Originally Posted by StuM
Even stating "19 Watts" is nonsense. That's a very precise number. At what time of day? For how many hours? Under what conditions?
Oh good lord. People, watts per hour is just another way of saying watt hour. It helps the folks, who are not engineers understand a bit easier.
I said "approximate" increase of 19 watts per hour. That was based on 65% of rated panel output and a 15% increase with a MPPT controller, which for a 12v panel is generous.
Why 65% of rated output? because that's what I've measured in Northern California
and southern Florida
with 100 watt 12v panels on sailboats at anchor
. Could be 60%, probably not more then 70%.
OP said two 100 watt panels. So they were nominal 12V, with Voc of 19v or higher. Really its the Vmp that is more important as Voc is voltage at zero current
flow. We really want current flow.
I assuming 14v with a PWM controller and 17v for a MPPT controler. Of course these numbers change all the time, but lets just use this for kicks and giggles.
A 100 watt panel will output about 65 watts during peak sunlight with a pwm controller. Lots of variables, but this is just an example, It's close in any case. Assuming a 15% increase in output efficiency with a MPPT controller the panel would increase output by 9.75 watts per hour. My real world data from Florida
, showed a 12% increase.
In any case 9.75 watts times two panels is 19.5 watts. I rounded this down to 19 as I knew my assumptions were a tad high. I'm an engineer
I can do that.
Of course I mean during the peak 5 hours of sun, when the panels are working the best. I probably did not mean at midnight.
By saying approximate, I know that any increase could be as little as 12 watts or as much as 25 watts, for two typical (perhaps older) 100 watt panels. Yes he could wire the two 12v panels in series and in theory increase peak solar output under ideal conditions. BUT in a sailboat with shading issues, odds are more times then not one panel would be shaded.
By shading, I of course mean with as little as one cell shaded more then 50 percent. As the panel cells are wired in series, shading one cell will cut panel output lots. Lots could mean up to 80 percent reduction, give or take a bit. Trying to keep it general ish.
BTW in southern Florida, ages ago, I saw a real world increase of 8 watts per hour or Watt hour, with a single
12v 100 watt panel and a MPPT controller (blue sky), during peak daylight. It was that piddling increase that made me decide to use PWM with an extra panel as it gave more bang for the buck.
Of course I used the word "Approximate" so folks on CF would not jump on my case, for trying to be to precise. Yeah, that worked out sooo well.