How efficient are home solar panels?

Is your house actually a space station? Is NASA your landlord? If so, then your home solar panels might be made by Boeing and deliver a sky-high 32% efficiency.

It costs about $10,000 to send a pound of anything into orbit, and packing the maximum power onto a small satellite is critical, so it makes sense to use the absolute best available technology.

By comparison, the solar panels I had installed on my house back in 2013 only manage about 16% efficiency - just half of what you’ll get from high-tech photovoltaic panels in space. I don’t know what space panels cost, but I can guarantee that my panels were much, much cheaper.

Nearly a decade later, technology has gotten a lot better: you can now get affordable solar panels that are 22.8% efficient.

That’s for a top-of-the-line panel, but what can you expect from a more average product? We’ll get into that, but first let’s define what we mean by solar panel efficiency.

What is solar panel efficiency?

The efficiency of a solar panel is calculated as the amount of incoming sunlight that the panel converts into electricity. Key things to remember are that it’s based on area, and not all solar panels are the same size.

This means that if you’re comparing, say, a 380 watt panel to a 400 watt panel, it’s possible that the lower wattage panel is more efficient, but has a lower power output because it’s smaller.

By comparing solar panels by their efficiency instead of their power rating, you’ll know how much electricity you’ll be generating per square foot.

When does high efficiency matter?

If you have adequate space on your roof to generate the amount of electricity you want, then efficiency might not matter to you. It’ll often be more cost-effective to go with a cheaper, lower efficiency panel.

However, you might have really high electric bills and want to generate as much of it as you can with solar panels. Or maybe you have a large roof, but it’s full of complicated angles, as you find with a hip-and-valley roof.

Or perhaps you’ve got a small section of roof that’s pointed south, and you want to get the most out of the panels in that ideal orientation.

In cases like this, a solar installation might benefit from high efficiency panels.

What types of panels are high efficiency?

Just a few years ago, home solar customers had a choice between relatively cheap but less efficient polycrystalline panels and more efficient monocrystalline. These days, you can still find polycrystalline panels marketed for the DIY consumer, but they’ve mostly been replaced in the home solar market by mono panels. And because of improving technology and manufacturing scale, prices have generally continued to trend down (except for recent inflation).

Manufacturers are competing to make better panels using other technologies. Heterojunction cells, half-cut cells, and PERC are some terms you might come across when comparing panels. All of these technologies have helped solar panel efficiency to continue to improve over time.

What about thin film solar panels?

The vast majority of panels sold are based on silicon, but thin film is a technology that has always shown promise. There’s one manufacturer that make mass market thin-film solar panels: Arizona-based First Solar.

They seem to be aimed at the commercial market - I don’t know of any residential installations that have used First Solar panels.

First Solar’s most efficient panel is their Series 7, a thin-film panel based on cadmium telluride (CdTe). They claim it reaches 19.3% efficiency - pretty good, but that lags the best silicon panels. However, they claim their panel is more efficient in the real world because it’s less affected by heat.

A quick note about real world efficiency versus the label

The advertised efficiency of a solar panel is based on ideal test conditions. In the real world, where your panel is mounted on your roof under a blazing hot sun and exposed to dust, pollen, and bird poop, you probably will never achieve that rated efficiency.

How much worse can you expect your panels to perform in the real world? About a 20% loss is a good rough estimate.

Fortunately, the datasheet for a solar panel, which you can find on the manufacturer’s website, will usually list another set of specifications that better represent real world performance.

The advertised efficiency of a panel is based on Standard Test Conditions (STC). This is the power output measured with a solar cell temperature of 25°C (77°F) and light intensity of 1,000 W/m² (watts per square meter).

The problem with STC is that solar panels perform worse when they’re hot, and panels get considerably warm when they’re sitting in the sun - on a bright summer day, they’ll be much warmer than 25°C.

Instead, you want to look on the datasheet for NOCT, PTC, or CEC ratings. These ratings are based on lower light levels and a hotter panel, and as a result they’ll often be around 20% to 25% less than the advertised STC rating.

You can read this article on solar panel specifications if you learn all nerdy details on this topic.

What is the highest efficiency solar panel that you can buy?

At the moment, the SunPower M-Series has the crown for the highest efficiency panel: the SPR‐M440 has an STC rating of 22.8% and a power output of 440 watts.

The REC Alpha Pure-R series of panels isn’t far behind, with a 22.3% efficiency rating and 430 watt output.

Here’s a list of the highest efficiency panels from some popular manufacturers:

How important is solar panel efficiency?

As you can see from the table above, the top-performing panels from the major solar manufacturers are all pretty close in performance. For many situations, other considerations - such as price - should be a bigger deciding factor.

However, if you find yourself in a situation where you have limited roof space, going with a higher efficiency panel can allow you squeeze as much power generation as possible with fewer solar panels.