Does a Passive House Really Save 90% on Energy Bills? Here Is What the Data Actually Says.
The 90% energy saving claim is everywhere in Australian Passive House marketing. It is also, in the way it is usually stated, misleading. Not because Passive Houses do not perform. They do. But because the number comes from a comparison that most people quoting it do not understand, and using it without context does more damage to the credibility of certified Passive House construction than any critic ever could. Here is the honest version.
A certified Passive House in Melbourne does not automatically save 90% of your total household energy bills. It does reduce space heating demand by 85 to 90% compared to a standard Melbourne home. Those are not the same thing. Space heating is one part of your total energy use. The 90% figure is real, it is just frequently misapplied. The correct claim, grounded in certified PHPP data from real Melbourne projects, is that a certified Passive House requires between 9 and 15 kWh per square metre per year for space heating, compared to 80 to 120 kWh/(m²a) for a standard Melbourne home. That is where the 90% comes from. It is accurate and it is significant. It is just not your whole power bill.
Carland Proof Point: The Forrest Passive House in Spotswood achieved a certified heating demand of 13.1 kWh/(m²a), compared to a Melbourne average of approximately 80 to 120 kWh/(m²a) for space heating. That is an 85 to 90% reduction in space heating energy, physically verified by blower door test and independently certified by Detail Green in March 2024.
Where did the 90% figure actually come from?
The 90% figure originates from research by the Passive House Institute in Germany in the late 1990s and early 2000s. At that time, the average heating load of existing German homes was approximately 100 W/m². The Passive House Institute set a certification limit of 10 W/m² for heating load. The reduction from 100 W/m² to 10 W/m² is exactly 90%.
The comparison being made was between a new certified Passive House and the average existing German home of that era. Not a new home built to modern code. Not an architecturally designed passive solar home. The average existing German stock, which was built largely in the post-war period with minimal insulation and no airtightness consideration.
The Australian Government's Your Home guide notes that research by the Passive House Institute has shown a 50 to 90% reduction in energy demand compared to local minimum building-code-compliant buildings in Europe, North America and China, but that a similar analysis has not yet been undertaken in Australia. That is an honest and accurate statement. The 90% figure has not been validated against Australian building stock with Australian data in a peer-reviewed form. Using it as though it has is intellectually sloppy, and it gives genuine critics of Passive House certification an easy target.
The Carland Constructions position is straightforward. Stop using the 90% figure as a marketing headline. Use the actual PHPP certified numbers instead. They are more impressive, more defensible, and more honest.
What does the data from certified Melbourne projects actually show?
Across the five certified Passive House and EnerPHit projects delivered by Carland Constructions in Melbourne's inner suburbs, the space heating demand results from the certified PHPP post-construction files are as follows:
| Project Name | Suburb | Type | Space Heating Demand | Passive House Limit |
|---|---|---|---|---|
| Pigeon Passive House | Yarraville | New Build | 9.0 kWh/(m²a) | 15.0 kWh/(m²a) |
| Forrest Passive House | Spotswood | New Build | 13.1 kWh/(m²a) | 15.0 kWh/(m²a) |
| Champion Passive House | Williamstown | New Build | 13.8 kWh/(m²a) | 15.0 kWh/(m²a) |
| Parade Passive House | Ascot Vale | New Build | 14.8 kWh/(m²a) | 15.0 kWh/(m²a) |
| Retrofit Rifle EnerPHit | Williamstown | Retrofit | 31.5 kWh/(m²a) | 30.0 kWh/(m²a)* |
The Passive House Classic certification threshold for space heating demand is 15 kWh/(m²a). The EnerPHit threshold for a retrofit is 25 kWh/(m²a) under the component method, with 30 kWh/(m²a) permitted under certain conditions. Every one of these projects meets its respective threshold. Every one was physically tested with a blower door after construction and independently certified.
A standard Melbourne home built to NCC minimum requires somewhere between 80 and 120 kWh/(m²a) for space heating, depending on orientation, insulation specification, window area, and airtightness. The average Melbourne home measures 19 ACH50 for airtightness (CSIRO and the Australian Building Codes Board, 2016). An NCC-minimum new home is better than that average, but not dramatically so.
The reduction in space heating demand between a certified Carland Constructions Passive House and a standard Melbourne new build is approximately 85 to 90%. That figure is real, it is data-backed, and it is drawn from certified PHPP files and blower door results, not marketing copy.
How much energy does a Passive House save in Melbourne? A certified Passive House in Melbourne reduces space heating demand by approximately 85 to 90% compared to a standard new build, based on certified PHPP data. The Forrest Passive House achieved 13.1 kWh/(m²a) versus an estimated 80 to 120 kWh/(m²a) for a standard Melbourne home.
What about total household energy? Is that where the claim falls apart?
Space heating is one part of total household energy consumption. The other parts include hot water, appliances, lighting, cooking, and plug loads. A certified Passive House does not automatically reduce your Netflix habit, your dishwasher cycle, or your hot water use. Total household energy consumption depends on occupant behaviour as much as building performance.
