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Grid-tied solar power systems and off-grid solar power systems both use modules and an inverter. They part company at the utility wire. A grid-tied system is allowed to run because the grid is there, and it must stop when that grid becomes an island. An off-grid system has no utility to follow, so its inverter has to make the AC supply, and something has to store energy for the hours after sunset. Confusing the two is how a house with a roof full of panels goes dark in a storm, and how a cabin is sold a grid inverter that will not start.

IEEE Std 1547-2018, in the National Renewable Energy Laboratory’s 2022 primer, requires a grid-connected distributed energy resource to detect an unintentional island and cease to energize it within 2 seconds. That behavior is type-tested, typically under UL 1741, with ride-through and voltage support enabled rather than switched off. It is the correct behavior of a listed grid-tied inverter. It is the wrong behavior if the design goal was power during the outage.

What a grid-tied solar power system does

While the utility is up, a grid-tied solar power system reduces what the building buys and may export the surplus if the tariff and the interconnection allow it. The inverter synchronizes to the grid. Ride-through keeps a large fleet from dropping off during a disturbance the bulk system wants it to survive. Tripping still outranks ride-through when voltage or frequency is actually out of bounds. The primer’s priority is disable, then trip, then ride-through, then power limits and voltage support. A grid-tied system can be excellent at a bill and useless in a blackout.

Backup is not included. Selected circuits stay on only if transfer equipment and a battery were designed for that island. Energy storage for the evening, without an outage, is a different job again: move midday kilowatt-hours into a later hour. The Department of Energy describes batteries as storing energy for later. That later hour does not appear because the modules are grid-tied. It appears because storage was specified. The split is in solar energy storage advantages.

What an off-grid solar power system has to include

An off-grid solar power system is the supply. There is no utility voltage to copy. The inverter forms the AC waveform, the battery covers the night, and a generator is often the plan for the week the weather exceeds the design. How many days that battery is asked to cover is a site choice, not a property of the word off-grid. Living with that arithmetic is already set out in living off grid with solar storage. This comparison only needs the boundary: if the loads continue after dark, storage is not optional.

The inverter size is the overlapping AC load, including motor start, at the temperature where the unit will sit. A grid-tied interactive inverter does not become off-grid because the array is large. Sizing the forming inverter is in how to size an off-grid inverter. The array and the battery still have to refill each other. A huge inverter on a small battery trips or dies quietly. A huge battery on an inverter that cannot start the well pump is the other failure.

Charge control sits on the DC side of many off-grid systems. A controller that pulls the array down to battery voltage is a different device from one that tracks the array’s own maximum-power voltage. Mixing those up is a common reason an off-grid system “never fills the battery.” The difference is in solar charge controller parameters.

Grid-tied vs off-grid solar power systems

Question Grid-tied solar Off-grid solar
What starts the AC The utility. The inverter follows it. The battery inverter. It has to create the supply.
Utility outage Cease to energize within 2 seconds under IEEE 1547-2018, unless a separate backup island was built. There is no utility outage. The site fails when the battery and generator do.
Night load Bought from the grid, unless storage was added to shift solar. Comes from the battery, sized for that night.
What “enough solar” means A bill and an export limit. Refilling the battery on a short day, with a plan for the worse week.

A hybrid that is grid-tied and can also island selected circuits is a third design. It must do the IEEE stop when it is still connected, and it must form an island only after it has disconnected. Buying “hybrid” on a label without the circuit list and the transfer hardware is how the table’s left column gets sold as the right column.

How to choose between grid-tied and off-grid solar

Choose grid-tied when the utility is reliable enough for your outage tolerance and the value is the bill. Ask whether export is paid, and whether any circuits must stay up. If the answer to the second question is yes, you are no longer buying a plain grid-tied system. Choose off-grid when there is no utility, or when extending the line costs more than the storage and the generator. Then size the battery for the night you will actually live through, and the inverter for the motors that start, and keep a log of generator hours so you can see whether the solar is the supply or a partial offset.

On either system the modules are live in daylight. Pacific Northwest National Laboratory states that plainly, and the shutdown for a grid-tied array is still the posted AC disconnect, then the DC disconnect, by someone trained. Off-grid DC connectors are not safer because the site is small. Monitoring is different too. A grid performance ratio does not show battery state of charge. An off-grid log that ignores generator hours hides the real supply. Those instruments are separate articles, and they should stay separate.

Frequently asked questions

Will a grid-tied solar power system run the house in a blackout?

Not by itself. IEEE 1547-2018 requires it to stop energizing an island within 2 seconds. Sun on the roof does not override that. Backup needs equipment that disconnects from the utility and carries only the circuits that were selected.

Is off-grid solar cheaper because there is no utility bill?

The utility bill is gone. The battery, the generator fuel, and the replacement of those parts are not. Off-grid is the right buy when the grid is absent or the line extension is the more expensive object. It is not a discount version of a grid-tied roof.

Can one inverter be both?

Only if it is built and listed to follow the grid when the grid is present and to form an island after it has disconnected. A standard interactive inverter will not form a supply. Read the certificate, not the brochure word hybrid.

Sources and references

Cease-to-energize, ride-through priority, and UL 1741: NREL and Sandia primer on IEEE Std 1547-2018, 2022. Live modules and array connection scale: Pacific Northwest National Laboratory. Storage as energy for later: US Department of Energy, grid modernization.