HOMER currently assumes 100% availability (excluding resource limitations) for every component, including the grid. We plan to change that in an upcoming version of HOMER, but until then here is a work-around to model an unreliable grid. Create a generator and label it as the “grid”. To do this, set the "grid" generator to have description "Grid" and abbreviation "Grid" as well.
The next step in the workaround is to set the cost of the "grid" generator to free. To make it free, set the cost matrix to have $0 for the capital, replacement, and O&M cost. You will also want to change the minimum load ratio to 0%, and choose a size to consider that matches your grid capacity. If you don't want to worry about grid size, you can make the size to consider something large (that is, 999999 kW).
You will also have to create a new fuel for the “grid” in the grid's fuel tab. The best way to do this is to create a fuel called “grid power” where 1kg is equivalent to 1kWh. Set the price per kg equal to your price per kWh. The energy content of the fuel will need to be 3.6MJ/kg and the density needs to be 1000kg/cubic meter. Set the Units dropdown menu to Kilograms. When HOMER reports “grid power” consumption in kg you will know that this is equal to kWh.
There is one more small change that must be made in order to complete this work-around. In the Generator Inputs Window for the "grid" generator, open the Fuel tab. In the fuel curve section, set the Intercept coefficient to 0 and the Slope to 1. This makes your “grid” a 100% fuel efficient machine, meaning you get to use every kWh that the "grid" delivers to you, none of it is lost through combustion.
The “grid” can then be scheduled off using the Generator Scheduling capability. You determine the duration and frequency of outages in the schedule tab of the generator inputs window. This allows you to have a different “outage schedule” every month. It could be as little as 2 hours per week of outage by setting 1 hour of outage on a weekend or you can have as much “outage” as you want. The only limitations are that it is not random, and the schedule must be consistent during each month.
We have built a sample starter file to help you get started. See attached.
Modeling the grid in this way does not allow you to sell electricity back to the grid. Developing an advanced grid module that can model an unreliable grid is a high priority for us and will be in the works soon.