Grid O&M cost
Hello, I designed a on-grid system which contain also a PV array and a wind turbine, I saw on report that net present cost at Grid O&M is negative value, like -13,814. I looked at the Homer Help Manual for this issue, it is mentioned that, grid O&M cost is equal to the annual cost of buying electricity from the grid (energy cost plus demand cost) minus any income from the sale of electricity to the grid. Is the negative value possible for the systems? What is the meaning of the negative value? Or did I a mistake?
A negative value would is possible if the money spent on electricity exceeds the money earned from selling electricity back to the grid.
The negative value would be the amount of money that you spend on electricity through the period, minus the money earned from putting electricity back into the grid.
This is a fairly common occurrence, unless a renewable project is designed with the intention of generating revenue from the grid it generally won't profit from grid buy back. Instead, renewable projects designed to supplement a demand tied to a reliable grid generally have value by costing less than the electricity from the grid at least some of the time. When grid electricity is less expensive or renewable resources aren't available, electricity will be purchased from the grid.
It might be helpful to see what the benefit of adding storage would be.