How efficient are solar panels and does it matter?

What does solar panel efficiency actually mean?

The rating on a data sheet is measured in a laboratory under standard test conditions: a fixed amount of light, a cell temperature of 25 C and a set light spectrum. Under those conditions, a panel with 20 percent efficiency turns one fifth of the light energy into direct-current electricity. The rest is lost as heat or reflected away.

A quick sum shows what the number means in practice. Standard test light is 1,000 watts per square metre. A panel of 1.9 square metres at 21 percent efficiency therefore peaks at about 400 watts (1,000 x 1.9 x 0.21). The same 400 watts from a 17 percent panel would need roughly 2.35 square metres. Efficiency is really a statement about how much roof area a given amount of power takes up.

How efficient are the common panel types?

Silicon cells dominate residential rooftops. Mono-crystalline cells usually rate somewhat higher than poly-crystalline ones, and thin-film panels tend to rate lower still, which is why they appear mostly on large flat roofs and in special uses. Newer cell designs push the figures up at the top of the market. Our guide to solar thermal versus solar PV explains why this rating applies only to electric panels.

Heat-collecting panels are a separate technology with a different yardstick. The flat-plate collectors that served Drake Landing Solar Community captured heat in a fluid rather than making electricity, so their efficiency is not comparable with a photovoltaic rating. The solar collection page describes how those collectors worked.

What lowers real-world output?

The laboratory figure is a ceiling, not a forecast. Several things pull a rooftop system below it.

• Temperature. Cells lose a little output as they heat up. The loss per degree is on the data sheet, and cold, clear Canadian days are often kind to panels, as discussed in solar panels in winter.
• Orientation and tilt. A roof facing away from the sun gathers less light. See the best solar panel direction in Canada.
• Shade, dirt and snow. Partial shading can drag down a whole string of panels.
• Wiring and inverter losses. Converting direct current to alternating current costs a few percent, and the choice of equipment matters, as covered in solar inverter types.
• Ageing. Output declines slowly over the years, which the lifespan and degradation guide covers.

When is a higher-efficiency panel worth it?

It earns its premium when roof space is the limit. A small or heavily broken-up roof may fit only a handful of panels, and a higher rating lets that area reach the system size you want. If the roof is large, a lower-rated panel at a lower price per watt can produce the same annual energy for less money.

So compare quotes on price per watt and on expected annual production for your roof, not on the efficiency figure alone. The price per watt explanation shows how to do that, and the guide to production estimates shows what a trustworthy forecast includes. For the wider question of payback, see whether solar panels are worth it in Canada.

Warranty terms matter as much as the headline figure. A performance warranty states how much output the maker guarantees after a set number of years, and the exact terms are in the paperwork for the product you are quoted.

How can you get more from the panels you have?

The cheapest gains come from the basics. Keep the array clear of shade from new trees or additions, clear snow when it is safe to do so, and check the monitoring app for sudden drops that point to a fault. A bit of rinsing in a dry spell removes dust and pollen. None of this changes the panel rating, but it keeps real output close to what the design promised.

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