An expert review of Sunpower solar panels
Key Advantages of Sunpower Solar Panels
Sunpower have the most efficient solar panels used widely in the residential market. They are an American company with significant investment in their brand and their solar panels also have the highest PTC ratings compared to their nominal power rating at STC primarily because of their excellent temperature coefficient.
Key Disadvantages of Sunpower Solar Panels
They are a company that relies purely on their revenue from the solar business and so this makes their long term viability less certain than companies that are part of strong diversified electronics manufacturers such as LG and Panasonic.
We also have concerns that despite the fact their panels are awesome their price tag can sometimes make them a financially less efficient option in terms of maximizing financial yield from a solar system. Still, they make a great solar panel!!
Sunpower Solar Panels Summary
The Sunpower X series of monocrystalline panels are the most efficient solar panels used widely in residential solar installations in America with their X21-345 panel claiming an impressive 21.5% average panel efficiency. They also make an E Series of monocrystalline panels in the range of 320 to 327 watts that also offer an impressive maximum efficiency of 20.4%
Sunpower also has a range of polycrystalline solar panels (not surprisingly known as the P Series) that have efficiencies much closer to industry averages of around 17%. They tend to market these to cost sensitive commercial and utility scale customers. Sunpower’s X series of solar panels are clearly the best solar panels on the market both in terms of efficiency and in terms of temperature coefficient.
Factors used to rate solar panel brands and how Sunpower Solar Panels rank on each of these
We use the following factors to measure solar panels brands against each other
Sunpower Solar Panels Warranty
Sunpower offer a 12 year structural warranty referred to as a "product warranty". This is slightly longer than the 10 year industry standard. This warranty is limited to physical and mechanical defects
They also offer a linear power output warranty for 25 years with a 2% degradation in the first year and then a 0.6% per year degradation allowed for in following years.
This linear power output warranty is a feature only offered by the better brands of solar panels. Other lower quality brands offer what is known as a step down warranty where they only warrant 90% of rated power at standard test conditions for 10 years and then 80% of rated power from your 11 to 25.
So on this note the Sunpower solar panel warranty is better than industry standard but when I actually read their warranty document they refer to allowing for a measuring tolerance of plus or minus 3% which effectively means they are only really guaranteeing 3% less than the number quoted in their guarantee.
The thing to note about solar panel power output warranties is that the panel has to be tested at standard test conditions meaning if you want to try and claim on the power output warranty you have to take the solar panel off your roof and take it to a test laboratory. In practice no normal consumer would be able to do this rendering warranties almost useless and putting us back to square 1, make sure we have a good panel top start with.
Efficiency of Sunpower Solar Panels
Sunpower offer the most efficient solar panels widely available on the market with their X Series panels claiming an impressive 21.5% average panel efficiency.
You can read more about the efficiency of solar panels and why it is not something on which we place great weight when ranking the overall attractiveness of one solar panel over another.
Power output of Sunpower solar panels
Solar Panels are rated at a particular wattage based on their power output at STC (standard test conditions) which involved 1000 watts of irradiation per square metre at a temperature of 25 C. (77 F).
However, real world conditions are much different to this. Generally speaking cells will operate at temperatures well above 25C and panel output will typically fall as temperature rises.
The rating system of PTC (Photovoltaics for Utility Scale Applications Test Conditions) is an attempt developed by NREL (National Renewable Energy Laboratories) to rate different panels at how much they will produce in real world temperatures.
The temperature coefficient of the Sunpower Neon II solar panels is a -0.38% fall in production per 1 degree increase in temperature. The lower this number the better as this means output falls less as temperature rises. The Sunpower coefficient is around average amongst the better brands of panels but nowhere near as good as the temperature coefficient of -0.29% from Sunpower X Series panels.The rate at which power output falls for each 1 degree increase in temperature is called the temperature coefficient. A better panel will have a lower temperature coefficient such that although its output wills till fall as temperature increases it will fall at a lower rate.
How much do Sunpower solar panels cost?
Value or cost efficiency is a big factor because at the end of the day solar is a financial decision for most people rather than predominantly an environmental choice. In many parts of the US electricity costs are quite low and so value systems around $3 per watt fully installed are needed to give consumers the 6-10 year payback they need for solar panels to be attractive. As such we see it as a positive factor if solar panel brands have a relatively low cost of manufacture and a low selling price in the US.
It is hard to get visibility on the wholesale price of Sunpower solar panels but retail systems seem to be available with a tier one string inverter for around $3.50 per watt for a cash purchase. This is around $0.40 cents per watt more than other leading brands and they are sold as a premium offering.