IEA Helios — Building Energy & Flexibility Simulation

01 / Location and weather

Location and weather

Search for a city or enter coordinates, then fetch a year of hourly weather from NASA POWER for that location.

City / location Type at least two letters; use the arrow keys or Tab to reach the matches. Place names from Open-Meteo geocoding.

Latitude (deg)

Longitude (deg)

Weather year

02 / Geometry

Building geometry

Choose a footprint mode. Floor area, roof area, wall area and volume are auto-computed from wall and envelope dimensions.

Building name

Geometry mode

Simple box

Plan length (m)

Plan depth (m)

Plan azimuth (deg)

North is 0 degrees, east is 90, south is 180, and west is 270.

Storeys

Floor height (m)

Total wall area: 0 m² · Footprint: 0 m² · Floor area: 0 m²

Manual wall-by-wall definition

Required: at least 4 walls, 1 roof, and 1 floor

Building proportions & defaults

Storeys

Floor height (m)

Wall heights use the storey count and floor height by default. You can customize each wall below.

Walls (4)

Auto-link angles (rotate envelope when any azimuth changes) Auto-sync dimensions & areas

With Auto-link enabled, changing any wall's azimuth rotates the remaining walls by the same amount. Supports polygons with four or more walls. Adjusting wall lengths auto-sizes floor and roof elements from the polygon area.

Total wall area: 0 m²

Roof (1)

Roof elements auto-update from wall dimensions. A flat roof uses 0 degrees and a vertical surface uses 90 degrees.

Total roof area: 0 m²

Floor (1)

Ground floor slab elements auto-update from wall dimensions.

Total ground floor area: 0 m²

Conditioned floor area (ground floor × storeys): 0 m²

Building footprint & solar compass preview

Real-time geometry & orientation layout

Walls (N/E/S/W vectors & normals)

Floor & Roof footprint hub

Cardinal solar azimuth (0° N, 90° E, 180° S, 270° W)

03 / Openings and glazing

Openings & glazing

Specify windows, doors, glazing properties, shading geometries (overhangs, side obstructions, blinds), and frame fractions.

Visual variable reference guide and diagrams

Allocation mode

Automatic glazing & door allocation

Window-to-wall ratio (20%)

Whole-window U-value, frame included (W/m²K)

Glazing solar g-value (0–1)

Frame fraction of gross window area (0–1)Used to determine solar glazing area; whole-window U already includes the frame

Door count (doors on Wall 1)

Door U-value (W/m²K)

Shading type

Shading depth (m)Used when overhang shading is selected

External obstruction angle (°)horizon obstruction e.g. adjacent buildings, trees (0–89°)

Shading transmission override (0 to 1)Optional transmitted fraction when no shading type is selected

Total wall area: 0 m² · Total gross window area: 0 m²

04 / Envelope and construction

Envelope & construction

Define opaque thermal layers, U-values, solar absorptance, shading of the opaque surfaces, and thermal mass for all walls, the roof and the floor.

Construction heaviness Effective heat capacity = class × floor area (ISO 13790:2008 Table 12).

Ground contact temperature (°C)Coupled floor boundary temperature

Infiltration / Air changes (ACH)Natural air infiltration rate

Surrounding ground albedo / reflectance (0 to 1)Examples: grass or asphalt 0.20; light concrete 0.40; snow 0.70

05 / HVAC and systems

HVAC performance presets

Choose a screening preset or enter seasonal heating efficiency and cooling COP. Presets do not model equipment capacity, dispatch, or part-load behavior.

Seasonal performance assumptions

Heating performance preset Air-Source Heat Pump with high seasonal COP

Cooling performance preset Split system or VRF with seasonal cooling EER / COP

Heating energy source Set from the heating preset; change it if your system differs. Used for energy costs and the electricity balance.

Cooling energy source All cooling presets run on electricity.

Heating efficiency / COPThermal conversion efficiency or seasonal COP (can exceed 1.0 for heat pumps)

Cooling COP (Coefficient of Performance)Seasonal operating COP / EER of cooling equipment

HVAC supply air humidity controlTarget maximum indoor relative humidity (%)

06 / Internal gains and setpoints

Internal gains & setpoints

Setpoints, occupant loads, plug loads, lighting power, and specialized electrical and thermal end-uses.

Heating setpoint (°C)

Cooling setpoint (°C)

Occupants (average people present)

May be fractional, e.g. 2.5 for a household often out. Sensible gain 75 W per person.

Moisture generation (g/h) Breathing, cooking, washing and drying. Auto uses 125 g/h per occupant (BS 5250 / IEA Annex 14: about 5–7 kg a day for two people).

