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REopt Lite Frequently Asked Questions

The following topics contain frequently asked questions (FAQs) about the REopt Lite™ web tool. Additional information about REopt Lite can be found in the REopt Lite User Manual, REopt Lite Fact Sheet, and REopt™ Technical Report.

Methodology and Features

REopt Lite uses a mathematical optimization model to determine the optimal economic size of PV and storage subject to a site's characteristics and constraints. REopt solves a series of simultaneous equations to find the system sizes and dispatch strategy that minimize the site's life cycle cost of energy.

More information can be found in the REopt Lite User Manual, REopt Lite Fact Sheet, and REopt Technical Report.

REopt Lite is a publicly available web-based subset of the more comprehensive REopt desktop model that offers the same optimization and integration capabilities. REopt Lite includes only PV, wind, storage, combined heat and power, and diesel generation while REopt includes a broader set of technologies such as solar water heating, solar ventilation air preheating, biomass, landfill gas, waste-to-energy, geothermal heat pumps, and dispatchable loads.

REopt Lite is a free-to-use publicly available web tool that utilizes optimization and integration to recommend optimal PV, storage, and wind system sizes and dispatch strategies for behind-the-meter applications. Other tools differ in the way they are accessed, the cost to use, the technologies evaluated, and other features.

Developers can access REopt Lite programmatically through the REopt Lite Application Program Interface (API) and open source code. Analysis using the full REopt model is currently only available through NREL.

Use Cases

REopt Lite accesses residential, commercial, and industrial utility rates from the Utility Rate Database (URDB), allowing users to evaluate the cost-effectiveness of PV, storage, and wind for various rate types. A residential building owner could upload hourly interval data or simulate electricity usage based on total annual electricity consumption and one of DOE’s commercial reference buildings. Currently, specific load profile shapes are not available for residential buildings, so a reference building with a similar electric load schedule would need to be selected for an estimate. Residential building owners may also need to adjust the default cost and financial assumptions. Results for a non-business homeowner will be approximate.

The building loads available in REopt Lite will be updated as DOE’s commercial reference building profiles are updated; other building loads such as residential buildings may be added in the future. Check the REopt Lite development page for updates.

REopt Lite evaluates the economics of behind-the-meter (BTM) PV, storage, and wind for a commercial building. There is no defined limit on the technology system sizes; however, REopt Lite does not currently evaluate the economics of front-of-the-meter systems.

REopt Lite evaluates the economics of PV, storage, and wind based on the technologies’ ability to provide energy and demand savings over the life cycle of the project. It does not currently include other value streams.

Defaults and Assumptions

The default values for PV, battery, and wind costs, along with all other default inputs to the model, are documented and referenced in the REopt Lite User Manual. These values will be reviewed and updated (if needed) on an annual basis.

The default values in REopt Lite are for lithium-ion battery technology and are documented in the REopt Lite User Manual. However, inputs such as battery costs, roundtrip efficiency, and minimum state of charge can be adjusted to model different battery technologies.

Excess power is credited at retail rate ($/kWh) for systems sized up to the net metering limit, and $0/kWh for systems sized over the met metering limit. The default net metering limit (0) can be adjusted by the user to reflect that of the site’s local utility.


REopt Lite leverages a mathematical optimization model to determine the cost-optimal size and dispatch of a PV, storage, or wind system subject to technology costs, the site’s electricity load, cost of electricity, solar or wind resource and other financial inputs. In some locations, where the utility costs and solar or wind resource are low, PV or wind might not be cost-effective at this time and the cheapest option at this time might be to continue to purchase grid electricity.

Make sure there are no dashes or blank cells. Make sure that your load profile is a .csv file with exactly 8,760, 17,520 or 35,040 entries (one for each hour, 30-minute, or 15-minute time interval of the year). Download the load profile template for an example.

REopt Lite utilizes rates from the Utility Rate Database (URDB), which characterizes over 47,000 utility rates across the United States. However, it does not contain every rate from every utility. REopt Lite searches rates within a 25-mile radius of the address entered. If a rate does not show up for your site, use the custom electricity rate option to enter your own rate.