For nearly two decades, the building industry has pursued a simple objective: Reduce annual energy demand. Produce renewable energy onsite. Achieve Net Zero Energy.

But what if annual net-zero energy does not mean annual net-zero carbon?

In our recent paper published in Building and Environment, we tested a fundamental assumption behind many Net Zero Energy Buildings (NZEBs): Does balancing annual electricity consumption with annual renewable electricity generation automatically result in zero operational emissions? The answer is no.

Using hourly dynamic grid emission factors across major G7 cities, we found that identical net-zero energy buildings can exhibit markedly different carbon outcomes depending on when electricity is consumed and produced.

📌 Key finding:

A building can be net zero in energy while still generating operational carbon emissions. Why? Because electricity grids are dynamic. The carbon intensity of electricity changes every hour depending on the generation mix, renewable availability, storage deployment, imports, exports, and fossil fuel backup generation. Our analysis showed that conventional annual emission accounting can significantly misrepresent building performance. In some locations, static annual carbon factors underestimated operational emissions by more than 100%. In others, they overestimated emissions by a similar magnitude. This has important implications.

As Europe accelerates electrification through heat pumps, district energy systems, electric mobility, and renewable integration, annual energy metrics alone are becoming insufficient. The next frontier is no longer "How much energy does a building use?" but rather "When does a building use energy?" "How carbon-intensive is the grid at that moment?" "Can the building shift demand to cleaner hours?" This is exactly why concepts such as

✔ 24/7 Carbon-Free Energy ✔ Hourly Carbon Accounting ✔ Demand Flexibility ✔ Grid-Interactive Efficient Buildings ✔ Building Energy Storage ✔ Vehicle-to-Building Integration

are rapidly gaining attention across Europe and North America. The future low-carbon building will not simply be energy efficient. It will be carbon-aware. It will respond to real-time grid conditions. It will store energy when carbon intensity is low and avoid consumption when carbon intensity is high. In other words, the transition from Net Zero Energy Buildings to Net Zero Carbon Buildings requires moving from annual performance metrics to hourly carbon intelligence. The future of net-zero buildings is not balancing annual kilowatt-hours, but matching energy use to low-carbon electricity every hour of the year.

📌 This paper was developed by Deepak Amaripadath, David Sailor, Aurora Luigia Teresa Bertini, Mike Barker, Shady Attia 阿提亚 沙帝, S. (2025). Are net zero energy buildings necessarily also net zero emission buildings? Time-integrated analysis using dynamic grid emission factors. Building and Environment, 283, 113367.

📘 Full article: https://orbi.uliege.be/handle/2268/334041

📚 Learn more about our research https://www.sbd.uliege.be/

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