Free Building
Energy Modeling
Load Calculator
RenBEM is a free building energy modeling and load calculator. Enter your envelope, windows, occupancy, and ventilation, and get peak heating and cooling loads plus a 24-hour hourly analysis in seconds. Built on real weather data and transparent formulas. Free to use, and if you need a full simulation, leave it to us.
RenBEM v1.1
Free heating and cooling load calculator with hourly analysis by Renwaves
Location & local climate
Choose a city or use your current location to estimate local outdoor design temperatures.
No location selected. Default outdoor temperatures are in use.
Location estimates use the 1st percentile of daily minimum and 99th percentile of daily maximum temperatures from the latest 10 complete years available. These are screening estimates, not official building-design temperatures.
Building basics
Advanced settings Envelope, glazing, design temperatures, occupancy & ventilation
01 · Design conditions
02 · Envelope & glazing
03 · Occupancy & outdoor air
Heating load breakdown
Cooling load breakdown
Cooling total includes sensible and latent components. Breakdown values are shown in kW.
Hourly heating and cooling analysis
Select a historical day to estimate hourly heating and cooling loads using local weather data.
Select a location first, then choose a date.
| Hour | Outdoor °C | Solar W/m² | Heating kW | Cooling kW |
|---|
Method: hourly outdoor temperature and global horizontal solar radiation are applied to the current simplified load equations. Occupancy, ventilation, lighting, and equipment are held constant for all 24 hours. Thermal mass, solar orientation, shading schedules, and HVAC controls are not simulated. Historical weather availability depends on the selected location and date.
Features of the free RenBEM load calculator
Explore the main factors behind a building’s heating and cooling demand and turn your assumptions into a clear first estimate of peak loads.
Location-Based Climate Data
Search for a city or address to estimate winter and summer design temperatures from local weather history.
Flexible Building Inputs
Adjust floor area, envelope U-values, glazing, shading, occupancy, ventilation, and internal gains.
Peak Heating and Cooling Loads
Get peak loads in kW and W/m², with a breakdown that shows where each load comes from.
24-Hour Hourly Analysis
Pick a past day and see heating and cooling loads hour by hour, using real temperature and solar data.
Instant Browser Workflow
Run it in your browser with no software to install and no account. Export the peak load breakdown as a CSV.
Clear, Practical Results
Use the numbers for an early screening check, then move to a detailed simulation when you need one.
How RenBEM Calculates Heating and Cooling Loads
RenBEM estimates peak loads from the building envelope, outdoor air, solar gains and internal gains. Heating and cooling are calculated separately. Hourly results apply the same equations to the selected day’s weather data.
📐 Formula
Heating load = Envelope heat loss + Ventilation loss + Infiltration loss
📐 Formula
Cooling load = Envelope heat gain + Window solar gain + Internal gains + Outdoor-air load
A transparent first-pass engineering model
Heating includes transmission, ventilation and infiltration losses. Cooling also accounts for occupants, lighting, equipment, and outdoor-air sensible and latent loads.
Scope and Limitations
RenBEM provides a first-pass estimate of building heating and cooling loads. Its current scope and key limitations are outlined below.
Single-Zone Model
Evaluates one building zone. Room-by-room zoning and interactions between zones are not included.
Peak and 24-Hour Analysis
Estimates peak loads and hourly loads for one selected day. Annual energy use and 8,760-hour profiles are not calculated.
Simplified Thermal Response
Does not simulate thermal mass, dynamic heat balance, construction-layer behavior, or moisture buffering.
Simplified Solar and Envelope Effects
Detailed solar position, orientation-specific shading, thermal bridges, and ground coupling are outside the current model.
User-Defined Design Inputs
You provide design temperatures, envelope areas, U-values, and peak solar conditions. Results depend on these assumptions.
HVAC Systems Not Simulated
HVAC plant efficiency and system performance are not modeled. Results are not a certified load calculation and should not be used alone to size equipment.
For more detailed analysis, explore our building energy simulation service and HVAC calculation service. The open-source EnergyPlus engine is a reference for detailed dynamic simulation; it is not used in the current RenBEM version.
A Quick Guide to Heating and Cooling Load Estimates
RenBEM is a free building energy modeling tool that runs in your browser. Choose a location, enter your building details, and estimate peak heating and cooling loads in kW alongside an hourly analysis for a day you pick. No installation or account is required.
Use local weather history to set outdoor design temperatures for your site.
Explore how envelope, glazing, occupancy and ventilation change your loads.
Use the estimate for an early check before moving to a full simulation or HVAC design.
RenBEM, answered
Quick answers about how the calculator works and what it can and cannot do.
01How accurate is RenBEM?
RenBEM is a screening tool. It uses simplified steady-state equations and the inputs you provide, so treat the results as early estimates, not design values. Real loads also depend on thermal mass, schedules, and orientation, which a full simulation models.
02What inputs affect the results?
Floor area, volume, wall, roof, floor, and window areas and U-values, glazing SHGC, solar exposure, shading, occupancy, lighting, equipment, ventilation, and infiltration all change the result. Winter and summer design temperatures fill in from your location, and you can edit them.
03Where does the weather data come from?
When you search a location, RenBEM pulls about ten years of daily temperatures from Open-Meteo to estimate winter and summer design temperatures. The hourly analysis uses Open-Meteo’s historical hourly temperature and solar radiation for the date you pick. These are screening estimates, not official design conditions.
04How does the hourly analysis work?
Pick a location and a past date. RenBEM applies the load equations to each of the 24 hours of real temperature and solar data. Occupancy, ventilation, lighting, and equipment stay constant, and thermal mass is not modeled, so treat it as a quick daily profile rather than a dynamic simulation.
05What does RenBEM leave out?
It does not calculate annual energy use (kWh over a year), an 8,760-hour profile, thermal mass, solar position, or HVAC plant efficiency. It models one thermal zone only. For multi-room buildings or annual energy, a full simulation is the safer route.
06Is RenBEM free, and is my data saved?
It is free to use. The calculator runs in your browser and does not send or store your building inputs. Location searches and weather requests go to Open-Meteo. Export a CSV if you want to keep a copy of the peak load results.
07Can I use it to size a heat pump, boiler, or chiller?
No, not on its own. Equipment selection needs room-by-room loads, operating conditions, manufacturer data, and local standards. You can request a full building energy simulation or an HVAC calculation from our engineers.