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cheap solar panels at Home Depot, with a catch (mouse.mousetrap.net)

submitted 3 months ago by [email protected] to c/[email protected]

6 comments fedilink hide all child comments

TL:DR: I saw a set of cheap panels with weird specs at Home Depot. I bought some to experiment on and to use as portables to augment my mounted solar.

Home Depot is selling 200w of panel for $114. That's $0.57/Watt. Not amazing compared to used panels (typically $0.33/Watt) but HD is all over the place and has free ship-to-store.

It also comes with mounting brackets and one of those single-stage PWM controllers. I'm not bagging on that kind of controller, but it's not a great fit for this particular set of panels.

#THE CATCH

The panels are a weird design, apparently 24 cells in series. Normal "12v nominal" panels have 36 in series for ~18Vmp. These have a Vmp of 12.0v, so I think we would call them "8v nominal".

This makes them practically unusable in parallel for charging lead or LiFePO4.

You could run the panels in series on the PWM controller since it has a 50v input max and the series Voc would be 30v. But, due to the way PWM works the panels would be running at in the 14v range at the most. This is way, way off the 24.0Vmp of the series array. I'd expect a max harvest of ~120w with that kind of setup. If these were normal panels in parallel and on PWM I'd expect a max of ~160w. We can go into the math on that if anyone wants.

The best case scenario IMO would be to run the panels in series with an MPPT controller. This would get us closer to ~170w max harvest.

some other thoughts:

The panels might work well enough in parallel for 3S Li-NMC because of that chemistry's lower voltage
HD has a 10% discount program for veterans if you provide them with a bit of documentation.

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[–] [email protected] 7 points 3 months ago (3 children)

Is there a good online tool for calculating the cosign of solar Zenith so I can know how much solar my my panels could be making in ideal conditions? I see that you've used it in your blog.

[–] [email protected] 3 points 3 months ago* (last edited 3 months ago)

It's cos(your latitude-23.44°), or just 1 in the tropics, unless I'm misunderstanding the question. I assume this is for a panel mounted level? Make sure your calculator isn't expecting radians!

If you mean ideal at a specific time and date, you can use the formula from the Wikipedia page. Substituting in 23.44°*sin(years since last March equinox*360°) for the (rough) solar declination, and longitude+UTC for hour angle, you get:

sin(latitude north) * sin(23.44°*sin(year fraction*360°))  +  cos(latitude north) * cos(23.44°*sin(year fraction*360°)) * cos(longitude+UTC time).

If you're just looking to avoid writing that out every time, I'll happily implement it in the common programming language of your choice. If you really care about the deviation caused by the elliptical orbit of the Earth you'll need more complicated math.

Edit: That should be signed latitude on the second equation, and March equinox, not solstice. Fixed.

[–] [email protected] 2 points 1 month ago* (last edited 1 month ago)

I've found these Android apps useful:

Trail Sense and it's Solar Panel Aligner tool (given a time length or all day, calculate and assist alignment of panels to maximize production)
RV Solar Forecast to see theoretical maximum possible
Sun Surveyor to see where I can put my portable panels to get the maximum light without shading

[–] [email protected] 2 points 3 months ago

Is there a good online tool for calculating the cosign of solar Zenith

There are an online calculators like this one from NOAA. This fork adds the ability to update the time with a click rather than manually. There are others but I haven't played with them much.

Since I travel constantly I've been on casual lookout for an app that does the calcs for us based on local time / position but I haven't seen any. Several apps show the solar zenith angle and we can take the COS of that manually with a calculator with trig functions. The standard android calculator app will do it.

so I can know how much solar my my panels could be making in ideal conditions?

Yeah, it's an imperfect tool for our purposes but better than nothing. Combining it with data from a solar irradiance meter would be great but right now I can't justify ~$100 to devote to the cause. :-)