The Passive House savings calculator above compares two versions of the same building: one at a baseline annual heating demand and one at a target you set, typically the Passive House criterion. It converts both into delivered energy through your heating system's efficiency, then into running cost at your tariff and into emissions at your factor. Every rate on this page is an input, because energy prices, grid carbon and construction costs vary by country, by year and by contract, and a published table would be wrong for most readers on the day they read it.
Arb Digital builds free calculators that keep their assumptions in the open. This one deliberately publishes no prices, no assembly build-ups and no U-value tables. It complements the heat loss calculator, which returns an instantaneous envelope heat loss in watts for sizing a heating system, whereas this page works with an annual demand figure to produce a running-cost and emissions comparison. Those are different questions and the two tools do not overlap.
What the Passive House Standard Actually Requires
Passive House, or Passivhaus, is a performance standard rather than a construction method. It sets criteria on the building's modelled performance and leaves the designer free to meet them however they choose. The Passive House Institute's building certification criteria set an annual space heating demand limit of 15 kWh per square metre of treated floor area per year, or alternatively a peak heating load limit, along with an airtightness limit tested by a blower door and a limit on total renewable primary energy demand.
The criteria are met in practice through very high levels of insulation, elimination of thermal bridges, high-performance glazing, careful control of solar gain and mechanical ventilation with heat recovery. None of those is prescribed by the standard, and the balance between them depends heavily on climate. A design that certifies in a cold continental climate is not the same design that certifies in a mild maritime or a hot humid one.
In North America, Phius operates a related but separate certification with climate-specific performance targets rather than a single fixed number. Whichever route applies, the target figure you enter here should come from an energy model of your actual building, produced in the certifying body's tool by someone trained to use it. The 15 in the field is the published criterion, not a prediction about your project.
How to Use It
- Enter the treated floor area. Use the area definition your energy model uses; mixing a sales area with a modelled demand per square metre produces a meaningless total.
- Enter both heating demands. The baseline should come from an assessment of the building as built or as designed, not from a rule of thumb.
- Enter the system efficiency or seasonal COP. This converts heat demand into purchased energy, and it changes the cost answer more than almost anything else.
- Enter your own tariff and emission factor. Take the factor from your grid operator or national agency and note its year, because grid carbon moves annually.
- Enter the extra capital cost if you want a payback. That figure has to come from a real quote or a quantity surveyor. Leave it at zero to see the energy and carbon comparison alone.
The Arithmetic, With the Default Figures Worked Through
Annual heat demand is floor area multiplied by demand per square metre. Delivered energy is that divided by the system efficiency. Cost is delivered energy multiplied by the price, and emissions are delivered energy multiplied by the factor. The saving is the difference between the two cases.
With the defaults: 150 m² at 120 kWh/m²/yr is 18,000 kWh of heat a year; at 15 kWh/m²/yr it is 2,250 kWh. Divided by an efficiency of 0.9, the delivered energy is 20,000 kWh and 2,500 kWh respectively. At 0.25 per kWh that is 5,000 and 625 a year, so the saving is 4,375 a year. At 200 g CO₂e per kWh the emissions are 4,000 kg and 500 kg, a saving of 3.5 tonnes a year. An extra capital cost of 30,000 divided by a 4,375 annual saving gives a simple payback of 6.86 years.
Simple payback is exactly that: undiscounted, with no fuel price inflation, no maintenance, no equipment replacement and no residual value. It is a screening number. The payback period calculator handles uneven cash flows, and any serious appraisal should discount future savings rather than adding them up at face value.
Heat Demand and Delivered Energy Are Different Numbers
The single most common error in this comparison is treating the kWh per square metre figure as electricity or gas purchased. It is not. It is the heat the building needs at the internal boundary, and the energy you buy to supply it depends entirely on the appliance. A resistance heater buys roughly one kWh for each kWh of heat. A condensing boiler buys slightly more than one. A heat pump with a seasonal coefficient of performance of three buys a third.
That has a consequence people frequently miss. Improving the fabric and installing a heat pump both reduce purchased energy, but they do it multiplicatively, so the cost saving from insulation is smaller once a heat pump is fitted than it was with a boiler. Run the heat pump preset and watch the annual saving fall even though the fabric improvement is identical. Neither measure is thereby a bad idea; it simply means the two cannot be appraised independently.
The reverse is also true: a very low heating demand makes small, cheap heating systems viable, which can offset part of the fabric cost. Passive House designs frequently eliminate a conventional distribution system entirely because the peak load is small enough to be met through the ventilation air. That saving belongs in your capital cost field as a reduction, and forgetting it overstates the extra cost of building well.