This is the legitimate critique of the 90% headline figure when applied to total energy bills. If a household uses 8,000 kWh per year in total, and 2,000 kWh of that is space heating, reducing space heating by 90% saves 1,800 kWh. That is roughly 22% of the total bill, not 90%.
The honest answer is that a certified Passive House dramatically reduces the energy required to maintain thermal comfort. It does not eliminate appliance loads, hot water energy, or cooking energy. What it does do is create a building that maintains stable indoor temperatures with minimal active heating and cooling input, which means the heating and cooling portion of the energy bill approaches zero for most of the year.
In the Forrest Passive House, the total annual heating energy requirement is 1,958 kWh. For context, that is roughly what a standard 60-litre electric storage hot water system uses in a year. The entire space heating requirement of a 149 m² certified Passive House for a full Melbourne year costs approximately as much to run as keeping a single conventional hot water system alive. That is not 90% of the total bill. It is still an extraordinary result.
Does a Passive House reduce your total energy bill by 90%? No. A certified Passive House reduces space heating energy by approximately 85 to 90% compared to a standard Melbourne home. Total household energy savings depend on occupant behaviour and appliance loads, which a certified building standard cannot control. The space heating saving is real, verified, and significant. The total bill saving will vary by household.
What about passive solar design without certification? Is that just as good?
This is the argument that deserves the most direct response, because it sounds reasonable and it is the one most often made by practitioners who deliver good work but stop short of certification.
The argument goes like this: a well-designed passive solar home, built with quality insulation, good windows, and thoughtful orientation, can achieve comparable performance to a certified Passive House without the cost and overhead of certification. Why pay for the standard if the outcome is similar?
The answer is simple. You do not know if the outcome is similar until you test it.
NatHERS models a building at design stage and issues a star rating based on that model. It assumes the insulation was installed correctly. It assumes the windows were installed to specification. It assumes the airtightness is somewhere in the range of what was intended. It does not verify any of these things after construction. It does not do a blower door test. It does not check whether the thermal bridges were detailed correctly. It does not confirm that the building performs the way the model predicted.
The average Australian new home built to a 7-star NatHERS rating measures approximately 5 to 10 ACH50 on a blower door test, according to available Australian airtightness data. The NatHERS model assumed something considerably better. The gap between the modelled performance and the actual performance exists in almost every home built without post-construction verification, and nobody ever finds out because nobody ever tests it.
A certified Passive House is tested. The Forrest Passive House was tested mid-construction before the linings went on, so any leaks could be found and fixed while they were still accessible. It was tested again after construction. The result was 0.41 ACH50. The model predicted it would meet the 0.6 ACH50 threshold. It did. Comfortably.
The question is not whether good passive solar design can produce a good home. It can. The question is whether you can prove it performed the way it was designed without testing it. You cannot. Certification is not the goal. Verified performance is the goal. Certification is how you prove it.
Is passive solar design as good as a certified Passive House? A well-designed passive solar home can achieve excellent performance, but without post-construction blower door testing and independent verification, there is no way to confirm whether the finished building performs as designed. Certified Passive House construction requires physical testing after construction and independent third-party verification. Passive solar design does not.
What should you actually ask when someone makes a performance claim about a home?
Whether the claim is 90% energy savings, 7 stars, high performance, sustainable, or anything else, the questions are the same.
Has the finished building been tested for airtightness with a blower door? What was the result? What is the Melbourne average for comparison?
Has the building been independently certified by a third party who checked the PHPP inputs against the actual materials installed?
Is there a post-construction PHPP file showing the certified heating demand, cooling demand, primary energy demand, and humidity exceedance frequency?
Can the builder show you those numbers for more than one project?
If the answer to any of those questions is no, the performance claim is a prediction, not a proof. Predictions are based on models. Models assume the building was built correctly. Nobody checked.
Carland Constructions has five certified projects in Melbourne's inner suburbs. Every one has a blower door result. Every one has a post-construction PHPP file. Every one was independently certified by a third party. The numbers are published because they are the point. Not because they are marketing. Because they are the evidence.
How do I know if a builder's energy performance claims are reliable? Ask for the post-construction blower door result and the certified PHPP heating demand figure. If the builder cannot provide both for at least one finished project, the performance claim is a design prediction, not a verified outcome.
The 90% figure is not the problem. Using it without data is.
The Passive House standard is rigorous, independently verified, and physically tested. The results from certified Melbourne projects are real and significant. A space heating reduction of 85 to 90% compared to a standard Melbourne home is not greenwash. It is the certified outcome of a building system that has been modelled, built, tested, and proven.
The problem is not the number. The problem is quoting the number without the data behind it, without the blower door result, without the PHPP file, without the certification. When those things are absent, the claim is marketing. When they are present, the claim is evidence.
Carland Constructions publishes the evidence because the evidence is the only thing that matters.
If a builder tells you their home saves 90% on energy but cannot show you a blower door result and a certified PHPP file to back it up, ask yourself what their number is actually based on.