Since its invention nearly 50 years ago, solar electric power
has been a reliable but relatively expensive technology. Now,
however, the cost to produce electricity from sunlight is
coming down. Climbing utility rates, combined with the rebates
that some states offer, are making photovoltaic (PV) power
systems cost-effective in many parts of the country.
In 1975 I started a company in Northern California that
specialized in solar thermal hot-water systems. While we still
install those systems, the bulk of our business these days
comes from designing and installing PV energy systems for
residential, commercial, and municipal clients. Sum total,
we've installed over a megawatt (one million watts) of solar
In this article, I'll explain how PV power systems are designed
and installed, so that contractors know what to expect when a
client decides to buy one.
What Customers Want
Clients have different reasons for wanting to install PV power.
For some, it's a way to act on environmental values or to
become self-sufficient. Others look at the rising energy costs
and believe that, in the long run, it will be cheaper to
produce their own power.
Financial incentives. Many
states, local governments, and utilities provide rebates and
tax breaks to property owners who purchase PV systems. The
state of California currently offers a cash-value rebate of
$2.80 for each watt of solar capacity installed. A typical
4,000-watt residential system costs about $8.40 per watt
without a rebate. The rebate brings this down to $5.60 per
watt. In New Jersey, the rebate is $5.50 per watt. Lower
rebates are offered elsewhere; for a database of state
incentive programs, go to www.dsireusa.org.
Service life. The systems we
install are designed for a 30-year service life. For the
customer, it's like paying up-front for 30 years' worth of
electricity. We can design the system to offset all or part of
the client's electricity needs. Most customers opt to entirely
eliminate their utility bills.
Solar modules come with 20- to 25-year warranties stating that
the module will still produce 80 percent of its rated output at
the end of the warranty period. In fact, the module will
continue to function indefinitely, but the output will drop
over time. Power inverters typically require one major service
or replacement during the life of the system.
Many people are confused about what solar can and can't do. One
misconception is that solar modules must be installed on racks
that project way off the roof (see Figure 1). Solar modules do
perform better when oriented perpendicular to the sun's path,
but they definitely look better when installed flush to the
roof. This reduces output, but can be offset by using money
that would have gone into the rack to increase the number of
Figure 1.Here, an installer bolts a mounting rail
to the roof. Modules usually sit on top of rails, but with this
low-profile rack they will drop between rails and be a couple
of inches closer to the roof than is typical.
Battery backup. Another
misconception involves what happens at night or when the
electrical grid is down. There are two types of solar electric
systems: grid-tied systems without battery backup, and
stand-alone systems that may or may not be tied to the grid but
have battery backup. If the utility grid is reliable, we
recommend against the added cost and complexity of a
battery-backup system. The only time we recommend batteries is
when the customer absolutely requires uninterruptible power or
is in an area where a grid connection isn't feasible.
Nonbattery grid-tied systems will not provide power when the
grid goes down. For safety reasons, the inverter in a
nonbattery grid-tied system automatically shuts down when the
grid is out, thus preventing electricity from backfeeding into
power lines and injuring linemen who are making repairs.
Net metering. In most states,
net metering legislation allows individuals to feed excess
solar energy into the grid. The utility meter spins backward
when clients are putting electricity in and forward when they
are taking it out. In a case like this, where there is no need
to store energy, the customer can install a less expensive
Some customers believe the utility will pay them if they put in
more power than they take out. Unfortunately, utilities will
not pay for unused credit. The best a customer can do is pay
nothing for electricity used. As a result, there is no
financial incentive to design a system that produces more power
than the client needs.
The most visible components of a standard grid-tied PV system
are the solar electric modules; these roof-mounted devices
convert sunlight to electricity. The modules produce DC power,
which passes through a DC-rated disconnect switch on its way to
a power inverter. The inverter converts the DC power to AC and
sends it to the main breaker box via an AC-rated disconnect and
standard circuit breakers (Figure 2, next page). Battery-backup
systems require additional components, including controllers,
power centers, solar subpanels, and batteries.
