2.1.2. 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
PV Array DC/AC
PV Array Inverter
Circuit
Combiner
Main Service
Panel
Utility
Utility
Ground-Fault Switch
Protector
AC
Fused
Switch
DC
Fused
Switch
Grid-Interactive PV System w/o Battery Backup
PV Installation Guide
June 2001 Page 6
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 2.1.1., a battery backup system may include some or all of the
following:
1. batteries and battery enclosures
2. Battery charge controller
3. separate subpanel(s) for critical load circuits
2.2. Mounting Options
There are several ways to install a PV array at a residence.
Most PV systems produce 5-to-10 Watts per square foot of
array area. This is based on a variety of different technologies
and the varying efficiency of different PV products. A typical 2-
kW PV system will need 200-400 square feet of unobstructed
area to site the system. Consideration should also be given for
access to the system. This access space can add up to 20% of
needed area to the mounting area required.
2.2.1. Roof mount
Often the most convenient and appropriate place to
put the PV array is on the roof of the building. The
PV array may be mounted above and parallel to
the roof surface with a standoff of several inches for
cooling purposes. Sometimes, such as with flat roofs,
Figure 1 Roof Mounted PV System
PV Array
PV Array
Circuit
Combiner
Ground-Fault
PV Array Protector
Switch
Backup
Power
System,
DC/AC
Inverter,
and
Battery
Charge
Controller
Main Service
Panel
Utility
Utility
Switch
AC
Fused
Switch
Backup
Battery
Charge
Controller
Battery
System
Critical Load
Sub-Panel
PV Installation Guide
June 2001 Page 7
a separate structure with a more optimal tilt angle is mounted on the roof.
Proper roof mounting can be labor intensive. Particular attention must be paid to the roof
structure and the weather sealing of roof penetrations. It is typical to have one support bracket
for every 100 Watts of PV modules. For new construction, support brackets are usually mounted
after the roof decking is applied and before the roofing materials is installed. The crew in charge
of laying out the array mounting system normally installs the brackets. The roofing contractor
can then flash around the brackets as they install the roof. A simple installation detail and a
sample of the support bracket is often all that is needed for a roofing contractor to estimate the
flashing cost.
Masonry roofs are often structurally designed near the limit of their weight-bearing capacity. In
this case, the roof structure must either be enhanced to handle the additional weight of the PV
system or the masonry roof transitioned to composition shingles in the area where the PV array
is to be mounted. By transitioning to a lighter roofing product, there is no need to reinforce the
roof structure since the combined weight of composite shingles and PV array is usually less than
the displaced masonry product.