How big is one solar panel?

A current residential module is a little over 20 square feet. Silfab's SIL-440 QD measures 1,721 by 1,133 mm, which is 1.95 square metres or about 21 square feet, and produces 440 W. Canadian Solar's HiKu CS3W-455MS is 2,108 by 1,048 mm, about 23.8 square feet, and produces up to 455 W. Both figures are from the manufacturers' own published specifications.

Why does the calculator give a smaller answer than the panel sizes suggest?

Because a real array is not a solid rectangle of glass. Dividing your roof area by a bare module footprint gives about 21 watts per square foot, but published rooftop array density is 15 to 20, and the difference is the room an installer actually needs: clearance from the ridge and eaves, gaps around vents and plumbing stacks, space to walk, and whatever your municipality asks for roof access. The panel-count line is shown so you can see the theoretical ceiling; the density line is the one to plan with.

Do I measure the roof flat or along the slope?

Along the slope, if you can, because that is the surface the panels sit on. A measurement taken flat from above, off a satellite image or a site plan, understates a pitched roof by exactly one over the cosine of the pitch. On a 6:12 roof, which is 26.6 degrees, that is about 12 percent. The calculator will apply the correction for you if you tell it the measurement was taken flat.

Does the whole roof count?

No, and this is where estimates go wrong most often. Only the faces you would actually put panels on count, which in practice usually means the south, east and west slopes. Then take off vents, chimneys, dormers, skylights, valleys and anything else fixed to the roof, plus the clearances around them. This page deliberately asks for the usable area rather than applying a rule-of-thumb fraction, because no source publishes one that is true of Ontario houses generally.

What if my roof is not big enough for the system I want?

Higher-wattage modules are the first lever: the same footprint carries more capacity, which is why panel wattage has climbed steadily. Beyond that, an array smaller than your consumption is not a failure. In Ontario the credit for exported power is reduced to zero after twelve months, so a system sized at or a little under what your house uses is often the better value anyway. A ground mount is the other option, but note it does not qualify for the provincial rebate route.

How much roof do solar panels need?

Roughly 50 to 67 square feet of clear roof for every kilowatt of array. This works it out both ways: what fits on the area you have, and how much area the system you want would need. It handles square feet and square metres, and it will correct a flat measurement for the pitch of the roof.

Last updated August 25, 2026By The SolarPanelFinder Rooftop Desk

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The short version: a typical 8 kW array wants 400 to 533 square feet of clear, unshaded roof. If you have one good face of about 500 square feet, you have room for a system that covers an average Ontario household.

How much solar fits on the roof you have?

Usable roof areaThe clear run you could put panels on, not the whole roof.UnitsIs that the slope, or the footprint?A flat measurement gets corrected for pitch.Roof pitchIgnored for a slope measurement.PanelWhere the house is

500 sq ft (46 sq m) of usable roof

7.5–10.0 kW of solar

That is the realistic planning range, using the 15–20 watts per square foot Green Building Canada gives for rooftop arrays. Bare panel footprints alone would suggest more, and the gap between the two is the room a real array needs around vents, valleys and edges.

By array power density (15–20 W/sq ft)Green Building Canada, What Commercial Solar Panels Cost in Canada in 2026: 7.5–10.0 kW
Bare Silfab SIL-440 QD (440 W) footprints at 21.0 sq ft eachSilfab Solar, SIL-440 QD product page: 23 panels = 10.12 kW
Bare Canadian Solar HiKu CS3W-455MS (455 W) footprints at 23.8 sq ft eachCanadian Solar, HiKu CS3W-MS datasheet: 21 panels = 9.56 kW
What 10.12 kW would generate a year in TorontoNatural Resources Canada, their stated margin of about 10%: 10,593–12,947 kWh

This asks for the USABLE area on purpose. Vents, chimneys, dormers, valleys, skylights, the north face and whatever your municipality requires for roof access all take area away, and no source publishes a single fraction that is true of Toronto houses, so this page does not apply one. Measure the clear rectangles and add them up.

The other way round: how much roof does a system need?

System size

For an 8 kW array

400–533 sq ft

37–50 square metres of clear, unshaded roof, at the same 15–20 watts per square foot.

The method

Two ways to count, and why they disagree

Module footprints give a ceiling. Array density gives a plan. The gap between them is real.