Solar modules. Most people
refer to individual solar modules as "panels," but,
technically, a panel is a group of modules that are wired
together. The roof-mounted portion of the system is called an
Solar modules are produced by a number of manufacturers and
come in many sizes and shapes. Right now, our best-selling
model is a particularly efficient 185-watt unit from Sharp. It
weighs 37 pounds, measures 62 inches by 32 inches, and is 2
inches thick. The edges are framed in aluminum and the face is
covered with tempered glass.
Figure 2.The drawing above indicates where
components would be located on a typical grid-tied
nonbattery-backup system. The diagram at right shows how
individual components are connected. Note how the main service
panel receives power from both the inverter and the utility
Strings. Modules are ganged together on the roof and wired in
series. Each group of series-wired modules is called a string,
and often there is more than one string per inverter. A typical
residential system might contain 21 185-watt modules in three
strings of seven.
Most solar modules produce either 12 or 24 volts. Higher
voltage does not equate to more power, however, because a
12-volt unit operates at higher amperage than a 24-volt unit of
equal wattage. Other things being equal, output is determined
by the number and type of modules. The modules usually account
for 75 percent of the total cost of a PV system.
The installation site will dictate the overall size of the
system. Considerations include roof orientation, shading,
available roof space, aesthetic concerns, and the capacity of
the existing service panel.
Life span of roof. Solar
modules usually have to be removed to reroof a building.
Fortunately, the 30-year design life of a solar energy system
is similar to the service life of most roofing products. It
makes sense to synchronize reroofing with the installation of
PV modules. We recommend reroofing the building if there are
obvious signs of wear and tear or if the roof has less than six
years of service left.
Location. It's extremely
important to avoid placing modules where they could be shaded
by trees, power lines, and other obstructions. Ideally, modules
would face due south. In some cases, though, it's possible to
install them facing slightly east or west; the drop-off in
production is not always that great.
In this area, an array could be oriented to the southwest and
still produce 90 percent of what it would produce if it faced
due south. An easterly orientation would be a poor choice
because we get a lot of morning fog.
Solar modules can be installed on nearly any type and pitch of
roof. They are usually attached to racks — metal rails
supported a few inches above the roof's surface by mounts that
are bolted to the rafters (Figure 3).
Figure 3.Post mounts are designed for new
construction and should be installed, flashed, and shingled
over before the rails and modules go on. Rails and modules are
already attached to the mounts upslope and will soon be bolted
to the mounts at left in this photo.
New construction and reroofs.
It's easiest to install mounts on new construction or on a roof
that has been stripped; this way, we can fasten them without
having to worry about leaks. On these jobs, we put post mounts
on the sheathing and lag them to the rafters. We then flash the
posts with the same flashings used on plumbing vents. This
obviously produces more penetrations than usual, but the roofer
should be able to warranty his work, because everything is
Tile. On tile roofs, we use
an aluminum mounting channel called a Tile Trac (Professional
Solar Products, 800/847-6527, www.prosolar.com). The channel bolts to the
deck and a threaded post extends up and passes through a
3/8-inch hole in one of the tiles. This penetration is caulked
and, because it's high on the tile and covered by a module, it
Existing roofs. Retrofits are
trickier. On tile roofs, tiles must be removed and reinstalled
over the mounts. On flat roofs, some of the membrane must be
removed and patched back in.
We install many systems over existing composition shingle
roofs. The preferred method is to fasten mounts through the
shingles. Tile Trac works well for this application. It comes
in 8-inch sections, which we install up the slope on a thick
bed of sealant (Figure 4). We've put a lot of effort into
finding appropriate products for this application and have
settled on tripolymer sealants like Geocel 2300 (Geocel Corp.,
800/348-7615, www.geocelusa.com) because they are durable,
adhere well, and are compatible with asphalt shingles. As an
added precaution, we divert water from the penetration by
installing a piece of step flashing just upslope.
Figure 4.It's common to retrofit systems to
buildings with existing shingle roofs. In the photo above, an
installer caulks the back of a Tile Trac mounting bracket
before bolting it through the shingles to the rafter below.
Metal flashings (right) are installed upslope to divert
rainwater from the penetration.
