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Thread: Solar panels
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Old 05-18-2019, 06:32 PM   #14
Chance
Senior Member
 
Brand: Still Looking
State: Texas
Posts: 6,187
THOR #2121
Quote:
Originally Posted by Long & Winding road View Post
Well maybe your right....and im not expecting too much out of my system.

I dont want to overthink this either. I want to start kinda small (200 watts) and add one or two more panels as I go. Plus Im limited on room for batteries and roof space for pannels.

But dont they say "for every 100 amh battery you need 100 watts of solar" as a general rule? Of coarse they are refering the to "entire" battery knowing that your only need to charge for 50% of that.

AND most are figuring the full 5 hours of sunlight a day where you and me are being more realitic with 3 hours a day to be safe. If every day was sunny and you live in the southern states you should realize 5 hours a day (but around 3 in the winter).

So 100 watt solar x 5 hours = 500 watts and if you figure 50 amh of useable juice then you get back around 41.2 amps per a day with that setup (as a general rule). But in reality Im sure its less.

Im just going to start slow. I dont really need solar .... it just something I want to do.

This is really my test project for when I add solar to my house in 5 to 10 years. I like to be close to net zero at home by the time im ready to retire.
Analyzing, or quantifying using available technical information, isn’t the same as overthinking it. It just helps make an informed decision

I wasn’t suggesting you only get 3 hours of sun. Or 5 hours for that matter. I was just stating that it appears that the panel manufacturer estimates the equivalent of 3 hours if at peak panel capacity for an average day.

Most panels don’t get close to the maximum peak rating for numerous reasons, starting with the fact that most are installed flat. Then there is the sun intensity which can be considerably below that used for rating the panel. And of course there is some inefficiency between panel output and what gets to batteries. This stuff isn’t rocket science, but has to be accounted for. It may actually take 9 hours to get the “equivalent” of three peak hours.

The 100 Watts of panel capacity per 100 Amp-hour of battery capacity “rule of thumb” seems like a good place to start, but if meant for extended periods of boondocking, the batteries may run down after a few days. If 100 Watt panels only produce ~ 300 Watt-hours, or 25 Amp-hours per day on average, then it can be estimated that 100 Amp-hour battery discharged to 50% will only get half of that back from the sun. After 2 “average” days you’d have to run generator to charge batteries.

I should note that if system isn’t 12V or if batteries are lithium, then the 100W/100Ah rule of thumb doesn’t work as intended.

Attached are performance curves for panels from same manufacturer mentioned previously. It’s interesting that solar intensity is shown from 200 to 1,000 Watts per square meter. That’s quite a range, suggesting we shouldn’t expect a lot of precision in any estimate.
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