Electrification, Decarbonization, and the Leapfrogging of Zero Net Energy

California’s New Path to a Low-Carbon Future

In February of 2018, the city of Palo Alto released what I regard as a groundbreaking report, Buildings Baseline Study and Roadmap for Zero Net Energy Buildings. This report, while valiantly attempting to adhere to the state’s previously outlined Zero Net Energy (ZNE) goals, quietly subverts them and subtly reveals our more urgent priority of reducing carbon emissions.

Palo Alto’s report arrived one month before the California Energy Commission’s (CEC’s) proposed updates to the 2019 Title 24 Building Energy Efficiency Standards were announced. This CEC announcement made headlines nationally due to a new requirement to include solar panels for all residential buildings by 2020. However, more significant than the bling of the required PV was a careful adjustment from an earlier push toward ZNE buildings to a beyond-ZNE target.

Courtesy of One Sky Homes

Figure 1. Palo Alto Utilities Data 2016 (inside pie chart), with Residential Appliance Saturation Study 2009 (RASS) and California Commercial End-Use Survey 2006 (CEUS) used to determine end use breakdown by building sector. Source: DNV-GL.

This nimble leapfrogging over ZNE targets was easy to miss, as it was almost apologetically tacked onto the very end of the FAQ, disguised under the subtitle Do the 2019 Residential Standards Get Us to Zero Net Energy? The answer concludes: “Looking beyond the 2019 standards, the most important energy characteristic for a building will be that it produces and consumes energy at times that are appropriate and responds to the needs of the grid, which reduces the building’s emissions.”

Learning from Palo Alto

There’s much to unpack in these two documents. The Palo Alto study, based as it is on measured data supplied by the municipally owned utility, gives a detailed look at current building energy use—and provides good insight into which policies would most impact the building sector’s carbon emissions. An introductory graph illustrating overall source energy use for Palo Alto’s buildings shows that natural gas accounts for just over half of total energy use. Figure 1 breaks Palo Alto’s building energy use down into its end uses, showing commercial buildings consuming the lion’s share of this energy.

While Palo Alto may not be average in terms of cost of living, its built environment could certainly be considered typical. For this reason, this report provides good generalized insight into the energy use of Every City, California.

The Big Reveal

By far the most eye-opening information exposed in this report is in the bar chart graphs illustrating annual building energy use, electricity use, and carbon emissions (see Figure 2). Applying the frameworks of Zero Net Carbon, Zero Net Energy, and Zero Net Electricity to these graphs forces us to ask: What is the target we’re really aiming for with our buildings?

Figure 2. Summary of Building Energy Usage (Total BTU). Source: Data compiled from RASS and CEUS data by DNV-GL.

This is precisely where the CEC’s move away from ZNE in favor of a carbon reductions focus starts to make sense. This graphic comparison enables us to see that if we look only through the total Electricity Use lens (right side of Figure 2), we’ll be led toward heavily promoting daylighting and lighting efficiency measures, which will probably do little to reduce carbon emissions.

The total Energy Use (middle) lens, the basis for ZNE, would encourage a broader distribution of possible policy incentive options, with improved impact on carbon emissions reduction. However, it’s clearly the total Emissions lens (left) that we simply cannot ignore. This shows—even in the mild climate enjoyed by Palo Alto—that the highest building carbon emissions are from energy used for space heating,1 with water heating following closely behind. This means that if we focus only on reducing these two end uses, we’ll significantly reduce our carbon emissions from buildings.2

So how do we achieve this most effectively?

Fortunately for us, the CEC’s FAQ already has much of this covered. Special mention is made of the new residential standards encouraging “demand responsive technologies including battery storage and heat pump water heaters.” (PDF) This is big. It signals a clear move toward the electrification of buildings—a big step for California, where gas has been the fuel of choice for many years. With this combination of electrification, storage, and heat pump technology, hot water energy use will mostly be covered by renewable energy.

Less clear in the CEC’s FAQ is a solid plan for how California will be reducing building space heating demand. Not too coincidentally, space heating demand happens to be a particular specialty of the Passive House standard, which makes it a great approach for meeting the CEC’s policy goals to encourage buildings that best match renewable-energy production capacity, place least strain on the grid’s resources, and quickly and substantially reduce building emissions.

Targeting Loads

To illustrate how the Passive House approach plays out in a reasonably typical residential building situated close to Palo Alto, let’s take a look at the measured energy use of a home I designed with One Sky Homes. This single-family home has a treated floor area of 2,342 square feet, R-28 walls, an R-46 roof assembly, windows of R-3.3, a floor slab of R-14, and an airtightness reading of 0.3 ACH50. It’s an all-electric home, utilizing heat pump technology for both hot water and space conditioning, and boasts a 7.5-kW PV array installed on the south-facing roof, which powers both the house and an electric vehicle.

Time of Use Matters

The daily energy use of this house has remained remarkably stable, with a net annual use well into plus energy territory. However, the bigger picture of energy use versus generation provides the most insight here, and points to the same conclusions for energy use priorities as those in the Palo Alto report’s bar graphs comparing total Electricity Use, Energy Use, and Emissions.

A full year’s worth of daily outdoor temperature, electric usage, and energy production (see Figure 3) shows that without battery storage, this house-and-EV package still requires a utility. Even in our sunny California climate, despite an incredibly low overall demand (and an electric vehicle), the 7.5-kW array is unable to meet all of its energy needs between the months of November and February. (Imagine how much larger the wintertime gap between production and demand would be if the house were not so well insulated and airtight!) This not only means that time of use matters, but that seasonal use matters. As the Palo Alto report corroborates, our most critical variable is in fact winter space heating demand, which cannot be covered by short-term battery storage.

Figure 3. Alamo Passive House Total energy demand vs energy production, courtesy of One Sky Homes.

Refocusing on Emissions Reductions

Given the information shared in the Palo Alto report, and bolstered by the data collected on the One Sky home, I’m encouraged by the CEC’s shift away from total building energy use to carbon emissions reduction. Piecing all this information together, it’s clear that aligning our energy code with the targets set by the Passive House standard would help California to meet our long-term carbon emissions reductions goals.

—Bronwyn Barry

Bronwyn Barry is a registered architect and a Certified Passive House Designer.


Herrschaft, Blake, and Betty Seto. Buildings Baseline Study and Roadmap for Zero Net Energy Buildings. Palo Alto, CA: City of Palo Alto Development Services, February 2018.

California Energy Commission.2019 Building Energy Efficiency Standards, Frequently Asked Questions. California Energy Commission, March 2018.

California Energy Commission. 2016 Building Energy Efficiency Standards for Residential and Nonresidential Buildings. California Energy Commission, June 2015, 255–56


1 Space heating demand is also the highest energy user in commercial buildings, challenging the commonly held notion that commercial buildings in this region are cooling load dominated.

2 Early projections indicate that this same approach applies equally to our warmer southern Californian climates, where summer peak load reduction offers the same opportunities for full electrification.


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