The amount of energy consumed by plug loads, lights, and appliances dwarfs that of space heating, cooling, and water heating. It’s no surprise because this home includes a highly insulated and air-sealed thermal envelope as well as a highly efficient heat-pump water heater and energy-recovery ventilation (ERV) system.

In the design of my home, like most zero energy projects, the efficiency of the thermal envelope, water heating, and lighting was targeted first. So, the next step in improving energy efficiency in a home like mine would be to focus on appliances and plug loads. In fact, research indicates that cable boxes, game consoles, large screens, and other electronics are taking a big bite of the home energy pie each year.

Since selection of appliances and management of plug loads are not generally under the control of designers or builders, one design solution to address this challenge would be to include a home energy management system in zero energy homes.

The results also suggest another approach. With the price of rooftop solar systems declining rapidly, it would be wise to weigh the cost of adding more solar with the cost of other energy saving measures. At some point, adding a few more PV panels may be cheaper than building thicker walls, buying super-efficient windows, or installing energy management systems. Of course, solar electric systems might be limited by more than cost. Solar exposure and available roof area must also be considered. Here again, an energy model can help balance various design issues.

How the modeling programs compare

In the categories of space heating, water heating, and PV production, the two models used were in fairly close agreement. The methodology for calculating these parameters is well established, so the level of agreement makes sense.

One number that jumps off the comparison table, however, is the difference in cooling energy. REM/Rate estimated nine times more consumption for cooling than Build Equinox Zero. While the percentage was very high, the absolute number of kilowatt hours was very low due to the almost insignificant cooling requirement at this location.

REM/Rate’s estimate was closer to reality on solar electricity generation by predicting only 5% more kWh than actually was produced and was very close to the 5,767 kWh that the solar contractor estimated for my solar production.

As mentioned, the category of plug loads, lights, and appliances represents by far the largest energy use. REM/Rate predicted 26% more energy used in this category than Build Equinox Zero. This difference may result from the set of assumptions used by each program.

While total energy consumption varied significantly between the two models themselves and between both models and the actuals, virtually all of these discrepancies can be attributed to the lights, electronics, and appliances category. While the percent discrepancies may seem high, both programs offer useful information for design decision making.

Model evolution

On-site storage batteries and electric vehicles are entering the market in a big way. The next developmental step for energy modeling software should be to integrate these technologies into their calculations. The tight integration of solar, storage, and transportation, along with the usual energy saving measures, will be essential to reach zero energy/zero carbon in the residential sector.

As momentum is growing to move the housing industry to zero energy for all new construction, building cost-effective zero energy homes is crucial. Energy modeling will be a key step in achieving this goal. Based on my comparison of actual energy use in my home with these two different modeling systems, the modeling program selected may be less important than the practice of using energy modeling in an iterative manner to optimize performance and affordability of zero energy homes.

This post originally appeared at Zero Energy Project.


  1. lance_p | | #1

    Other Software?
    So I wonder how these two modeling programs, REM/Rate and Build Equinox Zero, compare to programs like BEOpt and EnergyPlus? Bruce, do you have your actual heating energy use information to compare just your heating loads to the model predictions?

    I used BEOpt to model our upcoming build and it sure would be nice to know if the program overshot the actual energy use by 67%!

    One thing that puzzles me about BEOpt is equipment load sizing. The hourly cooling load models visible in DView (the graphical output of BEOpt) indicate my peak cooling load to be less than one ton (12000 Btu/hr), yet the program recommends a two ton unit at just over 24000 Btu/hr? Does anyone have any insight into this?

    Lance Peters

  2. Expert Member
    RICHARD EVANS | | #2

    Solar Heat Gains
    To what extent do these modeling programs factor in solar heat gain through windows? I know that with PHIUS, "shade studies" are needed to help quantify that information. If these gains are omitted in REM/rate, etc, I suspect this would contribute to the estimations being higher than actual use.

  3. Jon_R | | #3

    > ... to reach zero
    > ... to reach zero energy/zero carbon in the residential sector.

    Zero energy and zero carbon are quite different things. I'd focus on the latter and treat the former as irrelevant.

  4. calum_wilde | | #4

    Jon R
    Ok. Seems like a great

    Jon R

    Ok. Seems like a great point. But how do we do that?

  5. Jon_R | | #5

    net zero carbon
    A good start would be to stop talking about "net zero energy". And to start updating tools to account for carbon (even if, for now, we often have to simplify and assume some fixed carbon/kwh).

  6. GBA Editor
    Martin Holladay | | #6

    Response to Jon R
    There are so many uncertainties in current methods of carbon accounting that I am a skeptic as to whether a discussion of "going net zero carbon" is useful. In most cases, it isn't.

    More discussion here: Carbon Emissions By the Construction Industry.

