Model assumptions and limits

Every energy model makes choices. Nobody knows how often your household opens a window, what each layer of your walls is made of, or what next winter will bring. A model fills those gaps with stated assumptions and gets on with answering its question.

Standards and professional tools work this way too, and they often fix the assumptions on purpose. An energy performance certificate assumes standard occupancy and use of a home, so that different buildings can be compared fairly (the Scottish Government explains it that way). PVGIS, the European Commission’s solar yield tool, starts from a default figure for system losses and lets you change it when you know better (see its user manual ).

EnergyFlow takes the same approach, and this page lists our choices openly. A run is built to compare options for your house under identical conditions: one heat pump against another, shading or none, a bigger battery or a smaller one. For that, it matters that the assumptions are stated and stay the same in both runs. A run is not a forecast of your next bill, and we do not present it as one.

The choices we make

Zones are floors. Each level is one thermal zone, a part of the house the model treats as one block of air with one temperature, with separate zones for spaces that behave differently, such as an unheated garage. This keeps the inputs short, and it means temperatures are floor averages, not room values.
Interiors are not drawn. Partitions and furniture are represented in a simple way, and the wizard does not ask you for them.
Windows are described by area per outside face. All windows share one glass specification: how well the glass keeps heat in (its U-value) and how much of the sun’s heat it lets through (its g-value). Where a window sits along a wall is not modelled. Shadows from trees, neighbouring buildings and roof overhangs are not modelled either. External blinds close when the sun on the window passes a fixed strength and the outdoor air is above 10 °C. Roller shutters use the same sun threshold but close only above 20 °C outdoors, so the rooms keep their daylight on cooler days.
Air exchange is simple. Airtightness, how much air leaks through the shell, is a single value that starts from the construction year, and you can replace it with a measured one. It is reported as n50, the air changes per hour when a blower-door test holds the house at a standard pressure. The stairwell is an opening between floors.
Window opening follows a fixed rule: a steady background flow of fresh air, plus extra airing on summer nights in a house with no cooling. It does not follow what your household does.
The ground is generic. Ground temperature, which sets how much heat the floor loses to the ground, follows one fixed monthly profile, between 7 and 18 °C over the year, whatever the location.
Schedules are part preset. Presence follows one of three daily patterns (when people are home), hot-water use follows the household size, household electricity is a yearly figure with a daily shape, and thermostat settings (the temperatures the heating and cooling try to hold) are day and night values that stay the same all year. A car’s trips are entered day by day for a typical week. Holidays, absences and changing habits are not modelled.
Equipment follows representative performance data for its type and size, not your exact product, and not a particular installation. Two heat pumps of the same size can differ in how fast their output and efficiency fall as the outside air gets colder, which the performance curve on a datasheet describes. The model uses a representative curve for the type.
Weather is a file. A typical year (an ordinary-looking year built from many real ones) comes from the last 15 years at your coordinates and never happened. A real year comes from reanalysis data, a weather model combined with observations from across the world, not from a station next to your house.
Electricity flows are worked out in whole hours, so short peaks inside an hour are averaged. Tariffs are the prices you enter or the ones we hold for your country.

Reading a result with this in mind

Each article in this section ends with the limits that apply to its own topic. Taken together, they mean that a model figure is an estimate for the house you described. Use it to compare scenarios, and to walk into a conversation with an architect, an energy consultant or an installer with better questions. They can check what a model cannot: the site, the products, the regulations and the installation. Before you order equipment or build anything, ask one of them to review your scenarios.