Basic Electric for Photovoltaics (PV) and Wind Turbine Energy Systems (WTES)
How does a grid-tied photovoltaic (PV) system work?
Photovoltaic means light (photo) to electricity (voltaic). Light, usually sun light shining on photovoltaic (PV) cells that produces or generate about 2 volts DC per cell. Several PV cells are connected in series that is positive to negative and form a PV module. The voltage of a module can be between 18 volts to 64 volts depending on the number of PV cells connected in series and internal circuitry of the module.
The most common type of PV systems in use today is what is referred to as a string inverter. A string inverter usually has several PV modules connected in series that is positive to negative to form a PV array.
The PV array voltage can be anywhere from 34 volts DC to 600 volts DC. The PV array DC voltage then goes to the inverter where the DC voltage is changed to AC voltage that is compatible with the utility power and the building lighting and electrical equipment. The electrical energy generated by the PV system can then be used to run the lights and equipment in the building and the excess energy sent to the utility. If no lighting or equipment in the building is consuming the electric energy then all the energy is sent to the utility company.
What is the difference between AC (alternating current) and DC (direct current) power?
AC or alternating current is electric current that flow in one direction the in the opposite direction for each cycle. In North America the utilities supply 60 cycles (60 hertz) AC meaning that the current changes direction (from positive to negative and back to positive) 60 times a second.
DC or direct current flows only in one direction; conventionally from positive to negative. Batteries and photovoltaic cells produce DC or direct current. Most wind turbines also produce DC.
What is the difference between a kW (kilo-watt) and a kWh (kilo-watt-hour)?
Let’s back up a little. A watt is the instantaneous unit of electrical power being consumed (light bulb) or generated (PV). A 100 watt light bulb consumes 100 watts of electricity power as long as the light bulb is lit. A kilowatt (kW) is 1,000 watts; ten 100 watt light bulbs will consume 1,000 watts (10 bulbs X 100 watts) or 1 kilowatt (kW). A PV system might produce or generate 1 kW.
A kilowatt-hour (kWh) is the electrical energy consumed or produced over a period of time. If the ten 100 watt light bulbs are on for one hour then 1 kWh of electricity energy is consumed (10 bulbs X 100 watts X 1 hour). If the ten lights are on for 6 hours, then 6 kWh of power is consumed (10 bulbs X 100 watts X 6 hours).
In a photovoltaic system, if we have a 3 kW photovoltaic (PV) system ideally installed and there is 3 sun hours* the system will produce or generate 9 kWh (3 kW x 3 sun hours*) Utilities buy and sell electricity to the consumer by the (kilowatt-hour) kWh.
*Not the same as 3 hour of sun light.
What is an inverter and how does it work?
An inverter is a piece of electrical equipment that changes direct current (DC) form the photovoltaic (PV) array or wind turbine (WTES) into alternating current (AC) that is compatible with utility power and the equipment found in the homes and businesses.
The inverter also performs a second vital safety function. The inverter will turn off if there is no utility voltage is present at the inverter. This prevents electricity from flowing from the inverter into the power lines protecting utility workers that might be working on the utility line from electric shock or electrocution.
What is UL (Underwriters Laboratories) and how does it work with solar energy?
UL or Underwriters Laboratory is one of 14 National Recognized Testing Laboratories. UL performs two vital services; first UL write the standards that all electrical equipment must be manufactured to; second UL is one of the 14 NRTL that tests electric equipment to make sure that electrical equipment conforms to the UL standards.
What inspections are required for Photovoltaics (PV), domestic solar hot water (DSHW) and wind turbine (electric) systems (WTES)?
Electrical inspections are required for both photovoltaic (PV) and wind turbine systems (WTES). See RELI for electrical inspectors that have been trained in photovoltaic systems.
Each municipalities has their own requirements for inspection of the mechanical aspects of photovoltaic (PV), domestic solar hot water (DSHW) and wind turbine (WTE) systems. Check with the local municipality for the requirements.
Do I need an AC (CO-GEN) disconnect switch outside by the electric meter? (PV) (WTES)
LIPA requires all power generating equipment, including photovoltaic and wind turbine systems, connected to a service to have a CO-GEN disconnect switch with 10 feet of the electric meter for the safety of its employees and the general public. (See LIPA Red Book Section 10.4.5 for requirements)
What electric codes govern Photovoltaics (PV) and wind turbine (electric) systems (WTES)?
All one and two family and town houses fall under the Residential Code of New York State (RCNYS)*. Section 33. States that the 2002 National Electrical Code applies.
The Building Code of New York State required that all other buildings use the 2005 NEC
*New York City- has its own building code that is based on the 2005 NEC with several amendments including the requirement that all PV systems must be listed by a NRTL as a system.
*Town of Brookhaven has a More Restrictive Local Code (MRLC) allowing Brookhaven to use of the 2008 NEC.
When will New York State be going to new codes?
The2010 Residential Code of New York State, the 2010 Building Code of New York State and 20909 The New York City Electrical Code have been vetted and waiting on acceptance in Albany. After acceptance in Albany, there will be a 60 day period is for the new codes become mandatory.
Both New York State codes will reference the 2008 NEC.
The 2011 NEC code will be going into effect January 1, 2011 but is not referenced by New York State.
Career Resources, Education and Training
Does LISEIA have a job board?
Yes! Go to the Jobs Board Page
How can I get training on solar energy?
Long Island and New York City has several schools that offer certified courses in solar energy.
Go to the Education Page *
*Webmaster note: education is available from a variety of sources state wide, contact information on training & classes page.
How can I keep up to date on solar energy?
At LISEIA bimonthly meetings hot topics and issues are discussed. There is usually a code presentation at the meeting and also the opportunity to discuss code issues with inspectors. In addition the LISIA members and associates are kept up to date via e-mails on hot topics.
Does LISEIA provide representatives for solar presentations for prospective customers, business and community groups, schools, Fire Departments, etc.?
Yes! Several contractors offer PV seminars for prospective customers. LISEIA also provides speakers for presentations to community groups, schools and Fire Departments.
-FAQ section above contributed by Jerry Flaherty of Electrical Inspection Service, Inc.