How to find the best solar panels for your home

Trying to find the best solar panels for your home can be an intimidating process. It’s not like buying a car, where you can go to the dealer and take a test drive, or taking a trip to the home center to peer inside refrigerators and pick out the one that fits best in your kitchen.

One solar panel looks an awful lot like any other solar panel, and the manufacturers’ data sheets are long lists of acronyms and numbers. How does a home buyer know which one is best?

Your installer will make a recommendation, but it’s important for you to understand the tradeoffs between different panels so that you can tell your installer which attributes are most important to you.

This article is going to review what we think are the most important things to look for when doing DIY shopping for solar panels or discussing with your installer which panels to use for your project: warranty, efficiency, and price. Once you understand how to find this information for any panel you are evaluating, you can do your own evaluations and compare panels even when they don’t appear on a “best of” list.

Getting the best price for solar panels

When evaluating prices, it’s important to look beyond the sticker price. Instead, the key number that you want to know is the price per watt of power. And even more than that, you want to know the real world power performance, not just the nameplate performance.

How can you tell? If you’re evaluating a solar panel, start by either asking your installer for a copy of the datasheet, or go to the manufacturer’s website and download a copy. The datasheet for your solar panel lists the published technical specifications, which we’ll use calculate price per watt. Be sure to get the model number right: most solar manufacturers have a long list of products, and model numbers often differ only by a single number or letter.

Understanding different solar performance numbers

The power output advertised for a solar panel is always its nameplate performance. On the datasheet, the nameplate performance is referred to as Standard Test Conditions (STC) output. The STC performance for a panel represents the maximum power the panel generates under standardized laboratory conditions. While this may tell you the theoretical best output of the panel, it doesn’t accurately represent how a panel baking for hours on a hot rooftop will perform, because a hot panel is less efficient. Because of this, we want to look beyond the STC performance.

Test standards that better represent real-world solar panel performance are PTC/CEC and NOCT/NMOT. These are closely related standards that differ slightly in the test setup. To learn more, read our article that explains solar panel specifications.

Normal Operating Cell Temperature (NOCT) and Nominal Module Operating Temperature (NMOT) are actually two different tests with slightly different test procedures, but for our purposes they are close enough that we will treat them the same.

NOCT/NMOT are more the severe standards that result in a lower estimated power output compared to PTC/CEC, so if it’s published on the datasheet, that’s the number you should prefer to use. Solar datasheets will not always have NOCT/NMOT specifications, but PTC/CEC testing is required in California for panels to be eligible for incentives in that state, so at a minimum you should see that listed instead.

Calculating real-world price per watt

Once you have either the PTC/CEC or the NOCT/NMOT ratings for a panel, simply divide the price by the rating to get a price per watt.

Let’s do a real example by taking a look at a popular panel: the LG NeON R series. This series comes in a range of power outputs from 350 watts to 365 watts. But that’s the nameplate (STC) performance, so let’s look up the tech specs for this panel to find the estimated real-world performance:

You can see that this panel has an NOCT rating of 264 watts. Now we just need the price. It’s best to ask your installer, but for this exercise we’ll look up the price online. One online retailer sells this panel for $485.

Quick math

If you divide $485 by 264 watts, you get $1.84 per watt. This gives you a number that you can compare to other panels that also publish NOCT/NMOT data. Think of it as similar to the unit prices you see listed in the grocery store. A lower price per watt is preferable if price is one of your primary concerns.

Real world solar panel efficiency

One of the primary numbers that solar panel manufacturers advertise is the panel efficiency. This is expressed as a percentage of the incoming sunlight that gets converted into electricity. For residential panels, this number is usually between 15-23%.

Why is efficiency important?

The main reason to care about efficiency is if you wish to minimize the square footage of your roof that you need to cover with panels to generate the amount of electricity that you want. A higher efficiency panel generates more power with less space.

The issue with advertised cell efficiency is that it is expressed as a percentage of the STC efficiency, which we know does not accurately represent how the panel will perform in the real world. Solar panels become less efficient as the temperature increases, and panels differ in their ability to handle high temperatures.

One way to better measure panel efficiency is to use the NOCT/NMOT power rating.

Calculating real world efficiency

NOCT/NMOT is a test in which the panel is illuminated with 800 watts of light per square meter. We can use this number to calculate what we’ll call operating efficiency.

To calculate this, take the NOCT/NMOT power output number, divide it by the area of the solar panel in square meters, then divide by 800.

For example, let’s take a look at the same LG panel above. The tech specs tell us that the panel is 66.93” x 40.0”, which is 2,677 square inches. To convert to square meters, divide by 1,550.

2,677 square inches ÷ 1,550 = 1.727 square meters

Take the NOCT power output of the panel, which is 264 watts, and divide by 1.727 square meters, and then by 800 watts/square meter to get the efficiency.

264 watts ÷ 1.727 m² ÷ 800 W/m² = 0.191 or 19.1%

You’ll notice that the advertised efficiency of this panel is 21.1%, so the poorer performance under high temperatures is reflected in the lower 19.1% operating efficiency that we calculated.

This is a simple calculation you can use for any panel that you are evaluating. Fortunately, because these numbers don’t fluctuate like price does, we’ve done the math for you on a selection of popular panels in the next section.

Operating efficiency of popular solar panels

What are currently the most popular solar panel manufacturers? The Solar Nerd website has a database of solar panel installations that you can view in your local area. Based on the most recent full year, here is the list of most popular solar panel brands in the US, based on the number of residential installations:

Starting from this list, we’ve compiled some of the most popular solar panels in 2019 and calculated their operating efficiency from their NOCT/NMOT power output.

