What does on-grid or grid-tie and off-grid mean when referring to a solar photovoltaic system?

An on-grid or grid-tie solar photovoltaic system is when your solar photovoltaic system is connected to a utility source or electrical service provider.  You pay for the amount of electricity you consume to the utility provider.  Grid-tie solar systems use inverters that meet the requirements of the utility and must not emit noise that can interfere with electrical devices such as reception for television and must retain acceptable levels of harmonic distortion for quality of voltage and current output waveforms.  On-grid solar systems are advantageous as the electricity is utilized by the owner during the day and any surplus is supplied to the grid; some utilities pay for this surplus electricity at a higher rate known as a Feed In Tariff (FIT).  During the evenings, when solar electricity is not generated, electricity is drawn from the grid avoiding the need for costly battery banks.

An off-grid or standalone photovoltaic system is when your solar photovoltaic system is not connected to the utility grid and you are producing your own electricity via solar, wind, microhydro, generator, etc.  These systems will generally have a battery bank in order to store the electricity for use when needed.