A “Grid-Tied” system simply means solar power that is interconnected with the grid through your home's breaker panel, and it operates only when there is grid power.
How it works
As solar power is being generated it goes directly into the home's breaker panel, powering your electrical loads directly and offsetting what would otherwise have been purchased from the grid/power company. Also, when there is more solar power than the home needs at any moment, the excess power can go to the grid, earning credit (which is called back-feeding the grid).
In other words, the home's breaker panel is powered simultaneously by solar power and grid power, and excess solar power can flow to the grid (resulting in credits issued from the utility).
In South Dakota, the interconnection policy is similar to net-metering but is instead known as net-billing. Each month, your power bill is reduced and any excess solar power sold to the grid is credited to that month's bill. More information about this can be read in our Local Utilities post.
Grid-tied solar requires an interconnection agreement with the utility, in order for the utility to install a bi-directional meter and compensate the generator (homeowner) for power generation according to PURPA guidelines.
It's also important to know that a grid-tied-only system means that there is no energy storage (battery), and no backup power if the grid goes down.
The basic grid-tie system serves one function for the homeowner: to offset a building's use of power from the grid. The purpose is not to sell power to the grid (since credits are worth less than the purchase price). However, being able to sell excess power is better than idle power with nowhere to go.
Since grid-tie-only systems don't include energy storage, they will operate only during the daytime. The capacity should be sized to cover the average daytime energy requirement and not significantly overproduce (unless the generation credit is worthwhile). Therefore, a properly sized system might cover around half of total usage.
Since it is not likely to align power needs with the sunshine every day, some solar power will inevitably back feed the grid as well. Strategies can be used by the homeowner to maximize solar power consumption, by running large appliances when the sun shines instead of at night, for example.