Choosing solar panels
Rigid or flexible, small or big, on the roof or on the ground: here's how to choose your panels, and a tool to find the right angle for your location and season.
Tool: What angle for your panels?Kinds of panels
Small “12 V” panel (100 to 200 W)
- For:
- a trailer, a van, a shed, a small camp.
- The upside:
- Light, easy to mount. Its voltage (about 18 V working) charges a 12 V battery even with a small PWM controller.
- Watch out:
- Pricier per watt than big ones. Above 400 W, big panels come out cheaper.
Big house panel (400 to 550 W)
- For:
- a cabin, a house, a ground mount.
- The upside:
- The cheapest per watt, and a power warranty of 25 years or more.
- Watch out:
- Large (about 1 m × 2 m) and heavy (20 to 30 kg). Its voltage (30 to 45 V) needs an MPPT controller.
Flexible panel
- For:
- a curved RV, van or boat roof, where weight matters.
- The upside:
- Thin and very light, it sticks almost anywhere.
- Watch out:
- Glued flat with no air underneath, it runs hot, produces less and ages faster. It doesn't last as long as a rigid one.
Bifacial panel
- For:
- a ground mount, above snow, gravel or light-coloured ground.
- The upside:
- It also catches light bouncing up from below: a few percent more, often more in winter snow.
- Watch out:
- On a roof or glued flat, the back sees nothing: it's worth no more than an ordinary panel.
Photo: Grendelkhan, CC BY-SA 4.0 (cropped) · Photo: RVWithTito, CC BY 2.0 (cropped)
Where to mount them
On the roof
Simple and out of the way, but the angle is the roof's, and snow piles up in winter.
On the ground, on a rack
You choose the angle and direction, snow is easy to clear, and air flows underneath. The best choice for winter.
Flat on a trailer
Handy in summer. In winter, with a low sun and snow, output drops a lot: a tilting mount helps.
On a wall
Vertical, snow doesn't stick and the low winter sun hits almost head-on. Weak in summer.
What angle for your panels?
Search for your address or tap the map at your spot, then pick the season that matters most to you.
Search for your address, or tap the map at your spot.
in degrees; negative south of the equator
Example for Montreal (45.5°). Place your spot on the map to get yours.
38°
Tilt your panels to about 38° from the ground. Face them due south.
The noon sun is 45° above the horizon.
Flat, a panel gets about 29% less at noon, at this time of year.
Well-known rules for a fixed angle: all year, about 0.76 × latitude + 3°; summer, latitude minus 15°; winter, latitude plus 15°. A few degrees either way change output very little. The full calculator counts real weather, shade and snow.
Words to know
- Watts (Wp)
- Power measured in a lab, at best. Outdoors, count on about 80% of that number.
- Efficiency
- The share of light turned into electricity: about 20 to 23% today. The higher it is, the smaller the panel for the same watts.
- Voc and Vmp
- Open-circuit voltage and working voltage. They decide the wiring and the controller.
- Power warranty
- The panel keeps at least a certain percentage of its watts (often 80 to 87%) after 25 or 30 years.
- Bypass diodes
- In the junction box: they limit the loss when part of the panel is shaded.
Classic traps
- Mounting panels where a branch, chimney or antenna shades them part of the day: a small patch of shade cuts a good part of the output.
- Buying a “1,000 W kit” with a small PWM controller: it wastes a good part of what the panels give.
- Wiring different panels in series: the weakest one decides for the whole string.
- Mounting panels almost flat where it snows: snow stays on them for days.
- Gluing a flexible panel straight onto a black roof: it overheats and loses power for good.
To calculate
Next step
Choosing a charge controller
MPPT or PWM, reading a “100/30”, and a tool that checks your series wiring before the cold fries your controller.