Detailed solar panel specifications

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Solar panels have many specs you can pore over – efficiency, temperature coefficient, performance ratio, and multi- or monocrystalline. Really, none of these things matter if you’re getting a good brand with a strong Australian presence to handle any warranty claims. The performance of the good brands is so close that you won’t notice the difference in day-to-day operation. What you will notice is if the solar panels fail and you struggle to get a replacement under warranty.

So you now have a list of brands, so how do you choose between them?

If you find a good installer (I’ll show you how, later), they’ll have brand preferences. They’ll usually install two or three brands: budget, mid-range and premium choices. They will have chosen their brands based on personal preference, experience, local climate, and, yes, what deals they can get with the solar panel manufacturer or importer.

If they’ve been around for a while and they plan to be around for a while longer, they want to sleep at night for the duration of the 25-year warranty. That means they’re unlikely to offer a solar panel that they don’t believe in.

If you find a genuine, local installer with plenty of good reviews online, offering reasonable (not jaw-droppingly low) pricing and panels that are on the chart, I would trust their judgement.

I could write thousands of words about solar panel specs, but in this section I’m just going to cover two key considerations.

Spec 1: Efficiency (and the only time it matters)

Salespeople love to wax lyrical about solar panel efficiency, but the truth is, it’s not that important to most buyers.

It makes sense that the most efficient panel must be the best, giving you the most power and, therefore, the greatest return on investment, right?

If you’re buying a washing machine or car then this has some truth. The more efficient your washing machine or car, the cheaper they’ll be to run.

Does that mean that the more efficient your solar panel, the more energy you’ll get?

No.

The efficiency of a solar panel measures how much of the sunlight that falls on it is turned into electricity. It varies from 15% to 23%, depending on the model and manufacturer. If solar systems were sold by the physical size of the panel array, the system with the highest efficiency would give you the most energy. But solar systems are not sold by their physical size; they are sold by their peak power rating in kW.

This means a 6 kW solar power system with 15% efficient panels will require 24 of them, while with 20% efficient panels only 18 will be needed. Both systems will give you the same amount of energy, but the lower-efficiency system will take up more space on your roof because you’ll need more panels.

Higher-efficiency panels are often more expensive per watt than lower-efficiency ones because they use the latest technology, and that costs money to develop.

That means the only economical reason to pay more for a higher-efficiency system is if your roof isn’t big enough for the system size you need.

As I mentioned before, there’s an argument to be made (and the super-efficient panel manufacturers, LG and Sunpower, love to shout it from the rooftops) that in the near future, roof space is going to become much more valuable. As batteries become cheaper, people will install home batteries and electric cars, all of which need charging. With limited roof space, the best way to prepare for this is to buy the most efficient solar panels you can now, leaving space for even more as you need them.

I think that’s a valid argument for people with an eye to the future – perhaps five to ten years away.

Let’s run the numbers:

• Home batteries: A typical grid-connected Aussie home needs a 10–15 kWh battery to meet peak demand and get through the night on solar. It should have at least 6 kW of solar on the roof to be able to charge the battery through the day while still powering the home – even in winter. If you can’t fit 6 kW of ‘standard efficiency’ (15%–17%) solar panels on your roof and you expect to buy a home battery at some point, spend more on super-efficient panels.
• Electric cars: At the time of writing, the most affordable electric car with a reasonable range – the Chevy Bolt (not available in Australia, unfortunately) – has a 60 kWh battery. The average car in Australia goes about 40 km a day, which would drain the car battery by about 7 kWh. To reliably generate an extra 7 kWh from solar, even in winter, you’ll want another 3 kW of panels.

If you believe you’ll still be in the same house and that you’ll have a home battery and an electric car in the near future, use panels with a high enough efficiency to fit 9 kW of solar on your roof. You may only be buying 3–6 kW at the moment, but I believe that electric cars and widely adopted home batteries are less than five years away, so my advice is to use solar panels that will allow you to upgrade to at least 9 kW in future.

The higher efficiency, more expensive panels are those towards the right-hand side of the chart¹².

Spec 2: Temperature coefficient

Australia gets hot. Contrary to popular belief, a solar panel’s efficiency reduces as it heats up. Solar PV panels love light but hate heat. When the sun is beating down and the panels get hot, their power output drops by around 0.4% for every degree the panel gets above 25ºC.

Note that this is the temperature of the actual solar panel, which is usually about 25 degrees above the air temperature. On a 40 degree day, when the panel is 65ºC, the power output will be reduced by about: 40ºC x 0.4% = 16%.

Some panels perform better in the heat than others. If you have to choose between a couple of panel models and you want to see which ones do best in the heat, you can compare a number on the specification called the ‘temperature coefficient of Pmax’. Be careful to look at that number, not any of the other temperature coefficients.

Figure 5.7 Excerpt from a solar panel data sheet showing three temperature coefficients.

The smaller this absolute¹³ number, the better the performance. For example, a temperature coefficient of -0.42% per ºC is better than a temperature coefficient of -0.49% per ºC because you’ll lose less efficiency for each degree the temperature rises.

As with most specs, good solar panels all have similar temperature coefficients, so there’s no need to go crazy about hunting down the lowest. A really good temperature coefficient may give you a few per cent more energy over the year – but are you really going to notice?

To summarise, my advice is simple: buy a solar panel on the chart, preferably from a reputable local installer. If you can afford to buy something towards the middle or right-hand side of the chart, you’ll be rewarded with around 5% more energy from your roof and in my opinion, less likelihood of making a warranty claim in the decades ahead.

FOOTNOTE: ¹²Be aware that there is one exception to the ‘more expensive, more efficient’ rule: Tindo panels. Tindo are Australian-made solar panels that are expensive but low-efficiency. This is not a criticism: they have been designed to prioritise durability over efficiency in Australian conditions.

¹³In maths, ‘absolute’ means ‘without the minus sign’.

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