On retrofits, it's important to avoid drilling unnecessary
holes through the roof. It's easy to locate rafters when the
tails are exposed, but when they aren't, we go into the attic
and drill a pilot hole next to the rafter where the first mount
will be located (Figure 5). We leave the bit sticking up and
locate other mounts by measuring off it. Rafters may not be
evenly spaced, so we always check the layout from inside.
Figure 5.An installer locates the first roof mount
by drilling a pilot hole next to a rafter. Other rafters are
located from above by measuring off the protruding bit. To
prevent leaks, the installer fills the hole with sealant and
covers it with the mount.
Loads. The local building
department will want to know how much load the solar array adds
to the roof. It's typically 4 pounds per square foot. We've had
some success convincing concerned inspectors and engineers that
the added load on the existing structure is okay because no one
can walk on an area that's covered with modules. Also, it
doesn't snow in our part of the country, so offsetting the
local requirement for 20 pounds of live load with 4 pounds of
dead load is usually a no-brainer.
Hardware. Our systems are
designed to last 30 years, so it's important to use hardware
that will not corrode or react. All of the components on the
roof are made from aluminum or stainless steel, including the
lags that secure mounts to the roof, and the nuts, bolts, and
washers that hold the other parts together. We've come across
corrosion problems that occurred when competitors combined
aluminum, copper, and galvanized steel in the same
Whenever possible, we rough in home-run wiring from the roof to
the inverter location while the studs are still open. On
retrofits, we typically penetrate the roof eaves (Figure 6) and
run conduit down the outside of the building. We always make
sure that the owner signs off on the location of the
Figure 6.The crew uses a standard roof flashing
with a self-sealing collar to waterproof the opening where
electrical conduit penetrates the roof.
Once the conduit is installed, the solar crew pulls home-run
wiring from the module locations to the DC disconnect.
Typically, it's 10-gauge or 12-gauge outdoor-rated stranded
Connecting the modules. We
fasten the modules side by side on the rack and wire them
together in series (Figure 7). Each module has a positive and
negative wire: The positive from one module connects to the
negative on its neighbor. We used to hard-wire between
connection boxes on the backs of modules; now we connect them
using wires equipped with a waterproof quick-connect
Figure 7.On a series-connected system, adjoining
modules are connected positive to negative in much the same way
as batteries in a flashlight.
Grounding. Grounding is
extremely important and is something every inspector is
familiar with. Section 690 of the NEC (which deals specifically
with solar power) requires all modules and metal racks to be
bonded to the house grounding system. It also requires a
ground-fault interrupt for roof-mounted systems. Instead of
using separate wires to daisy-chain the modules together, we
ground them to each other and to the rack with a continuous
copper wire (Figure 8). We may need to add a GFCI, but one is
usually built into the inverter.
Figure 8.Modules should be grounded to each other
and to the rack with a continuous ground wire. On this project,
the installer is using a length of tinned copper braid to
ground the array.
Once the modules are connected, there will be a leftover wire
at each end of the string, one positive and one negative. We
connect these wires plus the ground to the wires from the
DC disconnect. By code, a PV
power system must have an adequately rated disconnect switch so
the inverter can be isolated from modules for servicing (Figure
9). The disconnect can be located anywhere, as long as it
conforms to NEC regulations with respect to accessibility and
maximum height. Manual disconnects must be mounted in such a
way that the midpoint of the handle in its highest position is
no more than 78 inches from the ground.
Figure 9.This job was new construction, so the
wires from the roof came down inside. The DC disconnect is
usually outdoors, but on this project it's in the crawlspace on
the other side of the wall from the inverter. Note the home-run
wiring on the neutral and ground.
The positive wire from the modules lands on a terminal in the
disconnect. We run the negative straight through to the
inverter, because the fewer connections there, the fewer
problems there are likely to be (Figure 10). A copper wire
grounds the disconnect to the inverter and roof array.
Figure 10.Each additional connection increases the
risk for problems. The hot wire lands on the terminal of this
disconnect, but the neutral and ground run straight through
without being cut.