  7. Expert Member
    Dana Dorsett | | #7

    Wwith supplier choice it can be ZERO carbon/kwh @ Jon_R
    In many "decoupled" electricity states the delivery and energy supply are separated, and the ratepayer has the option to contract with particular supplier or electricity broker. I happen to currently be on a 1-year contract for 100% wind power through a broker, at a price LESS than the standard offering through my utility. (I had previously done a 3 year contract for 100% renewables which also came in slightly less cost than the standard mix through the utility over that time, which had spiked in price due to gas pipeline constraints from the Polar Vortex week of 2014 and the heavy reliance on natural gas fired generation for both peak and baseload power in the ISO New England grid region).

    So by some carbon accounting methods the fixed carbon/kwh constant is zero, making all my electricity use net zero carbon.

    By other carbon accounting methods it's not even close to net zero carbon, given that I'm hooked up to the ISO-NE grid, where roughly half of all Twh shipped per year on the transmission grid has carbon in the source fuel. The quasi real-time mix gets updated every 5 minutes here, but does NOT include behind the meter exports of PV onto the grid, only the utility scale PV output (that the grid operator can actually "see"?) :

  8. Expert Member
    Michael Maines | | #8

    Rick, I use the PHPP and
    Rick, I use the PHPP and BeOpt. BeOpt does include some controls over solar gain--nowhere near the level of the PHPP, but reasonably good.

    I agree with Jon that zero carbon is what is really important--that's a mix of embodied energy and operational energy. If you can get to zero energy and minimize embodied energy, that's a good start while the research and technology catch up to make getting to zero carbon as easy as it is to get to zero energy today.

  9. Jon_R | | #9

    > contract for 100%
    > contract for 100% renewables

    I do this too - it's a good/green thing - better than many things that would increase my energy efficiency or lower net kWh usage.

    Neighbor A is building a near net zero energy house. Neighbor B is building a code minimum all electric house and will pay for 100% net renewable utility power. Which is the greener neighbor?

  10. mdahlhausen | | #10

    net zero carbon
    Thanks to Jon R for pointing out that net zero energy and net zero carbon are different things.
    Just putting solar on your home doesn't make it net zero carbon. It matters what the grid composition is hour by hour. A good way to see this is the system visualizations of the Cal ISO grid (electric grid in most of California): You can see how emissions change over the course of a day. A typical residential electric use profile peaks in the morning and evening when there isn't much solar capacity available. That means less renewables and more gas/fossils in the supply mix, meaning overall grid emissions per unit energy are easily double during those hours. To seriously consider emissions, you want to use hour-by-hour emissions factors. There are some completed and ongoing projects to calculate these, e.g.

    As a general example, say a typical home uses 75% of its energy in the morning and evening when the emissions rate is an average of 250 grams CO2/kWh, 25% of it's energy during the day when the emission rate is an average of 150 grams CO2/kWh, and produces 100% of its energy during the day when the grid emissions rate is an average of 150 grams CO2/kWh. If the home uses 10 kWh in a day, that means total emissions are (0.75*10 kWh*250 g CO2/kWh) + (0.25*10 kWh*150 g CO2/kWh) = 2.25 kg CO2, minus (1*10 kWh*150 g CO2/kWh) PV offset = 1.5 kg CO2, for a total of 0.75 kg CO2. So you've reduced site energy use by 100% but carbon emissions by only 67%.

    Ideally, you'd like to design your home so that the electric use profile aligns with the solar PV production profile as much as possible, even using batteries or smart controls on your water heater etc. to move electric use to peak solar / lowest emission hours. For a grid with a large percent of solar PV, it may even make sense if you are under a time-of-use tariff and care about emissions to orient your panels to be west-facing to offset more of the higher-emissions grid mix later in the day.

  11. GBA Editor
    Martin Holladay | | #11

    Response to Jon R (Comment #9)
    Q. "Neighbor A is building a near net zero energy house. Neighbor B is building a code minimum all electric house and will pay for 100% net renewable utility power. Which is the greener neighbor?"

    A. Everyone will answer the question differently, because there is no clear answer to the question.

    To many observers, programs by utilities to sell their customers "100% renewable energy" are fraudulent programs based on accounting tricks. If your utility tells you that all of your energy comes from PV and wind, and your house requires electricity on a windless night, the utility is going to deliver you electricity that was generated by a fossil-fuel plant -- because the sun isn't shining and the wind isn't blowing.

    If you are an environmentalist who wants to reduce CO2 emissions, live frugally in a small, energy-efficient house or apartment building, and use as little energy as possible. Installing a roof-mounted PV system (if you own the roof over your head) is probably a good idea.

    It's also a good idea to be humble. I advise environmentalists to avoid boasts like "my lifestyle is carbon-neutral" or "my house is net-zero-carbon." Unless you are living in a small off-grid village in Africa, your lifestyle probably isn't carbon-neutral.