Each model line listed here has a range of efficiencies. This is because a manufacturer’s model line usually includes several panels with different efficiencies and price points. For example, the SunPower E Series has five different panels, each with different efficiencies.

Interpreting the results

You can see from this ranking of NOCT/NMOT-based efficiency that the LG NeON R is the clear winner, reaching 19.9% efficiency.

What’s interesting about calculating efficiency this way is that it reveals some important performance differences. For example, compare the NeON R to the SunPower X Series. The LG panel is advertised as 23% efficient, while the X Series is nearly as high with a 22.7% efficiency claim. But under NOCT test conditions, the X Series reaches only a maximum of 14.8%, while the NeON R series performs dramatically better, topping out at 19.9%.

Keep in mind that these are still test conditions, and your actual performance may still differ. In particular, manufacturers or 3rd party testing labs can introduce inconsistencies in their testing process that could result in incorrect numbers. Nonetheless, we think these numbers are the best tool available to the consumer to make an informed decision.

Which solar panel has the best warranty?

A solar panel warranty isn’t just one number. There are actually several different things we think you should know about. We’ll go through them, and then let you know which solar company we think offers the best overall warranty.

Product warranty

Solar panel manufacturers list two different warranties with their products. The first is a product warranty which covers defects in materials and manufacturing. The shortest warranty that you will typically see is 10 years.

If your simply stops working within this period, it will be covered under the manufacturer’s product warranty.

Power warranty

The other warranty period is the power warranty. Every panel we’ve reviewed offers at least a 25 year power warranty, but not every 25 year warranty is the same.

There are two numbers that are important: the length of the warranty, and the percentage of the original power output that it is warrantied for. Solar panels slowly degrade and lose efficiency over time, and this second number of the power warranty reflects that.

For example, on their monocrystalline X Series and E Series panels, SunPower offers a 25 year warranty with a 92% power generation guarantee. This means that SunPower is guaranteeing that these panels will still continue to produce at least 92% of their original power after 25 years.

In comparison, the Trina Solar ALLMAX series, which is also an efficient monocrystalline product, also comes with a 25 year power warranty. However, Trina Solar only guarantees that these panels will produce 80.68% of their original power after 25 years. This is a big difference in comparison to the SunPower product, and an important detail that you would miss if you only looked at the duration of the warranty.

Is labor included in the warranty?

If you end up exercising the warranty on your solar panels, most manufacturers will only ship you a replacement panel for free, and leave it up to you to take care of removing the failed panel and installing the new one. The labor costs for this work could easily exceed the sticker price of the panel itself.

This is why you want to read the fine print to see if labor is covered. Out of the list of popular solar panel companies that we listed above, only two include labor costs in their warranty: SunPower and LG.

What this means is that if your SunPower or LG panel fails within the warranty period, not only will they send a new panel, but they will take care of actually installing it for you. This is a pretty big deal, and should be a factor in your buying decision.

Wind and snow load ratings

Solar panels have strength ratings, listed in Pascals, that describe how robust the structure of the panel is under the forces of wind and heavy snow.

Wind load is also known as backside load, and is the measurement of how much force the panel can withstand from the back. This is important especially for ground mounted panels.

The other measurement, snow load, is the measurement of how much force the panel can handle from the front. It’s called snow load because rooftop panels in cold climates could get buried under several feet of snow.

We mention this alongside the warranty because we think that these strength measurements are closely related to the durability of the panel over the long term. A higher strength rating indicates stronger glass or a stronger frame. Some of the most common reasons for solar panel failure are broken glass or failed seals that allow humidity to enter the panel. A more robust panel, theoretically, should be able to better resist the elements of rain, wind, and snow over the couple of decades that it will be in service.

Summary of solar panel warranties

Which solar panel has the best warranty?

There are five solar panel manufacturers that have 25 year product warranties (not just power warranties), and four of those cover the labor cost of replacing the panels. With other manufacturers, you can get a free replacement panel if yours is defective, but it’s up to you to uninstall the broken panel and install the new one. Often, the cost of labor is more than the price of the panel. This is why having labor included is a big bonus.

LG and SunPower are two companies with 25 year product warranties. To learn about the others, read our article on the best solar panel warranties.

Takeaways

Now that you how to figure out what all these numbers on a solar datasheet mean, how do you make a decision about what is most important?

Every consumer should make their own decision based on what’s important to them. That said, we would rank things this way, from most important to least:

1. Price-per-watt.
2. Warranty.
3. Real-world efficiency.

Per-per-watt is very important because it reflects the price you pay upfront. There can be a very large difference in price-per-watt between a budget panel and a premium panel, and you may never financially recoup that difference over the life of the panel, even with the better higher efficiency or warranty that premium panels offer.

While warranty is important, we ranked it second because the failure rate of solar panels is quite low. According to research by the National Renewable Energy Laboratory, the failure rate of panels since 2000 is only about 5 failures out of 10,000 panels every year. This means that every year, a panel has only a 0.05% chance of failure. However, if you have 20 panels in your system (which is about average) that expected failure rate across the system becomes 1%. Over 25 years, you have a decent chance of seeing one of your panels fail.

We ranked efficiency last. Unless you are constrained by your roof, it’s probably financially better to go with a budget panel with lower efficiency than to spend possibly twice as much money to upgrade to a premium panel. As you can see from our graph above, the difference in NCOT/NMOT-based efficiency between different panels is not very great, and may not be large enough to make up the difference in upfront cost required to buy premium silicon.