Signs Your Solar Array Is Producing Less Than It Should

About 1,163 kWh a year for each kW of panels facing south at latitude tilt, swinging from about 128 in July to about 51 in December, by NRCan's estimates for Toronto. NRCan publishes these figures for over 3,500 Canadian municipalities. Each one is the expected lifetime average output, in kilowatt-hours (kWh), for each kilowatt (kW) of panels in a grid-connected system without batteries. A kWh is a unit of energy, the same unit your hydro bill uses. The kW figure is your panels' rated size, which your contract or monitoring app will show.

For Toronto, facing south at a tilt equal to the latitude, NRCan's monthly estimates per kW are:

• Winter: about 66 in January, 82 in February and 51 in December.
• Spring: about 109 in March, 113 in April and 122 in May.
• Summer: about 123 in June, 128 in July and 121 in August.
• Autumn: about 103 in September, 89 in October and 55 in November.

July's estimate is about two and a half times December's. That swing is built into every GTA system, so a low December is not, by itself, a sign of a fault.

The figures barely change across the region. In NRCan's municipal data, the yearly estimate at latitude tilt for the 15 GTA municipalities the site's calculators cover, from Milton and Burlington in the west to Oshawa in the east and Newmarket in the north, runs from about 1,152 kWh per kW in Milton to about 1,167 in Oshawa. The Toronto table works as a guide anywhere in the GTA.

Multiply its size in kW by NRCan's figure. Take a 5 kW system as an example, chosen for easy arithmetic. Facing south at latitude tilt in Toronto, it should average about 5,815 kWh a year (5 times 1,163), about 640 kWh in July (5 times 128) and about 255 kWh in December (5 times 51).

NRCan says lifetime average yearly output for most systems should be within about 10% of its figures. For the 5 kW example, that band runs from roughly 5,230 to 6,400 kWh a year. NRCan adds that a well-functioning system should typically do better than its averages in the first years, because the estimates cover a whole lifetime of slow wear. A young system that lands at the bottom of the band, or below it, is worth a question to the installer.

If your installer gave you a production estimate for your roof, use that as your baseline instead. It can allow for your roof's direction, angle and shade, which NRCan's regional figures cannot. The site's panel count calculator applies the same NRCan figure and 10% band when it sizes a new system from a household's yearly use.

Three, all of which your monitoring app can show: a full year against your baseline, a month against the same month last year, and sunny days against each other.

1. A full year against your baseline. NRCan's 10% figure describes lifetime average yearly output, so a whole year is the closest test you can run against it. One year can still land outside it with nothing wrong, because NRCan's figures average a whole lifetime of good and poor years.
2. This month against the same month last year. A gap in one month is a prompt to keep watching. A gap that repeats month after month is a stronger signal.
3. Sunny days against sunny days. A clear day with far lower output than clear days a few weeks earlier says more than a cloudy week.

Do not lean on one month. NRCan says its monthly values are only indicative and do not take account of temperature, snowfall or year-to-year changes in sunshine. A snowy January can fall short of the estimate with nothing wrong.

If your app shows each panel or each string separately, look for one that lags the others. A string is a group of panels wired together in series. Cloud cover reaches the whole roof at once, so a single panel or string that keeps lagging on clear days is worth a closer look: it can be new shade, snow left on part of the array, damage or a loose connection.

Very little each year. NRCan says well-functioning systems typically degrade at about 0.6% a year over an expected lifetime of 25 years or more, citing a 2016 study by Jordan and others. At that rate a system loses roughly 6% over its first ten years.

So ageing explains a slow drift in your yearly totals. It does not explain a month that falls by a quarter against the same month last year, or a sunny day with nothing at all.

A little over a year at the usual roof pitches, and more from season to season. NRCan's Toronto data gives yearly estimates for several fixed orientations:

• Latitude minus 15 degrees (a shallower tilt): about 1,174 kWh per kW, the highest of the fixed angles.
• Latitude tilt: about 1,163 kWh per kW.
• Latitude plus 15 degrees (a steeper tilt): about 1,096 kWh per kW.
• Flat, lying horizontal: about 1,013 kWh per kW.
• Vertical, flat against a south wall: about 802 kWh per kW.

Steeper panels, up to latitude plus 15 degrees, do better in winter. In December, NRCan estimates about 55 kWh per kW at latitude plus 15 degrees against about 45 at latitude minus 15. So two healthy systems on different roofs can follow different monthly patterns. The site's solar panel angle calculator sets out the month-by-month comparison in full.

NRCan's fixed-angle figures are all for arrays facing south. If your panels face east or west, ask your installer for the estimate it made for your roof rather than using the table.

When the shortfall is sudden, keeps repeating, or sits in one part of the array. GreenEnerCanada, an Ontario service company, lists a monitoring app that shows no production among the signs that you might need inverter service, along with a red light or warning on the inverter's screen. The inverter is the unit that turns the panels' direct current into household power.

The same service page lists other reasons to call for service, from snow that stays on the panels for days to scratching under them. The site's guide to finding solar repair help in the GTA goes through that list and the order in which to contact people.

Stay off the roof. Look from the ground, write down what you see and when, and save screenshots from your monitoring app, including the months you compared. Find your contract, the installer's production estimate and the warranty documents, because the equipment may still be covered.

Then contact the company that installed the system. If it has closed, the index lists 4 companies in its repair and maintenance category, and each listing profile links to the company's own website where it has a working one. Routine upkeep is covered in the guide to what solar maintenance involves in Ontario, and the repair cost guide explains why Toronto service firms rarely publish prices.