Divide a roof by the size of one panel and you get the number of panels that would fit if the roof were an empty rectangle and the panels could be laid edge to edge. On the Silfab module that works out at almost 21 watts per square foot. It is a real number and it is the right ceiling to know, because it tells you the most the roof could ever carry.

Green Building Canada gives rooftop array density as 15 to 20 watts per square foot, “depending on panel efficiency, tilt angle, shading and equipment layout”. That is lower than the module figure for reasons anyone who has looked at a finished array can see: panels stop short of the ridge and the eaves, they route around plumbing stacks and vents, they leave a path for someone to walk, and on a flat roof they need spacing between rows so one does not shade the next in December. Planning with the density figure and treating the panel count as the ceiling is the honest way round.

Pitch is the other thing worth getting right, and it is pure geometry rather than anything published. A roof measured from above, on a satellite image or a site plan, gives you the footprint, and the actual sloped surface is that divided by the cosine of the pitch angle. A 4:12 roof adds about 5 percent, a 6:12 roof about 12 percent, and a 12:12 roof about 41 percent. The calculator will do it if you tell it which kind of measurement you took, and it says so on screen rather than quietly inflating your number.

What it will not do is guess how much of your roof is usable. Every house differs, and no Canadian source publishes a usable fraction that would be true of Toronto housing stock generally. So the tool asks for the clear area and tells you what to subtract. Measuring the largest rectangle on each face you would actually use, and adding those, gets closer than any coefficient would.

Frequently asked questions

How much roof do I need for solar panels?: Between 50 and 67 square feet of clear roof for every kilowatt, so a typical 8 kW array wants roughly 400 to 533 square feet. That comes from the 15 to 20 watts per square foot Green Building Canada gives for rooftop arrays. The wide end of the range is a conservative layout with generous clearances; the tight end assumes efficient modules packed well.
How big is one solar panel?: A current residential module is a little over 20 square feet. Silfab's SIL-440 QD measures 1,721 by 1,133 mm, which is 1.95 square metres or about 21 square feet, and produces 440 W. Canadian Solar's HiKu CS3W-455MS is 2,108 by 1,048 mm, about 23.8 square feet, and produces up to 455 W. Both figures are from the manufacturers' own published specifications.
Why does the calculator give a smaller answer than the panel sizes suggest?: Because a real array is not a solid rectangle of glass. Dividing your roof area by a bare module footprint gives about 21 watts per square foot, but published rooftop array density is 15 to 20, and the difference is the room an installer actually needs: clearance from the ridge and eaves, gaps around vents and plumbing stacks, space to walk, and whatever your municipality asks for roof access. The panel-count line is shown so you can see the theoretical ceiling; the density line is the one to plan with.
Do I measure the roof flat or along the slope?: Along the slope, if you can, because that is the surface the panels sit on. A measurement taken flat from above, off a satellite image or a site plan, understates a pitched roof by exactly one over the cosine of the pitch. On a 6:12 roof, which is 26.6 degrees, that is about 12 percent. The calculator will apply the correction for you if you tell it the measurement was taken flat.
Does the whole roof count?: No, and this is where estimates go wrong most often. Only the faces you would actually put panels on count, which in practice usually means the south, east and west slopes. Then take off vents, chimneys, dormers, skylights, valleys and anything else fixed to the roof, plus the clearances around them. This page deliberately asks for the usable area rather than applying a rule-of-thumb fraction, because no source publishes one that is true of Ontario houses generally.
What if my roof is not big enough for the system I want?: Higher-wattage modules are the first lever: the same footprint carries more capacity, which is why panel wattage has climbed steadily. Beyond that, an array smaller than your consumption is not a failure. In Ontario the credit for exported power is reduced to zero after twelve months, so a system sized at or a little under what your house uses is often the better value anyway. A ground mount is the other option, but note it does not qualify for the provincial rebate route.

Sources

Where these figures come from

Green Building Canada, What Commercial Solar Panels Cost in Canada in 2026 - the 15 to 20 watts per square foot rooftop array density
Silfab Solar, SIL-440 QD - 440 W at 1,721 by 1,133 by 35 mm, 22.6% module efficiency
Canadian Solar, HiKu CS3W-MS datasheet - 430 to 455 W at 2,108 by 1,048 by 40 mm
Natural Resources Canada, Photovoltaic potential and solar resource maps of Canada - annual generation for whatever capacity the roof turns out to hold

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