Photovoltaic Systems

By Kenton Shepard

Photovoltaic (PV) is the term used to describe systems which convert sunlight directly into electricity. To collect sunlight, PV systems use either panels or shingles, although panels are much more common. Individual PV panels are called “modules” and a number of modules can be mounted together on racks to form an “array”.

An array may consist of modules connected together in a single string, or modules may be connected to form several strings within the array. This is important because shading one module in a string will shut down the rest of the modules in that string. If all the modules in an entire array are connected to form only one string, the entire array could be shut down by shading one module.

PV systems have no moving parts, have 20-year plus lifespans and no negative environmental impact once they’re installed. Typical panels are now around 15% efficient, although panels not yet publicly available approach 30% efficiency.

To help avoid this problem some systems provide a “combiner box” in which electrical connections for individual modules are made inside an electrical panel mounted in an easily accessible.

Two types of PV systems are available, grid-tie and battery back-up.

GRID-TIE SYSTEMS

A “grid-tie” system is one in which, at a home conventionally connected to utility company power supplies, a special electric meter is installed which can spin both forward and backward, often called a “net meter”. When the PV system is producing more power than the home uses, the meter will spin in reverse.  Utility companies review the results annually and if the system owner has produced more electricity than they have used, they typically receive compensation. Compensation agreements between PV system owners and utility companies vary.

Typical is a situation in which the utility will sell electricity to consumers at a price of say, 11cents per Kilowatt hour (KWh) but will pay only a wholesale price of 4 cents per KWh to PV system owners who produce an annual surplus.

One of the disadvantages of the grid-tie system is that if the home loses power from the utility company, the PV system is designed to shut down automatically also, meaning that the home will be without electricity as long as no power is supplied from the utility provider. This feature is designed to protect utility employees working on power lines, since power from the PV system not used at the home goes out the service wires, through the transformer on the pole and into the utility companies’ electrical distribution system.

Inspection of PV Systems

Complete inspection of a PV system requires a qualified specialist. PV systems can produce lethal amperages and Home Inspectors without special training specific to PV should under no circumstances attempt to perform complete PV inspections. Proper grounding and bonding methods are crucial, line-side taps are not unusual and dangerous conditions my not be obvious to inspectors who feel comfortable inspecting conventional home electrical systems but are not PV-qualified.

Sunlight strikes the modules and is converted to DC electrical power. Modules in the array in the center photo have had silver electrical connections darkened for aesthic reasons. On the right, the first 3 courses and the upper roof shingles are PV shingles.

The array should be mounted in an unshaded area of the roof.

The array should be mounted in a manner which allows air to circulate beneath the modules, since heat from the array must be able to dissipate or system efficiency will be degraded.

                        Stand-offs should be properly flashed                                                             Racks should have a ground wire visible.      

 In addition to the racks, individual modules in the array should be grounded using specific, proper grounding methods, the confirmation of which exceeds the scope of a General Home Inspection .           

Wires installed accross the face of the module collect electrons and channel them to larger conductors...

...which channel them through even larger conductors to a junction box which also supports a barrel-shaped, silver-labed, gel-filled, lighting arrestor.

A full PV inspection requires calculations to confirm adequate conductor size which lie beyond the scope of a General Home Inspection.

Conductors from the array should be contained in conduit. If the conduit enters the home interior it should be metal. If the conduit doesn't enter the home interior, it may be plastic.

DC power travels from the arrays to a DC disconnect which should be clearly, permanently marked. Power then flows to the inverter, on to the AC disconnect, then to the backfeed breaker. Two systems are shown here.

Inverters will have a display window with a readout showing how much power the system is producing. A variety of information may be displayed at these windows and some systems cycle through the information showing each for 5 or 10 seconds. Some systems change the display information when the panel is tapped with a knuckle.

The system should be provided with a clearly-labeled AC Disconnect which may be a separate panel or may be located within the main panel. If located in the main panel, this should be indicated by a clearly-visible label.

Line-side taps should have a 60-amp fused disconnect with two 30-amp fuses.

The backfeed breaker is allowed to be 20% of the main panel rating. This means that a service panel rated for 200 amps may have a backfeed breaker rated at up to 40 amps. (20% of 200 = 40).

Some systems may include a combiner box, which should be marked. Connections between individual modules are made here.

Grid-tie systems should have specially-marked electrical meter made to spin in reverse when the photovoltaic system is supplying more power than the home is using. The meter may be labeled “Net Meter”. It’s important to confirm that a meter which can spin backwards has been installed, because if a conventional meter has been installed, power from the PV system will cause it to spin faster, which will actually increase costs.

If the Net Meter is installed at a location out of sight of the AC disconnect, the meter should have a permanently-marked label attached which gives the location of the AC disconnect. A missing label or one that’s illegible is a defect. This one was marked with a felt pen which has faded and is in a defective condition.

From the meter, unused power travels back to the utility pole through the service wires, passes through the transformer and is added to power in the local electrical system to be used by neighbors.

BATTERY BACKUP SYSTEMS

Battery backup systems will differ only at the equipment at the inverter.

Battery backup systems will typically have a 50-amp backfeed breaker.

 Inspectors may see systems with two backfeed breakers, one of which is installed upside down. One breaker acts as a back-up and when two are installed, one is installed upside down so that only one will be turned on.

All photos by Kenton Shepard

INSPECTING PHOTOVOLTAIC SYSTEMS This 31-page PDF is not intended to qualify inspectors to inspect photovoltaic systems but will help them develop a basic understanding of what's involved in this type of inspection.