After playing phone tag the other day, John and I were having a conversation about all things solar and my Solar Shed phase 3 performance. He asked how well the modules were performing as these are all recycled salvage panels in the form of roof tiles.
I replied that they are doing well, having seen around 8kw with a 10kw nameplate rating. That concerned him and he seemed a little surprised when I made the point that modules never seem to make full rated power…not in the real world anyway.
You see, panels are tested and rated at an industry standard under conditions that are rarely encountered at most sites. It isn’t exactly a lie, but neither is it the real world. Never mind that mine are panels that have a permanent grime on them and were stomped and thrown off the roofs of their original installations. I have seen some panels with a second set of numbers on the label that represent a more likely scenario. These lower numbers are closer to what most folks will see.
First of all, there is the matter of the sky sometimes being reluctant to provide the industry-standard level of irradiation. In our area, we go for days at a time in winter with clouds and gloom. In the summer, with 95 degrees and 95% humitidy, we have a 10,000 foot layer of humidity that looks sorta like blue sky, but it blocks some of the sun. My Phase Two version of the solar shed had plenty of power on sunny days, but not enough battery to ride through the gloomy ones. After 4 days, I’d have to switch back to grid to save the batteries. Phase Three has enough modules to charge the larger battery pile, even on cloudy days. Getting the right balance of relatively cheap solar and expensive battery is tricky, but I think I have nailed it and have added more loads to the new 48v system.
Electrical resistance can take a small toll. Keep your cables fat and short for lowest resistance. Long skinny wires and parallel strings will give you a power loss. Each connector loses a little power. Since my system uses relatively small 34 watt modules, there are over 700 MC3 connectors up there!
Then there is the matter of the sun’s angle, relative to the panel surface. They test with the sunlight coming dead-on square with the panel. Tracking racks are available, but I don’t think they are worth the extra expense. If you have a fixed installation, then you will get that max output for an instant, twice a year. Right now, the sun is a little low to be hitting best power on my array. This summer, the sun will be high, but the days will be longer, so I am looking forward to sun-powered central air conditioning! Don’t let somebody tell you that you have to have your rooftop panels at a really ridiculous and ugly angle to improve your performance. I saw photos of a system mounted on an otherwise picturesque barn at such an angle as to make the image hideous. Mount your panels at the roof angle and just use more of them if you need to. Keep solar beautiful and fashionable. If I remember any of that sine and cosine stuff correctly, you’d have to be 60 degrees out of alignment to drop the output to half and on a cloudy day the diffusion of light by the clouds would make up for some of that loss.
I mentioned that these modules of mine have a permagrunge. Nothing cleans them! Then there is the layer of pollen that the pine trees are presently giving them between rain showers. In May, cropdusters at my place will give them a coating of red clay dust as they go roaring back and forth. That’s right, anything that blocks the sun blocks the power. The big solar farms are now playing with the use of robotic panel washers to keep the modules clean. One of those long-handled RV brush/squeegee thingamajiggers will probably do well enough for you.
There is also the temperature factor. Note on your label that your panels are rated at a certain temperature. Mounted close to the roof or even out in the open you are likely to find much higher temps on a sunny day. Output goes down as temperature goes up. The solar roof tiles I use probably benefit from being mounted on open purlins instead of a sealed plywood roof deck as code dictated in their original residential installations. Let your panels breathe!
So, if you use your panels on a cold, dry Himalayan mountaintop with sun trackers, you might come out even with the rated output of your modules. The good news is that they start making power as soon as the sky makes light, even down here in the hazy flatlands. My batteries are usually bulked up by 9:30 and it just keeps getting better until mid day. You’ll end up with 5 hours equivalent at full rated power if you live in Florida and a little less as you move northward up the map. Your mileage may vary! Since solar panels are no longer the expensive part of the system, add another string of 3 modules modules (John’s 305 watts special) for about a hundred bucks, each, for a little extra insurance and peace of mind. You won’t regret having “too much.”
(Update: Since this was written, the sun arose to a perfect angle on the Solar Shed’s roof. We’ve even had some unusually clear days. I isolated one bank rated at 2500 watts and actually saw 2475 watts on the meter at noon. Close enough! This will drop off as the sun continues toward a higher arc, but the days are getting longer so there will be plenty of power.
By: Neal Collier