How rooftop solar works

Rooftop solar is a simple system with very few moving parts. Sunlight hits the panels, the panels make electricity, and that electricity either runs your home or flows out to the grid. Understanding the pieces makes it much easier to judge a quote or a savings estimate.

The path electricity takes

1. Panels turn sunlight into direct current (DC) electricity. Each panel is a grid of silicon cells wired together.
2. The inverter converts DC into alternating current (AC), the kind your outlets and appliances use. It also watches the grid and shuts the system off if the grid goes down, so line workers stay safe.
3. Your electrical panel sends that AC power to whatever is running in your home right now: the fridge, the furnace fan, a laptop charger.
4. The grid takes anything you don't use at that moment. Your utility meter records what you export and what you import.

At night, or on a dark winter afternoon, your home simply draws from the grid as it always has. Nothing about your day-to-day use changes.

kW and kWh are not the same thing

These two units cause more confusion than anything else in solar.

• kW (kilowatt) is a rate of power. A system's size is described in kW, such as a 7 kW system. It is like the width of a pipe.
• kWh (kilowatt-hour) is an amount of energy. Your electricity bill charges you per kWh. It is like the water that flows through the pipe over time.

A 7 kW system running at full output for one hour would produce 7 kWh. In practice panels rarely run at full rated output, so annual production depends on sunlight, location and setup.

How much a system produces in Canada

In southern Canada, a well-placed system typically produces somewhere in the range of roughly 1,000 to 1,300 kWh per kW of panels per year. The exact figure depends on your location, roof and shading.

As a rough illustration, a 7 kW system producing 1,150 kWh per kW would generate about 8,000 kWh in a year. Many Ontario homes use somewhere in that general range, though use varies a lot with heating type, family size and air conditioning.

What affects output

Direction

South-facing roofs collect the most sunlight over a year in Canada. East- and west-facing roofs still work, usually producing somewhat less. North-facing roofs are usually a poor fit. Our guide on whether your roof is suitable goes into more detail.

Pitch

The angle of your roof matters, but less than people expect. Most common residential pitches perform reasonably well. Very flat or very steep surfaces lose some output.

Shade

Shade is often the biggest factor. A tree, chimney or neighbouring building that shades part of the array for a few hours can cut output noticeably, especially on systems with a single string inverter. Panel-level electronics can reduce that loss, which we cover in panels and inverters.

Latitude and weather

Further north means a lower sun angle and shorter winter days, but longer summer days help balance it out. Cloud cover and snow matter too. Our guide on solar in Canadian winters explains why cold weather is less of a problem than it sounds.

What solar does not do

• It does not keep your lights on during a power outage on its own. Grid-tied inverters shut down when the grid does, unless you add a battery and the right equipment. See batteries explained.
• It does not remove your electricity bill entirely. Fixed monthly charges and some delivery costs remain, and you will still draw from the grid at night.
• It does not pay off on every home. A shaded, north-facing roof or very low electricity use can make the numbers weak, and we will tell you so.

How SolarMyBill estimates your production

We look at your roof's location, direction and pitch, apply local sunlight data, and account for typical system losses. We then compare that production with your actual usage and rate plan, so the savings estimate reflects your bill rather than a generic average. Unfamiliar terms are explained in the glossary.

Next step

See what this means for your roof: check your roof.