The second type of utility-connected system, grid-tied systems with batteries or battery backup abilities, will allow you to provide power during utility power outages... With this system you will also have a PV array, grid-tied inverter, but you will require the addition of a battery bank, charge controller, which regulates the amount of power sent to the battery bank from the array, and misc. balance of system components... The power produced by this type of grid-tied system will not produce as much power to the electrical grid as a grid-tied system without batteries... This is due to energy produced by the PV array, must first goes through the battery bank, and any left over energy from the battery bank is sent to the electrical grid... Under normal operating conditions, the batteries in the battery bank are not cycled... Meaning that the batteries are not discharged of energy and then re-charged... Thus, the PV array maintains a float charge on the batteries in the battery bank... During a power outage, the inverter immediately disconnects from the grid and utilizes battery power to energized the backed-up loads electrical subpanel, which is isolated from the utility...

Interconnecting a PV system with the utility grid will require entering into an interconnection agreement with the local utility... The interconnection agreement specifies the terms and conditions under which the PV system will be connected to the utility grid... It includes the technical requirements necessary to ensure safety and power quality and other issues, such as the obligation to obtain all necessary permits for the system and having the PV system insured... The key to obtaining an agreement is to involve the utility as early as possible in the installation...

Recently, progress has been made in developing nationaly recognized standards for the utility interconnection of PV systems... Although these standards are not necessarily binding on utilities, many utilities are adopting the standards rather than developing their own... The below two standards are particularly relevant...

Institute of Electrical and Electronic Engineers, IEEE Standard 1547: Recommended Practice for Utility Interface of Photovoltaic Systems. (Institute of Electrical and Electronic Engineers, Inc., New York, NY)

Underwriters Laboratories, UL Subject 1741: Standard for Static Inverters and Charge Controllers for Use in Photovoltaic Power Systems (First Edition). (Underwriters Laboratories, Inc., Northbrook, IL - December 1997) Note: An inverter listed to UL 1741 with the words "Utility Interactive" printed on the listing mark indicates that the unit is fully compliant with IEEE 1547...

The Interstate Renewable Energy Council (IREC) recommends practices and guidelines regarding grid interconnection issues... IREC is a non-profit organization commited to accelerating the sustainable utilization of renewable energy resources and technologies... For more information, refer to their website at http://www.irecusa.org/connect.htm

Utility-connected systems present some unique issues in meeting the NEC or National Electric Code... All electronic devices and system components need to be UL listed for the proper application and rated to the correct voltage... DC system components must carry a specific DC rating... These items include circuit breakers, fuses, disconnects, mechanical connectors, etc.... See NEC 2005, 2008, Article 690 for specific labeling details for utility interactive systems....

Net Metering allows the exchange of any surplus energy produced by the PV system for utility energy credit to be used during periods when the PV system is not producing enough energy to meet the needs... This means that the electric meter spins "backwards" when power is flowing from the building to the utility, and spins "forwards" when electricity is flowing from the utility into the building... At the end of the month, only the net consumption is billed... It is the amount of electricity consumed, less the amount of electricity produced... The utility acts much the same as a battery, crediting the energy "account" for later use if production exceeds consumption... For example, during the middle of the day, the system produces three kilowatt-hours but the building uses only one kilowatt-hour... Thus, the "account" will be credited for two kilowatt-hours... Later that evening, two additional kilowatt-hours might be used and the "account" ends up with a net zero balance, owing the utility nothing for that day...

The net metering protocal is a benefit to small renewable energy systems... There are three main reasons net metering is important... First, as increasing numbers of primarily residential customers install renewable energy systems in their homes, net metering provides a simple, standardized protocal for connecting their systems into the electricity grid that ensures safety and power quality... Second, as many residential customers are not home using electricity during the day when their systems are producing power, net metering allows them to receive full value for the electricity they produce without installing expensive battery storage systems... Third, net metering provides a simple, inexpensive, and easily administered mechanism for encouraging the use of renewable energy systems, enabling important local, national, and global benefits....

Net metering provides a variety of benefits for both utilities and consumers... Utilities benefit by avoiding the administrative and accounting costs of metering and purchasing the small amounts of excess electricity produced by these small-scale renewable generating facilities... Net metered PV systems can potentially reduce the utility's peak load demand... Because peak load generation is often expensive ($15-20 per kwh) this results in significant savings for utilities... Consumers benefit by getting greater value for some of the electricity they generate, being able to interconnect with the utility using their existing utility meter, and being able to interconnect using widely-accepted technical standards...

The only cost associated with net metering is indirect; the customer buys less electricity from the utility, which means the utility collects less revenue from the consumer... The reason is that excess electricity that would have been sold to the utility at the wholesale or "avoided cost" price is instead being used to offset electricity the consumer would have purchased at the retail price...

In most cases, the revenue loss is comparable to the customer reducing their electricity use by investing in energy efficiency measures,such as compact fluorescent lights and efficient appliances... The bill savings for the consumer and corresponding revenue loss to the utility depends on a variety of factors, particularly the difference between the "avoided costs" and retail prices...

Note: The standard kilowatt-hour meter used by the vast majority of residential and small commercial customers accurately registers the flow of electricity in either direction... This means the "netting" process associated with net metering happens automatically... The meter spins forwards in the normal direction when the consumer needs more electricity than is being produced and spins backwards when the consumer is producing more electricity than they need in the house or building... Some utilities use a meter that records the number of times the meter spins, not registering if it is moving forwards of backwards... This type of meter will bill the homeowner for the PV power produced!!!

Currently, many US states have some form of net metering... Germany, Japan, and Switzerland also have net metering... Many US states net metering rules were enacted by state utility regulators pursuant to state implementation of the federal PURPA statue... In recent years many states have enacted net metering laws legislatively... For more information about states with net metering legislation and incentives for renewable energy systems, refer to the Database of State Incentives for Renewable Energy (DSIRE) located at: http://www.dsireusa.org