Understanding Net Metering


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Understanding Net Metering

Anti-Islanding Requirement for residential solar installations

Prior to 1980, the most common application of photovoltaic (PV) technology was off-grid, also known as stand-alone. But today, more than 95% of solar installations are on-grid, or “grid-tied.” These most common PV systems are also known as utility-interactive, grid-intertied, or grid-direct, and are intended to operate in parallel with an electric utility. They generate solar electricity and route it to the loads and to the electric utility grid, offsetting a home’s or business’s electricity usage.

Living with a grid-connected solar-electric system is no different than living with utility electricity, except that some or all of the electricity you use comes from the sun. If the PV panels are producing more electricity than you are using, the system will feed the surplus of the energy back to the utility. It may even spin your electric meter backwards, further reducing your monthly bill.  In many states, the utility credits a grid-tied customer’s account for solar electricity produced during each billing cycle, which is then applied to periods when the system produces less or electrical consumption is greater. This arrangement is called net-metering or net billing. The specific terms of net-metering laws and regulations vary from state to state and utility to utility.

on grid PV diag

A notable limitation of these battery-less systems is that they provide no outage protection, i.e., when the utility grid fails, these systems cannot operate. This is due to safety concerns over a condition known as “Islanding” in which a distributed (DG) generator continues to power a location even though electrical grid power from the electric utility is no longer present. Islanding can be dangerous to utility workers, who may not realize that a circuit is still powered, and it may prevent automatic re-connection of the utility network. For that reason, distributed generators must detect islanding and immediately stop producing power; this is referred to as anti-islanding.

The common example of islanding is a grid supply line that has solar panels attached to it at a PV net-metering-equipped residence. In the case of a blackout, the solar panels will continue to deliver power as long as daylight is sufficient. In this case, the supply line becomes an “island” with power surrounded by a “sea” of unpowered lines. For this reason, solar inverters that are designed to supply power to the grid are required to have some sort of automatic anti-islanding circuitry in them.  Since 1999, the standard for anti-islanding protection in the United States has been safety standard UL 1741, which is harmonized with IEEE 1547, IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems.  DC-AC inverters which are labeled as safety-listed to UL 1741 have ant-islanding protection as a required safety feature.