Solar Power – Part of the Green Energy Solution in Massachusetts
Fred Paris for: Co-op Power
Sustainable Energy Summit
Interactive (Grid-Tied) Solar
Overview An interactive, or what is sometimes called grid-tied, photovoltaic solar system can
be designed to provide as little or as much of the electricity a home requires.
System sizes can range from small starter systems to larger arrays sized to eliminate
a home’s electric bill. Once the system is installed a homeowner could expect to gain
solar energy for 20 years or more. There are many examples of systems 25 years old
or more still performing at more than 80% of their original energy output.
Interactive systems are for homes that are connected to a public utility. There are two
significant advantages to interactive systems; first, the occupants of the home need
never worry about where their electricity is coming from. If the solar resource is not
available or if the electrical load is higher than the array can support, the utility will
supply up to 100% of their energy load. Second, interactive systems do not require
batteries. Batteries typically add thousands of dollars in costs and require
knowledgeable and frequent maintenance.
Fred Paris email@example.com Co-op Power Sustainable Energy Summit 2009 Page 1 of 9
How Solar Works
How Solar When a home has an interactive solar PV system, the electrical devices in a home
Works will use the owner’s solar power first and then look to the utility to cover the
additional power required. That makes it easy to understand that the larger the PV
system the less energy a homeowner will buy from the utility, and the lower their
monthly bill will be. It also explains how a homeowner can start with a small system
and incrementally add capacity as they watch their energy bill decline.
The other significant benefit of an interactive PV system is net-metering. Under net-
metering arrangements - available anywhere in Massachusetts – if a home’s PV
system is generating more energy that is being used, the energy goes back to the
utility for full retail credit. So, if you are paying 18¢ a kilowatt-hour (kWh) the
utility will pay you the full 18¢ for every kWh you send back. Think about those
long hot summer days when the house is empty – it may all go back to the utility.
There are four key elements to a solar photovoltaic system: the solar modules, the
inverters, the production meter, and the typical electrical switches and wiring
associated with any electrical system.
The Solar When sunlight strikes a solar module, the characteristics of silicone layered with
Modules other materials causes electrons to start flowing. Get enough electrons moving and
you create electrical current. Give that current a pathway to follow, and then join
multiple modules and pathways together, and you can design and create systems of
any size. A small starter system may only have a few solar modules while some of
the largest commercial systems cover acres of land.
Multiple solar modules wired together form a solar array. An advantage of
residential and commercial arrays is that they can be added to incrementally without
discarding the components installed earlier. So a homeowner might install a small
system and later add more modules and another inverter to build any size they want.
Continued on next page
Fred Paris firstname.lastname@example.org Co-op Power Sustainable Energy Summit 2009 Page 2 of 9
How Solar Works, Continued
The Inverter(s) The inverter is the device that changes Direct Current (DC) to Alternating Current
(AC). Solar modules produce DC but our homes require AC. AC is also what the
utility delivers to the home. For most homes the inverter will be a breadbox-size
electrical component installed by an electrician near the circuit breaker panel.
Inverters monitor and run on power from the utility. This serves a couple of
purposes; first, if the utility goes down, the inverter shuts down. We do not want
solar systems feeding the grid with utility linemen at work. The inverter
synchronizes the AC sine wave to the utility’s sine wave, allowing for net metering.
Continued on next page
Fred Paris email@example.com Co-op Power Sustainable Energy Summit 2009 Page 3 of 9
How Solar Works, Continued
The Production Unlike the utility meter, the homeowner owns the production meter. The
Meter homeowner’s meter only measures the energy produced by their solar system. This
allows the homeowner to see exactly how much power their system is producing. In
most cases, a production meter is a required part of a residential solar system.
One of the additional benefits of a residential solar system is the production and
accounting for virtual Renewable Energy Credits (RECs). A discussion of RECs is
beyond the scope of this paper, but for every kilowatt of energy a solar system
produces, RECs are generated in the owner’s name. RECs are then sold as a
commodity. The production meter helps the homeowner validate RECs.
Switches and Everything gets connected together. The modules are wired together, then to the
Wiring inverter and the production meter. AC and DC switches are installed enabling the
owner or the utility to shut down the entire system as required by the National
Electric Code (NEC).
Fred Paris firstname.lastname@example.org Co-op Power Sustainable Energy Summit 2009 Page 4 of 9