Aids in the design of single- or multizone energy-efficient buildings where the loads are dominated by the dynamic interactions between the building's envelope, its environment, and its occupants. SUNREL is especially well suited for passive solar buildings and includes algorithms for Trombe walls, advanced glazings, schedulable window shading, active-charge/passive-discharge thermal storage, and natural ventilation. The program is a true simulation model based on time steps of one hour or less. The model representation of the building is a thermal network solved with forward finite differencing among other techniques. In addition, a simple graphical interface aids in creating input and viewing output. It�s not very pretty, but it is easy to learn and review precisely what inputs went into each simulation run.

SUNREL is an upgrade of SERI-RES, which was released in the early 1980s by the Solar Energy Research Institute (now known as NREL). The program has been used by researchers around the world and is the underlying building physics and mathematics package in TREAT, a program offered to all New York State energy auditors by the New York State Energy Research and Development Authority (NYSERDA).


design, retrofit, research, residential buildings, small office buildings, energy simulation, passive solar


SUNREL and SERI-RES have both been extensively tested as part of the IEA Solar Heating and Cooling Programme�s efforts to validate building energy simulation software. The program will report if stability or convergence criteria are not maintained, and energy balances can be observed from output reports available to the user.

Expertise Required

Knowledge of mechanical engineering, building heat transfer.


More than 100. Also the underlying engine for TREAT.


Engineers, energy consultants, scientists, designers.


Thermophysical and optical properties of the building, simplified geometric description, and hourly weather data.


Temperatures and energy flows; all components of the building energy balance at hourly, daily, weekly, monthly, or yearly integrations.

Computer Platform

PC compatible using DOS.

Programming Language



Passive solar; dynamic modeling of building fabric; relatively easy to learn and use; flexibility of thermal network approach.


Lack of HVAC modeling.



National Renewable Energy Laboratory


1617 Cole Boulevard
Golden, Colorado 80401
United States


+1 (303) 384-7503


+1 (303) 384-7540




Available for download for a fee from National Renewable Energy Laboratory's SUNREL web site.

Taitem Engineering offers TREAT, a residential tool which uses SUNREL as the calculational engine. Visit the TREAT software Web site for more information.

SUNCODE, a private-sector PC version of SERIRES is available for $650 from:

Ecotope, Inc.
4056 9th Avenue NE
Seattle, Washington 98105

telephone: +1 (206) 322-3753
facsimile: +1 (206) 325-7270