Stand-alone or grid-tied - which approach is best?


There are basically two PV approaches for residential systems. The battery supplied inverter for stand-alone and net-metering inverters/grid-tied inverters.

The battery supplied inverter takes its (typically 12 volt to 48 volt Direct Current -DC) power from a bank of batteries and conditions and inverts it to produce 120 volt AC power. The frequency is determined by the inverter's electronics - in North America it's 60 hz.

The grid-tied inverter takes its power from directly from the PV panels. The AC voltage output to either the home or the grid (if the home isn't using all of the power generated) is synchronized to the power on the grid - matching the grid's voltage and frequency exactly (or at least to the standards of the utility the PV systems is attached to.) In this manner power in excess of the home's immediate needs is fed into the grid. In Ontario the utility must buy back the power at the rate it sells it to you at the time the transfer takes place (net metering), or if you sign a contract with the Independent Electricity System Operator (IESO) you can get paid up to 29.4 cents per Kilowatt-hour you put back into the grid (Micro-FIT,as of May 2016).

The drawback to grid-tied PV inverters is that if the grid power goes down, so does the PV system. The PV system cannot supply power to the home even if it's sunny outside. There is no storage of power without batteries. A separate inverter and battery bank would be required to capture and use the power of the PV panels - and it would not be eligible for the Micro-FIT program.