Post
Lots of people talking about buying solar panels. Alas if you look online there's a lot of retailers with all sorts of panels for sale, not all of which are honest. Want an easy way to check if the listed power output is plausible?
Example using a 50w panel listed on Amazon. It's 370x820mm.
Step 1 calculate the area in square meters. (0.37*0.82=0.3034)
Step 2 multiply by 1000w. (0.3034*1000=303.4)
Step 3 assume 20% efficiency and multiply (303.4x0.2=60.68)
60.68>50w. It's Plausible.
@quixoticgeek why only 20%?:0
@vgarzareyna cos commercial solar panel efficiency is about 20%. Some manufacturers claim closer to 25%, but they typically lose some of that efficiency in the first few weeks of operation. As a ballpark for judging if a panel is too good to be true. 20% works and makes for easy maths.
@quixoticgeek that's really cool! thanks for the info. as I mentioned in a different response, i thought efficiency was way higher, but the cool part is we have a lot of margin for improvement
@vgarzareyna we're doing better than photosynthesis. That's about 1% efficient...
@quixoticgeek In the UK there’s lots of information in the form of data sheets. Overwhelming. What there isn’t is anything useful. Like how long they take to repay their cost. Getting a straight answer from the installers is tougher than getting a politician to say yes or no. And then there’s the battery! Getting advice on whether they’re going to repay their cost within your lifetime is close to impossible
@TerryBTwo it depends a lot on what your usage is like, how big a battery you can afford, do you want to charge an EV? Power a heat pump? Etc... There's a lot of variables. I think with a modern solar install, the average household is likely to pay back a suitable solar install in 3-5 years. But it does depend a lot. Gone are the days where ROI was double digits years.
@quixoticgeek For our panels it’s a fairly simple equation. What we used to use in mains electricity compared to what we can realistically get from solar + what we can earn in selling back. But the battery is a much more complex estimate.
@quixoticgeek Are you saying solar panel manufacturers are underreporting the max wattage of their panels? Because when I read the first part of this, I assumed you meant the inverse
@disorderlyf no they under report. The 60 is higher than the 50 rating for the panel, as the panel has a border round it and not 100% of the area is silicon. The 50w panel in the sample has a border of about 20-30mm round the edge that is structural, but not power generating.
But I've also seen panels the same size listed as 100w. And that's clearly not gonna happen.
@quixoticgeek @disorderlyf ...and probably they are counting on different weathers, latitudes and angles, as in not promising close to the optimum case.
@annantidote @disorderlyf No, there are standard test conditions for which panels are rated. As @quixoticgeek says it assumes insolation of 1 kW/m², also a specific spectrum of light (corresponding to a panel standing on flat dusty red ground in Texas with the sun at a particular elevation IIRC) and particular temperatures for the ambient air.
But, yes, panels can produce more than the “peak” output quoted. E.g., when a cloud has just blown away so the panel is cooler and therefore operating more efficiently and there's also more sunlight because of the reflections from the cloud just out of the line of sight. So “peak” would be better called ”nominal” output.
@annantidote @quixoticgeek I'm in BC, so I'm already expecting things to not be optimal, but I also might have a place to put it where it'll get as much sun as it possibly can.
@quixoticgeek Yeah, I'd been waiting for efficiency to get to the point where we actually could get 100w out of a panel that size, but it's looking like I might just want to shell out for a bigger battery and (relative to its capacity) drip feed it solar. I want a panel I could fit inset into a window since I'll likely be living in a rental for at least another year.
@disorderlyf if you're putting a panel behind a glass window. Assume about 50% less power from it. I have a 100w panel behind a double glazed window and get about 50w from it peak.
@quixoticgeek So, solar flux at the surface (of Earth) is approximately 1kW/m^2?
@BoydStephenSmithJr @quixoticgeek That's the approximate peak illumination at the peak of the day. 9am-3pm have reasonable illumination but most commercial panels have efficiencies in the low 20 percent it's a reasonable way of telling if they're bullshitting. You probably won't get that much because of the effect of heat on efficiency mind you.
@BoydStephenSmithJr essentially yeah. It varies of course across the planet. But panel rating is based on 1kw/m², and typical efficiency is about 20%.
@quixoticgeek @BoydStephenSmithJr
And, to return to an earlier theme, slightly more in earth orbit. Going to make cooling those orbiting data centres so much fun.
@OneInterestingFact @BoydStephenSmithJr exactly. Orbit is a stupid place for a datacentre.
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