Plug, lighting & base equipment loads

Lighting (kWh/m²·yr)Interior lighting energy

Appliances & plug loads (kWh/m²·yr)General electronics and standard plug loads

Fans & auxiliary pumps (W/m²)Circulation pumps, ventilation fans, distribution

Specialized building equipment & process gains

Server / IT / electronics room (W)Continuous IT racks, UPS, network closets

Kitchen & cooking sensible heat (W)Domestic or commercial kitchen sensible emissions

Elevators / vertical transport & motors (W)Mechanical transport, lifts, variable motors

Lighting: 0 W · Base Equipment: 0 W · Aux/Fans: 0 W · Total Sensible Heat Gains: 0 W

Annual HVAC screening adjustments

These factors scale reported annual HVAC demand. They do not create hourly schedules, unoccupied temperature drift, recovery loads, or separate room zones.

Reported heating demand factor (100%) Post-calculation multiplier for screening only

Reported cooling demand factor (100%) Post-calculation multiplier for screening only

Homogeneous conditioned-area factor (100%) Scales demand but does not model separate room conditions

07 / Energy management and renewables

Energy management and renewables

Optional. Switch on the parts that apply to this building. With every card switched off the results are the same as without this step.

Grid peak window

The hours when grid electricity is most expensive or most constrained, usually the early evening. Load shifting, battery peak shaving and time-of-use operation, and off-peak EV charging all use this window.

Peak window starts (hour, 0–23)

Peak window ends (hour, 0–23)16 to 20 means 16:00 until 20:00.

Building controls (BEMS)

Include

A building energy management system changes the heating and cooling setpoints by the hour. Helios simulates the building without these controls and with them (plus a few extra runs to find the smallest plant that still keeps the rooms comfortable) and reports the difference in plant size, the busiest grid hour, energy, cost and comfort. The results card explains each number in plain words.

Night setback

Lets the building run cooler in winter and warmer in summer overnight.

Heating setback (°C)Degrees below the heating setpoint.

Cooling setback (°C)Degrees above the cooling setpoint.

Starts (hour, 0–23)

Ends (hour, 0–23)

Pre-conditioning

Heats or cools a little extra before the peak window and before the setback ends, so the building coasts through with less plant running.

Hours before

Heating boost (°C)Degrees above the heating setpoint.

Cooling boost (°C)Degrees below the cooling setpoint.

Load shifting in the peak window Relax the setpoints during the grid peak window

Heating setback in the window (°C)

Cooling setback in the window (°C)

Plant sizing

Helios finds the smallest heating and the smallest cooling plant that keep the rooms comfortable with these controls, allowing this many uncomfortable hours in the year.

Maximum acceptable unmet hours (h/yr)Hours when the room is colder than the heating setpoint or warmer than the cooling setpoint. 50 h is a common design allowance; 0 sizes for every hour of the weather year.

Fixed cap on plant output (optional) Also test a fixed cap on heating and cooling output

Cap (% of the largest plant output without controls)The plant may never deliver more than this share of the most it delivered in any hour without controls. When the cap bites, the room temperature drifts; the results show how many hours are then uncomfortable.

All times in this step are local standard time for the location, without daylight saving.

Rooftop solar PV

Include

Photovoltaic panels on the main roof. Generation is simulated hour by hour from the weather file's solar radiation and air temperature.

Battery storage

Include

A home or building battery that stores electricity and releases it later.

Electric vehicle

Include

Electric cars charged at the building. Charging is added to the building's electricity demand.

Tariff

Include

Energy prices used for the annual cost and the cost change from demand-side management. Enter your own supplier's prices.

08 / Run and results

Run & results

Execute the hourly simulation model with the fetched NASA POWER weather dataset.

Idle

No calculated result for the current inputs.

Baseline standard

Reference standard

Heating Demand

—

Cooling Demand

—

Peak heating load

—

Peak cooling load

—

Estimated HVAC Input

—

HVAC Input / Thermal Demand

—

Baseline monthly heat and cooling demand (kWh)

Show the monthly values as a table

Current scenario

Live iteration

Heating Demand

—

Cooling Demand

—

Peak heating load

—

Peak cooling load

—

Estimated HVAC Input

—

HVAC Input / Thermal Demand

—

Current monthly heat and cooling demand (kWh) hover or open the table for exact values

Show the monthly values as a table

"Heat and cooling demand" shows the heat and cooling delivered to the rooms (thermal energy). "Electricity and solar" shows electricity used by the plant, appliances, lighting and EV above the axis and solar generation below it, so both sides are electricity. Both views are in kWh per month.

