"YOU SHOULD ONLY PAY A DOLLAR A WATT FOR SOLAR PANELS.”

This is a comment we hear often. What if I was to tell you that while you think you might be paying a dollar a watt, chances are, you’re not. There are a lot of panels being sold online that have their power listed at equal or less than a dollar a watt, people think they’re getting a great deal, unfortunately I have yet to find one where the numbers make sense.

So how do you tell?

It’s all in the math. In order to explain, you need to understand what cell efficiency is and how it is calculated. Have you seen claims of 18% cell efficiency? 22%? “Over 24%”? What does it actually mean? Basically, cell efficiency is calculated by exposing a cell/panel to 1000W of light on a light table. 

Here’s a shot of one of our panels being tested by an independent third party on one of these light tables.

Let’s run through an example from our 145 watt flexible panel* made using genuine SunPower Maxeon Gen III cells (I have coloured the numbers so you can see where each one comes from, you can also use the online calculator listed at the bottom of this article**). This panel has 40 cells with 3.625W per cell (145W / 40 cells = 3.625W).  

First you need a measure of the surface area of all cells in the panel. Length x width of each cell x No. cells = area of collector (Ac) in m2. In this example it’s 40 cells x 0.125m x 0.125m = 0.625m2 (the cells are 12.5cm x 12.5cm).

A calculation is then made to determine the percentage of the 1000W m2 which is hitting the panel. (i.e. 1000W x 0.625m2 = 625W) over the surface area of the cells. To calculate the watts hitting each cell calculate 625W/40 cells = 15.625W.  

From here you can then calculate the efficiency of the cell by dividing the power output of each cell (3.625W) by the power being captured by each cell (15.625W) or 3.625W / 15.625W = 0.232 or 23.2%

The equation for working out cell efficiency is this*:

NB. 1000 is the watts/m2 applied on the light table which is a global PV testing standard, this is E in this equation.

Knowing the cell efficiency is the easiest way to determine if a cell is actually going to produce the number of watts stated. If a panel is listed at 17% efficiency, and it’s stated size is 1200 x 1100mm, then the equation to determine how many watts the panel really is would look like this:

Pmax = (Cell efficiency/100) x (E x Ac) 
Pmax = (17/100) x (1000 x (1.2x1.1)) 
              = 224.4W

(Now this is being generous, seeing as the cells don’t actually cover the entire panel, if you take into account the frame then the watts would actually be less - generally around 10-15% is frame and backing panel. You can ask the retailer for the physical size of the cell if you want a more accurate result).

If this panel is listed at more than 225W (some level of rounding to the nearest 5w is expected), say 300W, then you know there’s something not right. Perhaps their cell efficiency is higher than they have stated? Unlikely…as you wouldn’t understate something like this. 

Now we’ll talk more about why this is unlikely further on, but chances are, it isn’t actually a 300W panel.

What happens if they don’t list their cell efficiency? Well you could ask. If the response is anything along the lines of “we’ll have to check with our manufacturer” or “we trust our supplier, but we don’t have this information” then it is highly likely they have not been independently tested by a third party, let alone tested by the retailer at all. 

Cell efficiency should be known, and proof should be available. Hopefully they get back to you with the information…in our experience though, many will just ignore you as either they don’t have the information, or they know that what they are advertising is total rubbish.

There is a work around, though, if you still aren’t sure that what is stated is correct. This isn’t as accurate but will still gives you a pretty good idea of whether the stated size makes sense. Using the original equation for working out cell efficiency, the above example (assuming it is stated as 300W) would look like this:

Cell efficiency = (300 / (1000 x (1.2 x 1.1))) x 100
                                 = 22.7%

From here there’s a bit of general knowledge required to understand where solar cell technology is at globally. Yes, there are cells that have 22% cell efficiency, SunPower are one manufacturer to produce these levels, LG is another one that is beginning to get these results also. But, on the whole, standard solar cells sit around the 15-18% mark (the higher the percentage, generally the higher the cost of the cell). If you do this calculation on a panel that is less than a dollar a watt, and the cell efficiency comes out to be above 18%, then the likelihood is pretty high that you’re not actually going to get what is being claimed, as the cost to purchase raw cells alone at these levels is above a dollar a watt in US dollars. You still have to make the panel (labour and materials), ship it, pay tax, distribution costs and make a profit. 

Cells of above 18% efficiency are not cheap (check out the article over here for a link to SunPower cell prices) – ask for proof, what cells are they using, who is the manufacturer, have their panels been independently tested, can they show you the reports proving the panels are actually capable of producing the watts stated.

We have personally run the numbers on over a dozen panels selling online for less than a dollar a watt. None have made any sense mathematically. Where cell efficiency is not stated we requested the information...I am still waiting for answers years later, whilst others responded with “we trust the suppliers information is correct” but none had actually tested or used them themselves.  

When shopping for a solar panel you want to know that you are getting what you pay for. Run the numbers and ask questions – if something doesn’t make sense, if something seems too good to be true, it more than likely is.

* NB. This panel has been superseded, see our current range over here. 
** You can also use this calculator to work out the cell efficiency

NB. We are talking about folding panels here, single glass panels are generally cheaper due to their reduced components/charger/ legs/bag etc.