Why we created HOMER Grid: Understanding the differences between HOMER Grid and HOMER Pro

Note: This article was updated in July 2018 to reflect changes in HOMER Grid 1.2.

In mid-March 2018, HOMER Energy released HOMER Grid – our first new desktop product since the launch of HOMER Pro in November 2014. HOMER Grid addresses a growing market that was not well served by HOMER Pro: behind-the-meter systems connected to a mostly reliable grid, where demand charges, energy arbitrage, self-consumption, or all three of these are driving the economics of storage, often in combination with renewables, such as solar or wind, backup generators, or combined heat and power systems.

Techno-economic Optimization of Hybrid Power Systems

Both products are techno-economic optimization models, a mouthful that means both have a goal of finding the lowest cost power system that meets all technical requirements from among hundreds or thousands of possibilities. They accomplish this with chronological simulation of each possible power system for a year, minimizing the cost at each of a series of time steps while also meeting the power requirements for that time-period. HOMER is an acronym for Hybrid Optimization of Multiple Energy Resources, so both products are at their best when the problem is how to select, size, and compare a combination of generation sources and storage. Both products have an impressive array of options and ways in which constraints and requirements can be configured, dozens more major features not listed here.

Why did we create HOMER Grid as a separate product and not an upgrade or add-on to HOMER Pro? And which product should be used for which modeling issues? If you are new to HOMER or do not care about the details, here is the answer to “which product do I need when”:

HOMER Pro is a Swiss Army Knife

HOMER Pro is a “Swiss Army knife” of hybrid optimization. It is an astoundingly robust modeling tool that has become a true global standard. Its roots were in the energy access world – the original problem for HOMER was to find the lowest cost means to either offset diesel generation with renewables and storage or to build a completely new mini-grid system. The easiest way to describe this is that HOMER was trying to find “the cheapest thing that would meet the load.” HOMER 2.0, released in 2003 (by the US Department of Energy’s National Renewable Energy Laboratory, where HOMER was originally created), added significant capability to the original HOMER – including grid-connected systems, hydrogen, multiple generators, and combined heat and power.

Energy access in India

The original implementation of grid in HOMER was simple. The grid was always on, and the price was fixed. It became more sophisticated over time, and with the first release of the Advanced Grid module, in HOMER Pro 3.1, we added real-time rates, scheduled rates, scheduled outages, separate rate periods with control options, and the ability to compare the cost of grid extension to the cost of a hybrid microgrid. With the release of HOMER Pro 3.2, in 2015, we added the ability to model unreliable grids, one of our top customer requests. We have continued to improve and refine the grid-connected modeling capabilities of HOMER Pro since then.

A Growing Importance of Demand Charges in Energy Costs

There was one use case, however, that simply did not fit the HOMER Pro approach, and that was the growing importance of demand charges and time-of-use rates. The issue with demand charges is that they can be set for a particular month, and then get  reset for the next month. The modeling goal for HOMER would therefore change every month, which is not a satisfactory result. If you try to model for only one month at a time, you may end up with a different system configuration in each month. Adding 12 more decision variables, i.e., one for each month, to the optimization creates too large a problem to solve in a reasonable amount of time.

We needed a new approach to “Dispatch Strategy,” the way in which a power system uses available power sources, to assist in this market. HOMER Grid allows HOMER to “look ahead” at the anticipated load and make decisions based not necessarily on “What is the cheapest form of reliable power in this time step?” but rather on “Which source of power should I use in this time step that will minimize my total cost including the demand charge?” But the issue is not that simple, either. The savings from smoothing out utility usage must be weighed against the cost of the offsets, i.e., the renewables, storage, and other local generation sources that are used to avoid grid power. There is then the issue of what to use when – storage, solar photovoltaics, or other available sources such as generators (diesel, natural gas, steam from CHP), or wind turbines.

HOMER Shines in Complex Modeling Problems

Sorting out the complexity required to understand economic and engineering options is where HOMER shines. To create HOMER Grid, we took the proprietary optimization approach from HOMER Pro and combined it with a dispatch approach that can handle demand charges and  time of use rates the way we believe they should be handled. To this we added access to an extensive tariff database via an API to Genability, giving HOMER Grid users easy access to complex tariffs (currently tariffs for the USA, Canada, and Mexico). So again, the simple answer to which product to use is based on the fundamental problem to be solved. HOMER Pro still serves many grid-connected markets extremely well, particularly those situations where the primary issue is backup power for a highly unreliable or part-time grid or where the choice is between extending the grid or providing an off-grid solution. For areas outside North America or tariffs not included in Genability, HOMER Grid includes an Advanced Tariff Builderthat lets the user define almost any tariff (or we can do it for you.)

But when the issue is modeling behind-the-meter projects, HOMER Grid is the best tool available, and it is only going to get better.

There is no other commercially available, supported modeling tool that considers such a broad range of potential technologies for lowering the overall cost of energy in grid-connected systems with demand charges.

We Care About Our Users

One final point I wish to make, for our loyal customers and HOMER Pro users, is that product usability was an important part of this decision as well. While it was theoretically possible to make this demand charge modeling problem one more tool in the HOMER Pro Swiss Army knife, it would have significantly increased HOMER Pro’s complexity, making HOMER Pro more difficult for people to use and understand.

HOMER Pro now has nine modules that can be added to its base and three “packages” to assist users in bundling together modules that are often used together. This requires significant time from our customers up front understanding which product configuration is the right one for them. We would rather our customers spend time modeling than sorting out software configurations. But complexity continues after a purchase, and the more complex a piece software becomes, the greater the likelihood of using it incorrectly, not knowing how to solve a particular issue, which serves no one. It is very important to us that HOMER software is used correctly, that it is giving the best possible answer, so that you and your clients are getting the best possible information by which to understand and evaluate choices in hybrid power system design.

Commercial solar tops a Des Moines market.

What is the future for HOMER Grid?

Capabilities we are exploring for future versions of HOMER Grid include multi-year modeling, accounting for ancillary services revenue streams, unreliable grids, and demand-side management options, such as controllable loads and demand response. We will be adding Genability tariffs for additional regions of the world as Genability adds them. We are also considering mechanisms for our customers to report expired or invalid tariffs.

What is the future for HOMER Pro?

HOMER Pro 3.12 is planned for August 2018 – the 12th major upgrade for this flagship product. This version will add:

• Predictive dispatch in off-grid systems
• OpenEI Load Profiles available worldwide (extrapolated by climate zone)
• Improved handling of concurrent licenses and proxy authentication

We will continue expanding HOMER Pro’s capabilities, primarily in terms of usability, fixing any new bugs that crop up, keeping up with market needs and technology changes that drive customer needs and requirements. As HOMER Grid’s capabilities grow, we can focus improvements to HOMER Pro on purely off-grid systems, either in basic energy access or island utilities with multiple generators.

Choosing a HOMER Version

The “choice” between HOMER Grid and HOMER Pro is not an “either/or” decision. We have created this comparison page to make it easier to directly compare the two products. Companies with a very focused market approach may need only one product, but large companies working in multiple markets will likely need both. HOMER Pro also continues to be our primary product for educational and research markets, because of its broad applicability to many modeling approaches (we did add academic and student pricing for HOMER Grid in version 1.2). Both products are available as APIs for companies that wish to develop OEM versions or custom interfaces.

Our approach to software development has always been to keep a close ear to the market and to listen to our customers. So, I invite you to let us know what you would like to see in HOMER Grid or HOMER Pro. We are listening.