The simulation engine

Every EnergyFlow run is calculated by EnergyPlus, the building energy simulation engine developed by the U.S. Department of Energy. We use EnergyPlus version 25.2.0 software and keep that version fixed, so the same house always gives the same answer.

What EnergyPlus calculates

EnergyPlus follows your house hour by hour through a full year of weather. For every floor it keeps account of the heat going in and out: through the walls, roof, floor and windows, with the sun coming through the glass, with the air that leaks in or is ventilated, and into and out of the building materials themselves. It also works out what your heating, cooling, ventilation and hot-water equipment does in response, and how much electricity or fuel that takes.

Inside each hour it takes four steps of 15 minutes. A house with a ground-source heat pump, or one where the heat pump or boiler also heats a hot water cylinder, takes twelve steps of 5 minutes instead. We keep hourly results: 8,760 values a year for the temperature of every floor, the heat and cooling delivered, and the energy the equipment uses.

You never write an EnergyPlus file. The wizard builds it from your answers: the floors, the envelope (the shell of walls, roof, floor and windows), shading, schedules, a weather file and the equipment you picked. Solar panels, batteries and tariffs are covered in how energy management is simulated.

Who develops it

EnergyPlus is open source and has been developed since 1997 with funding from the U.S. Department of Energy. The Department of Energy states that EnergyPlus is tested according to the ASHRAE Standard 140 methodology . That is the Department’s statement about EnergyPlus, not a claim about EnergyFlow. Standard 140 is the American standard method of test for building energy simulation software.

That statement is about the engine. What a run tells you about your house also depends on what you enter and on the choices made in setting up the model, which model assumptions and limits lists openly.

Licence

EnergyFlow is independent of the EnergyPlus developers and the U.S. Department of Energy, and neither endorses EnergyFlow. EnergyPlus is released under an open-source licence, and its terms  are public.

Limits of the model

• EnergyPlus is only as good as its input. Floor-level zones (one air temperature per floor), preset schedules and representative equipment data (typical performance for a type and size, not a named product) all sit upstream of it.
• A year of weather, typical or real, is not a forecast.