When I planned the caravan, I took a quick look about on online for solar panels. I saw that a 100W panel was about 1m long. The next size up was about 1.5 or 1.6m long. That was the basis for the caravan. If I'm committed to 1.6m, when my tent is 1.9m long, why not sleep under the solar panel?
A few months ago I looked quickly on Ebay and saw 300W and 400W panels. Small sizes too. Wow. This is great. Cheap too! I could get 4 panels on the roof. That's an amazing 1200 watts! I'll be swimming in power. Solar must have improved a lot. I was happy for weeks.
Then I went to order.
Money on the line, so I really dug into the details and specs to compare panels. A huge face palm moment. How did I not notice earlier!
I won't put a link here, but search Ebay and you'll find a lot of examples for cheap solar panels. I'm looking at flexible panels, but this also applies to rigid panels.
Taking one example on Ebay, advertised as 400W for $136.99. Looking through the nice pictures, lots of claims of high efficiency. High conversion rate. Panel size is 960x670mm. Nice and small. The seller has a lot of sales. High ratings and good reviews.
Scrolling down to "Item specifics" to get more details is the first hint of something not quite right.
Connector Type: MC4 Wattage: 300-399W
Power versus Wattage? What's this about? Both given as a range, but it is advertised as 400W. What would I really get out of it?
The panel is using monocrystalline cells.
Solar Technology: Monocrystalline
Googling, the top result: Monocrystalline cells are rated at 15%–24% efficiency. Looking at other results, 15%-20%, 15%-23%. Note this is for the cell. After you encapsulate it in plastic the rating will drop. But then searching for solar panel efficiencies, I also find 15%–24%. Cheap Ebay panels are unlikely to be 24%.
How to compare solar panels: using Standard Test Conditions.
Standard Test Conditions (STC) are the industry standard conditions under which all solar PV panels are tested to determine their rated power and other characteristics. When a panel is advertised as having a capacity of 350W for example, this is the power it is expected to produce under STC.
It tests a cell at:
Irradiance: 1000 W/m²
Air mass: 1.5
This is the cell, there are other measurements that give a more real world data for a panel.
But taking STC, and using the 1000W/m² and a low and high efficiency value, we have:
Panel Wattage = Solar Irradiance x Area x Efficiency
= 1000 W/m² x 0.64m² x 15%
= 96 W
= 1000 W/m² x 0.64m² x 24%
= 154 W
96-154W is a long way from the advertised 400W. Not even close to the 201W power rating either.
But I might not get even that. That was at 1000 W/m². Using the solar radiation in Sydney. On the day I was doing the calculations, the maximum was 961W/m² at midday. The average of 10am to 3pm was 833W/m². The day average was 569W/m².
Running the numbers again this time using 15, 17, 20 and 24% efficiency for the hours of the day. How many watts at the different hours at an efficiency.
| October 28 | |||||
| ---------- | ------------ | ||||
| Sunrise | 06:00 | ||||
| Sunset | 19:18 | ||||
| Total | 7398 wh/m² | ||||
| Panel m² | 0.64 | ||||
| Watts | |||||
| Time | W/m² | 15% | 17% | 20% | 24% |
| 07:00 | 144 | 14 | 16 | 18 | 22 |
| 08:00 | 376 | 36 | 41 | 48 | 58 |
| 09:00 | 594 | 57 | 65 | 76 | 91 |
| 10:00 | 777 | 75 | 85 | 99 | 119 |
| 11:00 | 901 | 86 | 98 | 115 | 138 |
| 12:00 | 961 | 92 | 105 | 123 | 148 |
| 13:00 | 826 | 79 | 90 | 106 | 127 |
| 14:00 | 771 | 74 | 84 | 99 | 118 |
| 15:00 | 763 | 73 | 83 | 98 | 117 |
| 16:00 | 612 | 59 | 67 | 78 | 94 |
| 17:00 | 445 | 43 | 48 | 57 | 68 |
| 18:00 | 213 | 20 | 23 | 27 | 33 |
| 19:00 | 15 | 1 | 2 | 2 | 2 |
What time you test and the efficiency of the panel matters. In the real world, that 400W panel is only going to give a maximum of 148W on that day if (and that's a bit if) it is 24% efficient and the temperature of the panel is 25C. Spoiler: It won't be 25C in the sun and it isn't 24% efficient.
All the high quality brand name panels I checked had Monocrystalline efficiencies of 17%, so using that, the 400W panel might give 105W at midday at most. Real world will be less. Flexible panels also degrade faster than rigid panels.
But all is not lost.
I created a spreadsheet of all the solar panels I was looking at. On and off Ebay. The off Ebay panels were by Renogy, Sunman and ATEM. Their specifications as advertised. Price, size, advertised watts, panel type, Voc and Vmp. I then calculated the watts at STC for all panels based on its size. No surprise, most Ebay panels were way less than advertised. But not all. All the non Ebay panels I included were better than advertised at STC.
I also calculated $/watt at STC for all panels. If area wasn't a concern this would be one deciding metric, but panel size was also important to me. I only have so much space on the roof. How could I get the maximum use of it?
With the different panel sizes, I could get between 1 and 4 panels. In the end, I decided on three Ebay panels advertised at 400W, but by my calculations are more likely to be 90W each. I got lucky that the seller was having a sale, so each panel ended up costing $70 delivered. There was a Sunman panel I really liked with watt output only slightly lower than the combined three I panels I ordered. But I could only get one on the roof and it's price was significantly higher. Annoyingly a couple of weeks later, they had a 60% off sale. At that price I would have purchased it. But too late by then.
I'm happy with the three I did order. All the reviews of flexible panels show they don't last as long as rigid panels. Flexing cracks the cells, lowering output. Over heating is a problem if they don't have an air gap underneath. As my caravan is insulating foam, I intend to mount them raised. So having cheap panels for a first go suits me. I think of it as an experiment to test the mounting system. When they fail, I'll know more about my usage and the mounting system. Then I might look at a more expensive options.