Figure 11.The inverter converts DC power from the
modules to AC household current. It's a sophisticated piece of
control equipment that is the brain of a grid-tie system. The
fins on top of the case are for cooling.
A series-connected string of modules produces high-voltage DC
power, which must be run through an inverter to convert it to
120-volt AC (Figure 11, previous page). One of the more common
inverters we install is rated for 2,500 watts, but inverters
are also available for both bigger and smaller systems (Figure
Figure 12.It's sometimes more economical to install
two smaller inverters than to go to the next larger
Most inverters can be mounted indoors or out. They often
generate an appreciable amount of heat and make subtle humming
noises during heavy operation. For these reasons, the inverter
should be mounted in a well-ventilated area, out of direct
sunlight, and away from walls adjoining living areas that get a
lot of daytime use.
AC disconnect. Our local
utility requires a lockable AC-rated disconnect between the
inverter and main service panel. It's there to isolate the PV
system from the utility. Although it seldom does so, the
utility has the right to shut off and lock out the PV system at
this disconnect to prevent power from backfeeding into the
grid. For this reason, the AC disconnect must be outdoors and
within 10 feet of the main service panel (Figure 13).
The inverter produces 120-volt AC power,
which passes through an AC disconnect on its way to the main
service panel. The hole through the lever is for a padlock to
lock out the system.
Main Service Panel
The solar electric system is usually connected to the grid
through the main service panel. AC power comes from the
inverter, passes through a circuit breaker, and lands on the
bus bar in the panel.
The electrical sub should talk to the solar sub to make sure
there is room for an extra breaker in the service panel and
that the bus bar is rated to accept the amount of power that
comes from the grid plus additional power from the PV system.
The existing service panel may be inadequate, in which case it
will have to be upgraded or replaced before the PV system can
Tie-downs. By code, breakers
that backfeed panels must be tied down to prevent them from
coming free. A few municipalities enforce this provision, but
we've convinced many that it's unnecessary, since all grid-tied
inverters automatically disconnect from the grid when the grid
goes out. This comes up because most residential systems are
tied to the bus bar by 15-amp or 20-amp breakers, a size not
normally available with tie-downs. Breakers can be adapted for
tie-downs, but it's awkward to do.
Gary Gerberhas been in the solar business since 1976
and is the owner of Sun Light and Power in Berkeley,
The first thing the solar contractor needs to do is
assess the clients' energy goals. Do the clients
want to produce 30 percent, 50 percent, or 100
percent of the power they use? Do they plan to add
on to the building or change something about the
way it's used? Once we know what the clients want,
we estimate how much energy the system will need to
Historical vs. projected
usage. The simplest and most accurate method
is to look at past utility bills and add up the
number of kilowatt-hours (kwh) used per year.
Divide this number by 365 and you have the average
daily power consumption. The chart below contains
sample data from one of our customers.
If there isn't any historical data, as happens in
new construction, we sit down with the customers
and list all the loads that are likely to occur.
The list includes information such as the
appliances they own, the number of hours per day
they are at home, and how often they use air
conditioning or heat. This method is most common in
The historical method is most accurate because it's
based on real data from the actual client. Data
from previous occupants or information gained
before the completion of a major remodel can lead
to inaccurate load assessments.
Knowing the average daily consumption, we can
roughly size the system based on the number of
sun-hours per day where the building is located. In
California, the yearly average is four to five
hours of sun per day. In New England, it might be
only three to four hours per day. The rough
calculation for a system in San Francisco is as
12.45 kwh per day ÷ 5 sun-hours per day =
2.49 AC kw system size
We perform the final calculation with an online
design tool called PVWatts (see
PVWatts uses test data from actual systems to
accurately project output based on such local
conditions as weather and the orientation of
modules on the roof. In my area, a 2.49-kw system
with a true southern orientation and an 18-degree
(4/12) slope will produce 4,441 kwh per year.
Because the client needs 4,545 kwh of electricity
per year, a 2.49-kw system will be undersized.
According to PVWatts, a 2.55-kw system will produce
4,548 kwh, which is just about right.