Bringing Solar Energy to Colorado

Consumer Center
Solar Electric Grid-Tied Systems

What about a grid-tied PV system? This type of system has a tie in with the electrical utility. When the PV system produces more electricity than the house can use, the excess electricity can be fed back into the utility grid. When the house requires more power than can be supplied by the PV system, power is drawn from the utility grid. Some grid-tied residential systems have a battery back up system, some don't. If for some reason the electrical utility grid goes down, those systems without a back up battery system will also shut down. A back up battery system will allow a grid-tied system to continue supplying power to the house after first disconnecting the house from the utility grid.

On this page we will show you what two typical grid-tied PV systems might look like. The first will be a system with no battery backup while the second will incorporate a backup battery system.

People who install grid-tied systems usually do so for non-economic reasons. PV systems are expensive and at today's typical electrical costs ($.08 - $.12 per KW-hr) and barring government or utility incentives, it takes many years for the system to pay for itself.

The first grid-tied system we will look at does not have a battery back up system. Power generated by the PV system is either utilized by the house loads or fed into the utility grid. At times when the house loads require power and the PV system cannot produce electricity, the electricity is provided by the utility grid. In essence, the utility serves the role of a backup battery system. This type of system is certainly cheaper to install and maintain than one with batteries. Batteries are expensive and do require maintenance and occasional replacement. However, if there should be a power interruption from the utility grid, the PV system also shuts down.

Grid-Tied Diagram
When power is fed into the utility grid, you should get paid for the electricity you produce. The amount of this payment is often up to the utility. Some electric utilities allow the customer to feed the electricity directly back into the grid and spin the electric meter in the reverse direction. Using this method, the electric utility is buying back the electricity at the same retail price that they are charging the customer, i.e., true net metering. Other utilities require a separate electric meter to monitor the electricity being fed into the electric grid. With this type of set up, the utility charges the customer the retail price and pays the customer a lower price for the electricity fed back into the power grid. The lower price can vary, but some states set this at the "cost avoided" price. Although some utilities like to also call this net metering (good PR), it is not true net metering.

Note the liberal use of circuit breakers and/or fuses. All circuit breakers and/or fuses must be rated for their intended DC or AC usage and UL approved. This is required in order to meet the National Electrical Code (NEC). If you are installing your own system, it is recommended you get professional help or ask your local electrical inspector since understanding the NEC is not an easy task and mistakes could easily result in a serious fire. In Colorado, the AC connections for grid-tied systems must be completed by a licensed electrician and the grid-tied connections approved by the involved utility.

This type of grid-tied PV system is a step up in complexity and cost from the system described above by the addition of a battery bank. The addition of a battery bank, however, does add a measure of independence in that with this system, if the utility grid goes down, the house can draw power from the batteries. Just how long the power can be supplied to the house depends upon the size or capacity of the battery system and how much power the house is requiring. When the utility grid power again becomes available, the batteries are automatically placed back into reserve status.
 Grid-Tied Diagram

There are PV mounting systems that will allow the modules to track the sun as it crosses the sky. Such trackers can increase the amount of yearly energy collected by a considerable amount. It may or may not provide an economical advantage for your chosen system but should, at least, be considered. PV cells are also available as a material resembling roof shingles and can be attached directly to the roof. You will need to look into whatever system appeals and is right for you.

It should be noted that the NEC requires a device called a ground-fault protection device be installed on any roof mounted PV system located on a dwelling whereas if the modules are ground mounted, the ground-fault protection device is not required.

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This page last updated on 10-3-05