How a building solar PV system works

A grid-connected solar photovoltaic (PV) system turns sunlight into DC electricity in its modules, converts it to AC in an inverter, and feeds it into the building's electrical system. The building uses that power first. When the system produces more than the building needs, the surplus flows out to the utility grid, and when it produces less, the grid makes up the difference. Most building systems in Canada work this way, with no batteries.

The parts of the system

Modules and arrays

PV modules, often called panels, contain cells that produce DC current when light falls on them. Modules are wired together in series into strings to build up voltage, and strings are combined into an array. Modules are rated in watts under standard test conditions, which describe a bright, cool day; real output is usually lower.

Racking

Racking holds the modules on a roof, a carport or the ground. On roofs, it is either attached through the roof membrane or ballasted with weights. Either way, the roof structure must be checked for the extra dead load, wind uplift and the way snow drifts and slides off arrays. A roof near the end of its life should be replaced before the array goes on. See snow loads on roofs.

Inverters

The inverter converts DC from the modules into AC that matches the building's supply. It also tracks the operating point that gives the most power as light changes, and it disconnects from the grid automatically if the utility supply fails, so it cannot energize lines that utility crews believe are dead. There are three common arrangements:

String inverters: one inverter serves one or more strings. Simple and economical, but shading on one module affects the whole string.
Microinverters: a small inverter at each module. Each module works independently, which helps with partial shading and complex roofs.
Optimizers with a string inverter: module-level electronics that manage each module's output, feeding a central inverter.

Larger commercial systems often use several string inverters, or central inverters for very large arrays.

Disconnects, protection and metering

The system includes DC and AC disconnects, overcurrent protection, grounding and bonding, and labels identifying the PV system and its disconnects for firefighters and electricians. The Canadian Electrical Code includes rules for PV systems, including requirements for rapid shutdown of conductors on buildings, so that the DC wiring on a roof can be de-energized quickly in an emergency. The utility usually requires a bidirectional meter, or a separate production meter, depending on its program.

Solar first, grid for the rest. The array makes DC, the inverter turns it into AC and feeds the building panel through a disconnect. Building loads use solar power first. Surplus flows out through the meter to the grid, and the grid supplies whatever the array cannot.

What affects output

Orientation and tilt. In Canada, south-facing arrays tilted at a moderate angle produce the most energy over a year. East- and west-facing arrays produce less in total but spread production over more of the day. Flat-roof systems often use a low tilt to reduce wind loads and allow more modules.
Shading. Even partial shading from rooftop equipment, parapets, trees or neighbouring buildings can cut output more than its size would suggest, depending on the inverter arrangement.
Season and latitude. Output is much higher in summer than in winter, when days are short and the sun is low. Prairie locations such as Alberta and Saskatchewan get relatively good solar resource; coastal BC gets less in winter.
Snow. Snow on modules stops production until it slides or melts off. Steeper tilts shed snow faster. Snow loss is usually a modest share of annual output because winter production is small anyway.
Temperature. Modules are more efficient when cold, so clear, cold spring days can be very productive.
Soiling and ageing. Dust and debris reduce output slightly, and modules lose a small amount of output each year.

How much of the power the building uses

Whether solar power is used on site or exported depends on when the building uses electricity. An office or school that runs during the day uses much of what the array produces. A residential building with peak use in the evening exports more during the day and imports at night. How exported power is credited depends on the utility program. See net metering and microgeneration in Canada.

Used on site or exported. The shaded area is solar power the building uses directly. When production rises above the load line, the surplus, shown solid, is exported. The white area under the load line, in the evening and at night, is imported from the grid. A building with more daytime load uses a larger share of its own production. The shapes are illustrative.

Solar PV reduces energy purchased but often does little for demand charges, because the building's peak may occur on a cloudy day or after sunset.

Does it work in a power outage?

A standard grid-connected system shuts down when the grid fails. This is a safety requirement, not a fault. To keep power during an outage, the system needs batteries and equipment that can isolate the building from the grid and run it independently. See battery energy storage for buildings.

Planning a system: what to check first

Roof condition, remaining life and structural capacity, or available ground and carport space
Shading across the year, including from rooftop equipment
Twelve months of electricity use and when it happens
The utility's interconnection rules and program limits
Electrical room space and service capacity to accept the inverter output
Fire department access paths on the roof, which affect layout
Insurance, and any landlord or condominium approvals

Operation and maintenance

PV systems have no moving parts other than tracking systems, which are rare on buildings, so maintenance is modest but not zero. Monitoring software shows production and flags inverter faults; someone should check it regularly, because a failed inverter can go unnoticed for months. Periodic inspections look for damaged modules, loose or damaged wiring and connectors, racking and roof attachment problems, and signs of overheating. Infrared thermography can find failing modules and hot connections. Work on PV systems must be done by qualified people: modules produce voltage whenever light falls on them, and the DC side cannot simply be switched off at the modules.

DC is always live in daylight. Opening a DC disconnect stops current flowing to the inverter but does not stop the modules producing voltage. Only qualified workers should work on PV wiring, following the system's shutdown procedure and the site's electrical safety practices.

How ehZee can help

Our solar PV system design work starts by asking why you want solar and what your building's use looks like. We assess the site, estimate production, design the array and electrical system, and support the utility interconnection. Drawings for permit are sealed by a licensed professional engineer where required, and installation is done by licensed electrical contractors under permit.

Common questions

Is solar worth it in a Canadian winter?

Winter production is low, but systems are judged on annual output. Long, bright summer days, especially on the Prairies, produce most of the year's energy.

Do I need to clear snow off the panels?

Usually not. Climbing on a snowy roof to clear modules is a safety risk and can damage them, and the energy recovered is small. Arrays are designed to shed snow on their own.

How long do PV systems last?

Modules typically carry long performance warranties and often outlast them, while inverters usually need replacing at least once during the life of the array. Plan for inverter replacement, and for removing and reinstalling the array when the roof underneath needs replacement.