Open any panel brochure and you will find a table of figures that looks like it was written for engineers. It was. The good news is that about six of those numbers matter to a homeowner, and you can read them in ten minutes.
This guide is part of our collection on panels and equipment.
Wattage: what the number means
A panel’s wattage rating is its output under standard test conditions: a set light level, a set cell temperature and a set angle in a laboratory. Real roofs are rarely that bright and are often hotter or colder, so the rating is a way to compare panels, not a promise of what you will see on a given afternoon.
In recent years, typical residential panels have been about 400 to 450 W each. A 7 kW system built from 420 W panels is therefore about 17 panels. Our article on how many panels you need shows how to turn your household use into a system size.
Bigger wattage does not always mean better quality. A 450 W panel can simply be a larger sheet of cells. Wattage tells you how much power one panel can make, and efficiency tells you how well it uses its area.
Efficiency: watts per square metre of roof
Efficiency is the share of sunlight hitting the panel that becomes electricity, and for current residential panels it is roughly 20 to 23 percent. You can estimate it yourself. Under standard conditions, sunlight is about 1,000 W per square metre. A panel of 1.9 square metres rated at 420 W produces about 221 W per square metre, which is roughly 22 percent.
When does this matter? When roof space is limited. On a small roof, a more efficient panel fits more watts in the same area. On a big roof or a ground mount, a slightly less efficient panel at a lower price per watt may be the sensible pick. Our guide to solar panel types compares the cell technologies behind these numbers.
Temperature coefficient
Panels lose some power as they heat up. The temperature coefficient, usually given as a percentage per degree Celsius, says how much. A figure around minus 0.3 to minus 0.4 percent per degree is common. A smaller number (closer to zero) is better.
Cell temperature on a hot summer roof can reach well above air temperature, so the loss can be noticeable. Canadian winters work the other way: cold raises voltage and efficiency, which is one reason a bright January day can give a strong output for the size of the array. It is a good reason not to dismiss solar in the north.
Degradation and the performance warranty
Panels slowly lose output over time. Commonly quoted degradation is about 0.4 to 0.6 percent a year, though individual products differ. The performance warranty, usually about 25 years, promises a minimum output at the end of that period, often described as a percentage of the original rating.
If a panel starts at 100 percent and loses 0.5 percent per year, it would be near 88 percent after 25 years, which is in a similar range to what many warranties guarantee. Compare the first-year drop and yearly rate between products, and read the full conditions in our guide to solar panel warranties.
Reading a spec sheet
Here is a short checklist of what to look for, in the order that tends to be most useful.
| Line on the sheet | What it tells you | What to look for |
|---|---|---|
| Rated power (W) | Output under test conditions | 400 to 450 W is common |
| Module efficiency (%) | Output per area | Higher helps on small roofs |
| Temperature coefficient | Heat loss per degree | Closer to zero is better |
| Dimensions and weight | Fit on your roof | Check against layout |
| Maximum system voltage | Compatibility with the inverter | Match your design |
| Operating temperature range | Cold and heat limits | Covers Canadian winters |
| Snow and wind load ratings | Mechanical strength | Check against local codes |
| Certification marks | Safety testing | Look for CSA or equivalent |
| Warranty terms | Product and performance cover | Length and conditions |
You do not need to calculate everything. You do need to ask the installer for the data sheet for the exact model in your quote and to check the model number on the delivered panels.
Why system output differs from panel ratings
Even the best panel never delivers its rated watts all year. Losses come from heat, dust, wiring, the inverter, shade and the angle of the sun. A system’s yearly output is usually estimated in kilowatt-hours per kilowatt installed, and in southern Canada that often falls roughly between 1,100 and 1,400 kWh a year, depending on location and design. The inverter choice also plays a part, as covered in our comparison of string inverters and microinverters.
Price per watt is the real comparison
Because panels differ in size and power, compare quotes by installed price per watt and expected yearly production, not by panel count or headline efficiency. A slightly more expensive, higher-efficiency panel can still be the better value if it lets you reach the production you need on a limited roof. Ask each installer to show the data sheet, the warranty terms and the modelled output for your roof, and make sure the equipment carries recognised certification such as CSA marks.
Quick answers
What is a good solar panel efficiency?
For residential panels, roughly 20 to 23 percent is typical in recent years. Higher efficiency matters most when roof space is tight, since it means more watts in the same area.
Is a higher wattage panel better?
Not by itself. A higher wattage panel may simply be bigger. Look at efficiency, warranty, temperature coefficient and price per watt together.
How much do solar panels degrade each year?
Commonly about 0.4 to 0.6 percent a year. Check the performance warranty for the guaranteed output after 25 years.