One 200w panel vs two 100w panels

[Growinupinfl]

Hello to all,
This is my 1st post and I feel I've only achieved Newbie status through watching Will's videos. I found some Q&A on here comparing different size panels, but doesn't seem to answer some of mine. Just like everyone i want a capable 1st system with minimum cost of upgrading in the future, but low price is paramount initially.. this is to power a 33' travel trailer, even if just to power the all the 12vdc system and charge the battery. I have found some used Sharp 200w panels used for similar price as 100w Renology panels from HD ($75-$90) for each panel. Looking at the specs (see pics) I see the 200 panels are 36v and say about 8A vs the 100w that are 18v and 5-6A. So my question is are the watts (200 vs 100w) or the amps (8 vs 5) more important to consider? Is the 200w panel really giving 2x the power for the same money Is? Is the higher voltage going to be wasted on a 12v system? I hadn't decided on PWM or MPPT controller yet (yes I know Will preaches MPPT) because I will have such a small system initially. I suppose I would require MPPT with the 200w panels due to voltage mismatch (36v panel output for a 12 v system). I currently have 1 lead acid 180 ah battery (crappy) that cannot run minimal 12v power (fridge and water pump) for 8 hours without dying. Yes I'm looking to upgrade batteries too. But think beer tastes on a tap water budget!! I'm sure I've left off many necessary details. I'll clarify what I can... I'm not even thinking about an inverter yet, but I can dream!! ????

 

[snoobler]

With an MPPT controller, all you care about is a Vmp > 150% of battery voltage. The maximum Voc must be under the MPPT's voltage limit though possibly less due to cold weather voltage spikes (10% under is good enough in most cases), and an input current that doesn't exceed the input current of the MPPT.

If you go over the voltage limit, bad stuff happens.

If you go over the current limit, the MPPT just ignores it and only takes up to the maximum current allowed.

If you used the 208W panel, you must get an MPPT, or you're throwing away lots of power. You'd only get 126W max out of it.

Also note that with PWM, you will NEVER get peak power from a panel. If a panel's optimal is at 17V, and you're running it at 14.4V, you're only going to get about 14.4/17 = 85% (approximately) of its power in perfect conditions. At lower charge voltages, you get less.

With 180Ah lead acid, you need to limit charge current to 18-27A, so 18*12 = 216W and 27*12 = 324W, but 416W would probably be okay if you are getting to absorption voltage before you see peak current AND you'll have a decent margin for running your loads while charging your battery. 200W will be marginal for supplying needed charge current AND powering your loads.

If you are routinely running your battery to zero with 8 hours of run-time, you should turn it off after 4 hours. You're just hammering the battery, and in a year of daily cycles, the battery will be on its last leg.

IMHO, get 208W panels and use 30A MPPT. With 416W of solar and 180Ah of batteries, on sunny days, you're likely going to be able to run your fridge 24/7. BEER ME BRUH!

 

[Growinupinfl]

We bought 3 of these panels (see 'Sharp's pic above) and I want to buy a controller that can accommodate them so I don't have to buy another in the future. I think I will require a 50a-60a and 150v input controller to cover the rated 624w (208x3) panel output. Many 40a controllers state max input of 520w for 12v or 1040w for 24 v. I believe I need to consider the 12v limit because it is feeding my 12v battery and RV system? Or am I looking at the 36v pv for max input?? I've been reading so many different things online, I think I'm confused.. And if I do use a 50a controller then I believe I need to use 8ga or 6ga wire from controller to battery, but standard from panels to controller is 10ga?? Ugh!! I know 600w may be overkill for my current battery situation. In the short term we plan on putting another battery in parallel to increase our reserve. May upgrade to lithium fe04 later...

 

[snoobler]

Charge controllers are rated for charging current. If you see a 40A charge controller, it can charge the battery at 40A max.

40*12 = 480W, but it will go higher as the battery voltage will be higher than 12V
40*24 = 960W, same higher limit as above.

So, yes. You need to consider the 12V power ratings if you are charging a 12V battery. 50A would be fine with 8awg.

With MPPT, you can use much higher panel voltage and lower current, so 10awg from the panels to the SCC is fine.

~600W is too much for your current battery. ~400 is much more reasonable. You could get the 600W now in anticipation of your added batteries, but you should leave the extra disconnected or limit the charge controller charge current. You always want to have your peak charge current 10-15% of your battery capacity to keep from damaging them.

 

[rin67630]

~600W is too much for your current battery. ~400 is much more reasonable.

That must not be true at every geographical location.
At high latitudes, you frequently get less than 50% of the rated power, and very frequently MUCH less!