What This Comparison Leaves Out
Space heating is only part of a building's energy use. Domestic hot water, cooking, lighting and appliances are untouched by this calculation, and in a very efficient building hot water can exceed space heating as an annual load. The water heating calculator covers that separately, and the energy use intensity calculator takes the whole-building view across all end uses.
Cooling is also outside the comparison. In warm climates a highly insulated, airtight envelope changes the cooling load as well as the heating load, and the sign of that change depends on shading, glazing orientation and ventilation strategy. Treating a heating-season saving as the whole story is a temperate-climate assumption that does not travel.
Embodied carbon is not counted either. The insulation, triple glazing, airtightness membranes and heat recovery unit that deliver the operational saving all carry emissions from their own manufacture. Over a building's life the operational saving usually dominates, but the ratio has shifted as grids have decarbonised, and a whole-life carbon assessment is now the honest way to compare two specifications. The emission factor you enter here covers the operational side only, and for a US grid the EPA eGRID database of grid emission rates publishes subregional figures with the year stated.
Comfort, Air Quality and the Things Energy Does Not Measure
The measurable output of this tool is money and carbon, and those are not the main reasons most occupants notice the difference. A well-insulated envelope keeps internal surface temperatures close to air temperature, which removes the radiant chill from windows and external walls and allows the same comfort at a lower air temperature. Draughts largely disappear, because airtightness removes the pressure-driven infiltration that causes them.
Mechanical ventilation with heat recovery is a requirement of the standard rather than an optional extra, and it is what makes airtightness safe. A sealed building without controlled ventilation accumulates moisture and indoor pollutants; a sealed building with continuous filtered supply and extract does the opposite. Airtightness and ventilation are a single decision, and separating them is genuinely hazardous.
Thermal bridge detailing and airtightness are also where designs fail on site rather than on paper. A modelled performance is a claim about workmanship as much as about materials, which is why certification involves a blower door test on the completed building. If your target figure has not been tested, it is a design intention.
Arb Digital builds free tools like this one because useful pages earn attention. If you want tools, calculators or content built for your own audience, we can help.
Browse All Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Treating heating demand as purchased energy — divide by the system efficiency or COP first, or a heat pump's saving is overstated threefold.
- Mixing area definitions — treated floor area, gross internal area and sales area are different, and the demand figure is per unit of the area its model used.
- Assuming a saving applies in any climate — heating demand targets are climate-specific, and a cooling-dominated site is a different calculation entirely.
- Adding thirty years of savings at today's prices — that ignores discounting, fuel price movement, maintenance and equipment replacement.
- Quoting an untested design figure as performance — airtightness and thermal bridging are built, not specified, and certification tests the finished building.
Related Free Tools From Arb Digital
Size the heating system with the heat loss calculator and check fabric with the insulation calculator. Take the whole-building view with the energy use intensity calculator, add hot water with the water heating calculator, and check tariffs with the electricity bill calculator. For on-site generation see the solar panel calculator, and for appraisal the payback period calculator. The full free online tools hub lists everything.
Frequently Asked Questions
The Passive House Institute's building certification criteria set an annual space heating demand limit of 15 kWh per square metre of treated floor area per year, with a peak heating load limit as an alternative route, alongside airtightness and primary energy criteria. Phius in North America uses climate-specific targets instead of a single number.
Because they vary by country, by tariff, by contract and by year, and a published figure would be wrong for most readers. Every rate here is an input so that the result is traceable to assumptions you can defend and update.
Heating demand is the heat the building needs. Delivered energy is what you buy to supply it, which is the demand divided by the efficiency or coefficient of performance of the heating system. A heat pump with a seasonal COP of three buys about a third of the heat it delivers.
Because the fabric improvement and the efficient appliance both reduce purchased energy, and they multiply rather than add. The same fabric saving is worth less in purchased kWh once the system already delivers three units of heat per unit bought. Both measures still reduce energy use.
No. It compares space heating only. Domestic hot water, cooking, lighting, appliances and any cooling load are outside the calculation, and in a very efficient building hot water can be the larger annual load.
No. The emission factor you enter applies to operational energy only. Insulation, glazing, membranes and heat recovery equipment carry manufacturing emissions of their own, and comparing two specifications properly requires a whole-life carbon assessment.
The heat loss calculator computes an instantaneous envelope heat loss in watts from areas, U-values and infiltration, which is what sizes a heating system. This page starts from an annual demand figure and produces a running cost and emissions comparison between two build standards.
This tool is provided for educational and screening use only. It is not an energy assessment, a certification calculation or financial advice, and it makes no claim about what any building will cost or save. A real building must be modelled by a qualified energy assessor or certified Passive House designer using the certifying body's own tool, and capital costs must come from a quantity surveyor or contractor.