Building controls (BEMS): what changed

Helios ran your building through the same weather, solar, battery and EV twice: once with the thermostat fixed at your setpoints all year (without controls) and once with the schedule you set in step 7 (with controls). The main results above are the run with controls. Times are local standard time (no daylight saving).

How the numbers were made

What the controls changed

Plant size

Without controls against with controls

Words used on this card

Plant: The heating and cooling equipment: boiler, heat pump, chiller or air conditioner. Its size in kW is the most heat or cooling it can deliver in one hour.
Setpoint: The room temperature the thermostat is asked to hold: one for heating (for example 21 °C) and one for cooling (for example 26 °C).
Without controls: The heating and cooling setpoints stay the same every hour of the year. The plant is as big as it needs to be, so the rooms never fall outside the setpoints. This is the "before" column.
With controls: The same building run with the BEMS schedule from step 7: a night setback, pre-conditioning before the setback ends and before the grid peak window, and easier setpoints inside that window. This is the "after" column and the main results above.
Setback: Letting the rooms run a few degrees cooler in winter (or warmer in summer) at night. Temperature differences are given in °C.
Pre-conditioning: Heating or cooling a little harder for a few hours before the setback ends and before the grid peak window, so the building can coast afterwards. It costs energy in those hours and can create the busiest hour of the year.
Grid peak window: The hours you set in step 7 when grid electricity is most expensive or most constrained (for example 16:00 to 20:00). It is a period of the day, not a power figure.
Busiest grid hour: The single hour of the year with the most electricity taken from the grid, in kilowatts. It can fall outside the grid peak window.
Peak heating load, peak cooling load: The most heat or cooling the rooms needed in any one hour, in kilowatts of heat, not electricity. The same number appears on the tiles at the top, in the plant table and in the table at the bottom of this card.
Smallest plant that keeps comfort: Helios shrinks the plant until the rooms are outside the setpoints for no more than the number of hours you allowed (step 7). Worked out for both runs, so the comparison is fair.
Uncomfortable (unmet) hours: Hours in the year when a room is colder than the heating setpoint or warmer than the cooling setpoint because the plant could not keep up.
Plant cap: An optional fixed limit on how much heat or cooling the plant can deliver in any hour (step 7), set as a share of the largest output it reached without controls. When it is ticked, the main results above are for the capped plant and this card shows the comfort cost.
kW and kWh: kW (kilowatts) is how much power is used in one moment; kWh (kilowatt-hours) is how much energy was used over the year.

Annual heat balance of the rooms (kWh)

Every flow of sensible heat the model sees over the year. Gains and losses close to the heat stored in the fabric.

Losses Gains

Live parameter scenario adjustments

Wall U-value 0.20 W/m²K

Roof U-value 0.15 W/m²K

Floor U-value 0.18 W/m²K

Whole-window U-value, frame included 1.1 W/m²K

Heating setpoint 21°C

Cooling setpoint 26°C

Heating efficiency / COP 3.2

Cooling COP 3.5

ACH (Infiltration) 0.5

Window-to-wall ratio 20%

Slider changes accumulate in the current scenario. Use Reset to Baseline before a single-parameter comparison.

Export & share scenario run

The complete run bundle packages all scenario inputs, controls, tariffs, hourly weather metadata, and verified simulation outputs into a single JSON file. Colleague analysts can upload this bundle to inspect the exact scenario state and all resulting metrics without rerunning.

Location and weather

Climate graphs & monthly solar profiles

Building geometry

Geometry mode

Simple box

Manual wall-by-wall definition

Building proportions & defaults

Walls (4)

Roof (1)

Floor (1)

Building footprint & solar compass preview

Openings & glazing

Visual variable reference guide and diagrams

Allocation mode

Automatic glazing & door allocation

Manual windows & doors per wall

Envelope & construction

HVAC performance presets

Seasonal performance assumptions

Internal gains & setpoints

Plug, lighting & base equipment loads

Specialized building equipment & process gains

Annual HVAC screening adjustments

Energy management and renewables

Grid peak window

Building controls (BEMS)

Rooftop solar PV

Battery storage

Electric vehicle

Tariff

Run & results

Baseline standard

Baseline monthly heat and cooling demand (kWh)

Current scenario

Current monthly heat and cooling demand (kWh) hover or open the table for exact values

Energy balance

Peak electricity demand: the busiest hour of the year

Building controls (BEMS): what changed

How the numbers were made

What the controls changed

Plant size

Without controls against with controls

Annual heat balance of the rooms (kWh)

Live parameter scenario adjustments

Indoor humidity

Export & share scenario run