HOMER Energy Support

Unreliable Grids

Last Updated: Jan 18, 2017 09:17AM MST

HOMER can model a range of grid-connected solar photovoltaic (PV) systems. These include those with feed-in-tariffs, net metering, as well as resilient solar + storage systems that ensure electricity even when the utility grid goes down.

Grid-connected PV systems can operate under a range of different tariff structures. The HOMER Pro Wizard is a great tool for creating models that demonstrate four common scenarios. Just open the Wizard, either from the schematic in an empty HOMER model or from the Help menu. Once you get to the Grid step, select the tariff structure, and HOMER automatically will set up your system for you.

You can also set up your grid-connected PV model without the Wizard:
I cannot sell electricity back to the grid: Set the Grid Sellback price to $0, to tell HOMER that you can’t sell electricity back to the grid. In some cases Grid Sales will still appear in your results instead of Excess Electricity, but you can count the Grid Sales as Excess.
I sell electricity with a feed-in-tariff: If the utility pays you a fixed price for each kWh that you export to the grid after serving local energy needs, you can set the grid Sellback Rate to that price.
I sell electricity using monthly net metering: In the Grid menu, choose “Net Metering” and choose monthly. At the end of each month, HOMER totals up your grid purchases and grid sales. If the grid purchases exceed the sales, you pay the Power price for the difference. If the grid sales exceed the purchases, the utility pays you the sellback rate for the difference.
I sell electricity using annual net metering: As with monthly net metering above, select “Net Metering” in the grid menu, but choose “Net purchases calculated annually”. The total grid purchases and sales are totaled yearly instead of monthly. This can result in lower payments or higher revenues in cases where some months have net purchases and some months have net sales.

Additional common PV grid-connected systems

Feed-in-tariff for all PV Production: In some locales, the grid purchases all of the power produced by your PV array, and you buy all your power from the grid. In this case, the PV sales to the Grid performance are independent from your Grid purchases to serve the load. This is best modeled with two separate HOMER models, a PV + Grid model and a Grid + Load model. Since the PV + Grid model will not contain a load, you will need to set the Maximum Annual Capacity Shortage (found in the Constraints menu on the Project ribbon) to 100%. If desired, you can include the annual revenue calculated in the PV + Grid model as a negative “System fixed O&M cost” (Economics menu), as well as the capital cost as an expense in the “System fixed capital cost” (also in the Economics menu), in your Load + Grid system.
Solar + Storage and Resilient Grid Modeling: In other situations, the primary goal may be resilience with an unreliable grid connection. Optimizing this kind of hybrid system really leverages HOMER’s power. You will need the Advanced Grid module to accurately model these systems. You can consider a PV array, a battery bank, and even a diesel generator.
To input utility grid outages, open the Grid menu, and under the Reliability tab you can add random failures or scheduled outages.

You can also tell HOMER to keep the battery fully charged, and only discharge it if no other power source is available, so that it has the maximum energy saved for when there is an outage. You can find this setting in the Grid menu (scheduled rates or real time rates) in the Demand Rates tab, as pictured below. You can try the optimization with and without this setting, to see if it helps reduce your costs.

When you optimize this model, the winning (lowest-NPC) system may include the generator, the battery bank, and/or the PV, or exclude some of these. Which combination of components is most cost-effective to support the load will, primarily depend on grid tariffs, and grid outage frequency and duration. Other factors, such as the solar resource available, component costs and fuel costs can also influence the economics. The HOMER software gives you the power to quickly compare all of these factors to determine which design will best serve your load requirements at lowest cost.
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