Photovoltaic (PV) power systems

Photovoltaic (PV) power systems convert sunlight directly into electricity. A residential PV power system
enables a homeowner to generate some or all of their daily electrical energy demand on their own roof,
exchanging daytime excess power for future energy needs (i.e. nighttime usage). The house remains
connected to the electric utility at all times, so any power needed above what the solar system can produce
is simply drawn from the utility. PV systems can also include battery backup or uninterruptible power supply
(UPS) capability to operate selected circuits in the residence for hours or days during a utility outage.

There are two general types of electrical designs for PV power systems for homes; systems that interact with
the utility power grid and have no battery backup capability; and systems that interact and include
battery backup as well.

Grid-Interactive Only (No Battery Backup)
This type of system only operates when the utility is available. Since utility outages are rare, this system will
normally provide the greatest amount of bill savings to the customer per dollar of investment.
However, in the event of an outage, the system is designed to shut down until utility power is
restored.

Typical System Components:

PV Array

A PV Array is made up of PV modules, which are environmentally-sealed collections of PV Cells—
the devices that convert sunlight to electricity. The most common PV module that is 5-to-25 square
feet in size and weighs about 3-4 lbs./ft Often sets of four or more smaller modules are framed or
attached together by struts in what is called a panel. This panel is typically around 20-35 square feet
in area for ease of handling on a roof. This allows some assembly and wiring functions to be done
on the ground if called for by the installation instructions.

Balance of System equipment (BOS) BOS includes mounting systems and wiring systems used to integrate
the solar modules into the structural and electrical systems of the home. The wiring systems include
disconnects for the dc and ac sides of the inverter, ground-fault protection, and overcurrent
protection for the solar modules. Most systems include a combiner board of some kind since most
modules require fusing for each module source circuit. Some inverters include this fusing and
combining function within the inverter enclosure.

DC-AC Inverter: This is the device that takes the dc power from the PV array and converts it into standard ac
power used by the house appliances.

Metering: This includes meters to provide indication of system performance. Some meters can indicate home
energy usage.

Other Components: Utility switch (depending on local utility)

Grid-Interactive With Battery Backup
This type of system incorporates energy storage in the form of a battery to keep “critical load” circuits in the
house operating during a utility outage. When an outage occurs the unit disconnects from the
utility and powers specific circuits in the home. These critical load circuits are wired from a subpanel that is separate from the rest of the electrical circuits. If the outage occurs during daylight
hours, the PV array is able to assist the battery in supplying the house loads. If the outage occurs
at night, the battery supplies the load. The amount of time critical loads can operate depends on
the amount of power they consume and the energy stored in the battery system. A typical backup
battery system may provide about 8kWh of energy storage at an 8-hour discharge rate, which
means that the battery will operate a 1-kW load for 8 hours. A 1-kW load is the average usage for
a home when not running an air conditioner.

Typical System Components:
In addition to components listed in above, a battery backup system may include some or all of the
following:
Batteries and battery enclosures
Battery charge controller
Separate subpanel(s) for critical load circuits