Which Way Should Solar Panels Face on a GTA Roof?

The south-facing side, if the roof has one, with east or west as the fallback. The City of Toronto's solar guidance says "the optimal orientation for solar panels is on a south-facing roof", and that east- and west-facing sloped roofs also work, with less output.

A north-facing slope is the one the City of Toronto's SolarTO Map rules out. Its criteria for a suitable rooftop include a slope of 45 degrees or less and a roof that does not face north. None of the sources used here gives an output figure for a north-facing roof. If a north slope is your only roof, ask an installer to model it and put the expected output in writing.

Less than a south face, and the two sources used here that give a figure disagree on how much. The City of Toronto's solar guidance says east- and west-facing sloped roofs give "an average of 15 per cent less output" than a south-facing roof, without saying at what tilt. The Canadian Renewable Energy Association (CanREA) says that for rooftop panels in Canada, east- and west-facing orientations "can provide around 70% of the output of a south-facing solar PV system", which works out to a loss of around 30 per cent, and it gives no tilt either. PV stands for photovoltaic, the kind of panel that turns sunlight into electricity.

Neither page says how its figure was worked out. Natural Resources Canada (NRCan), the federal department that publishes solar output estimates for Canadian municipalities, gives no east or west figure in those estimates: its estimates cover four fixed south-facing tilts, flat panels and panels that track the sun.

An east or west roof is workable, then, but the penalty for your own roof depends on its pitch, its exact direction and its shade. For a Toronto address, the City's SolarTO Map, covered below, models all three.

No. On the figures below, both change the output less than the direction does. The City of Toronto says systems on sloped roofs "are typically flush mounted to follow the slope of the roof", which means the panels sit parallel to the roof surface. It adds that "An angle between 10 and 45 degrees works well for most solar installations".

NRCan's figures for Toronto show how little the tilt moves the result. NRCan gives its estimates as expected lifetime average yearly output in kWh per kW: a kW is the rated size of the panels, and a kWh is the unit of electricity on your hydro bill. Besides a vertical setting, it publishes three sloped tilts for south-facing panels: latitude tilt, meaning panels tilted at an angle equal to the location's latitude, and 15 degrees shallower or steeper than that. NRCan's Toronto estimates for those three run from 1,096 to 1,174 kWh per kW a year, within about 7 per cent of each other, a smaller gap than either source's figure for an east or west roof. The site's solar panel angle calculator converts a roof pitch, such as 6:12 (6 units of rise for every 12 across), into degrees and shows an estimate for south-facing panels at that angle, drawn from NRCan's Toronto figures.

The town matters even less. NRCan's yearly estimate for south-facing panels at latitude tilt runs from 1,152 kWh per kW in Milton to 1,167 in Oshawa across the 15 GTA municipalities the site's calculators cover, a gap of about 1.3 per cent. A house in Mississauga, Markham or Oakville gets almost the same NRCan estimate as one in Toronto. The guide to signs a solar array is underproducing turns these estimates into a yearly baseline for a working system.

Look at a flat section of roof, an outbuilding, a south-facing wall or the yard. The City of Toronto lists walls, adjacent structures such as a detached garage or shed, and ground mounts among the options when a roof does not suit solar.

On a flat roof, the City says the panels "can be tilted to optimize energy production". It says most flat-roof installations need no holes in the roof, because the racking is ballasted, meaning weighed down with concrete blocks, and that ballasted systems add more load. The tilt matters there: NRCan's Toronto estimate for flat, horizontal panels is 1,013 kWh per kW a year, about 87 per cent of the 1,163 it gives Toronto for south-facing panels at latitude tilt.

A ground-mounted system "can be positioned to face due south and the angle can be set to optimize electricity production", the City says, although it calls a ground mount more expensive. It says a south-facing wall can also work if nothing shades it, with the panels placed high on the wall, though NRCan's Toronto estimate for vertical south-facing panels is 802 kWh per kW a year, about 69 per cent of its latitude-tilt figure.

Shade or obstructions on that slope can, and so can the roof's age. The City of Toronto says shading "can drastically reduce solar energy output", whether it comes from the building itself, such as plumbing stacks, roof vents and chimneys, or from nearby buildings and trees. It says smaller amounts of shading can be addressed with micro-inverters, which it describes as attaching to each individual panel, or with power optimizers.

Obstructions take space as well. The City says chimneys, skylights and vents "cannot have solar panels installed over them", so a south slope with a skylight may hold fewer panels than a clear west slope. It says you may be able to prune several branches before installing, but that removing trees is not advisable.

The roof's age can hold up the whole job. The City does not recommend installing solar on a roof "more than 10 years old (dependent on condition)", and if the roof will need replacing soon, it says it is best to re-roof and install solar at the same time or shortly after.

It should name the roof faces, the tilt and the output expected from that layout, so two quotes can be compared on the same terms. One installer may use only the south slope while another adds the west, and a single total hides that. Ask for these lines in writing:

1. Roof faces: which slopes carry panels, and how many panels on each.
2. Tilt: the angle of each group, and whether it is flush mounted or tilted up.
3. Expected yearly output: in kWh, for each face and for each kW of panels.
4. Shade: what the design allows for, and whether micro-inverters or power optimizers are included.
5. Obstructions: what the layout works around, such as a skylight, a vent or an overhanging tree.

CanREA says a professional assessment, including a measurement of how much sunlight falls on your roof, "is required to estimate your potential system production". The index does not check any of its companies' layouts or output estimates, so it cannot tell you which installer models a roof well. The guide to what a solar quote leaves out covers the cost lines worth checking next.

Start with a map, then get a site visit. For a Toronto address, the City's SolarTO Map "includes the effects of roof pitch, orientation and shading from surrounding structures and trees". It marks a roof area as suitable only with solar exposure of "at least 800 kWh per square metre" and "at least 30 square metres of suitable surface", along with the slope and direction criteria above. The City says the map is for information only and is a preliminary assessment tool, and it encourages users to contact installers for a detailed site-specific assessment.

Outside Toronto, a phone compass or a satellite map shows which way each slope faces. If the south slope looks small, the site's roof size tool works out how much clear roof a system of a given size needs, and how much fits on the area you have.

When you are ready for quotes, the solar panel installation category counts 143 of the 161 companies in the index. Each listing profile links to the company's own website where a working one is on record. Ask every installer for the five lines above.