  12. Jon_R | | #12

    > fraudulent ... all of your
    > fraudulent ... all of your energy comes from PV and wind, and your house requires electricity on a windless night

    I agree, but note that a net zero energy house suffers from the same issue (although using clearer terms).

    I don't think that tracking when energy is used and emission factors is all that hard - but it does require updates to some tools. And more awareness of the importance of it.

  13. Robert Opaluch | | #13

    Choosing 100% renewable energy from utilities
    Although technically the energy you use on a windless night might be from fossil fuels (even tho you specified 100% renewable energy from your utility), the utility would be getting more power from renewables overall, so it helps to select a renewable energy option from the utility.

  14. Expert Member
    Dana Dorsett | | #14

    There is no such thing as a windless night. @ Martin
    Electricity brokers selling 100% wind aren't tied to a particular wind farm at a particular location- they are trading PPAs from numerous wind farms (some nationwide). The wind is always blowing somewhere, just as the sun is always shining somewhere, even in the US. But getting PV or wind power from a facility in Guam to your house on St. Thomas could be a bit tough, eh? ;-)

    Even the notion that wind blowing in North Dakota at night is heating an all electric house in Maine on a night when it's calm in Maine is more than just a bit of a stretch, since the transmission connectivity between the MISO and ISO-NE grid operator regions is effectively nil, and crosses more than one independent grid operator boundary.

    In MA there are "green-up" options where the power is purchased via the utility (usually at a mark-up from their standard mix offerings), but there are also third party supplier options (what I've been doing) where the contract is through a (state regulator vetted) electricity broker, which is a competitive marketplace where it can often beat the standard mix pricing. By statute the utilities are not allowed to mark up their cost of power purchased from generators- they have to pass it through at cost, and are not allowed to have long term power purchase contracts, but have to solicit bids from their supplies every 6 (?)months. Electricity brokers do not have those constraints, and 100% renewables is often cheaper than even the standard mix offered by the utility (let alone the standard mix price plus a green-up cost-adder.) The state even operates a website for comparative shopping:

    Taking for instance ZIP code 01803 (where my office happens to be located) the standard mix for the energy portion of the bill is 12.888 ¢/kWh, but there are multiple 100% renewables suppliers selling fixed price term contracts for as low as 10.600 ¢/kWh (for a 6 month contract), and most (but not all) beating the standard utility price even on 1-year or 3 year contracts:

    Mind you, it's important to read the contract details- there are often cancellation fees and automatic re-subscription at an undisclosed price at the end of the term, which has led to quite a bit of abuse of the unwary. The company I did the prior 3 year contract at 12.99 cents had both a steep cancellation fee and automatic renewal unless one took action at the end of the term to opt out. The new-improved rate for the auto-renewal would have been 17.99 cents, with a $150 cancellation fee. Just one month on the new contract + cancellation would have more than wiped out any savings over the prior 3 years. There is some movement afoot at the state attorney general's office to put limitations on that sort of abuse, but also a (misguided IMHO) proposal to get rid of the option for third party electricity sales altogether. This type of business can be regulated by the state (and it probably needs to be tightened up .)

    In MA the utility's only part in third party electricity sales is simply the metering & billing- they aren't purchasing that power and reselling it to the ratepayer (the way they do their standard mix or green-up mix), and are not allowed to count third party green power against their state mandated minimum renewables mix. These sorts of things vary by state (a LOT)- there is no single model for retail electricity sales are regulation in use, so clearly YMMV.

  15. GBA Editor
    Martin Holladay | | #15

    Response to Dana Dorsett
    With every passing year, we're getting closer to a future in which "the wind is always blowing somewhere." But we're not there yet.

    In the meantime, as you report, "the notion that wind blowing in North Dakota at night is heating an all electric house in Maine on a night when it's calm in Maine is more than just a bit of a stretch."

  16. Expert Member
    Dana Dorsett | | #16

    Yes, it's evolving pretty fast! @ Martin
    Some of the transmission line projects for shipping large amounts of wind eastward from the upper midwest are coming, as well as other transmission lines from the grain belt to the southeast, hooking up to the TVA regional transmission grids.

    Whether building ever more transmission line infrastructure will prove more cost effective than offshore wind for New England is an open question. Neither is very cheap right now, but wind is on a double-digit learning curve. In the lease areas off MA the wind really is always blowing, it's only a matter of how hard. Estimates of wind farms south of Martha's Vineyard operating at ~60% capacity factors may prove to be too conservative, since the technology is still evolving rapidly, with year on year gains in capacity factor along with year on year falling costs. Some of the 10Mw class turbines currently being tested off Scotland (they're "only" 8.8Mw, actually) produce as much power in one rotation as the average house in the UK uses in a day, and are expected to bring a quantum-step reduction in the levelized cost of power from offshore wind. The state mandated offshore wind projects in MA are still in the bidding phase, and are likely to come in far cheaper than even the most aggressive cost estimates being discussed just 3 years ago.

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