Lowest OCV An MPPT Is "Worth It"?

MEXICOWANDERER wrote:
400 watts at 17.5 volts? I am trying to clarify my question :)

You get panel Isc with PWM pointing at the sun with sun high enough for STC. Simple. It is all proportional too so you can figure your "expected amps" from one known amount. I use 8.2a for a 130w panel (my Sharp poly-- watch out for monos with high Voc but low Isc. You want Isc)

So 400/130 x 8.2 = 25a. You want a 20% margin in controller amps over that to meet times when panel does above rated. So you need another 5a so use a 30a controller.

My Solar30 (cost $34) with the fancy readout for v, a, ah, etc, and adjustable voltage --not found on the $13 jobs) is 30a and would do you perfectly. I really like mine anyway! :)

If you go MPPT with either 12s or 24s that total 400w you get an extra 10% on average, so now you get 25a + 2.5a = 27.5a. But you can still use a 30a controller since most/all MPPT controllers 'clip' the amps to their ratings, so you don't have to worry about the 20% margin rule.

My Eco Worthy 20a MPPT ( cost $102) controller is limited to 42v input Voc, so it can only do one serious sized 24v panel to maybe 250w. Works great with a 230w

So 400w using 12v panels is four 100w panels that do 6.3a each Isc. cost is say $150? (Mr Wiz is right again--watch for sales--maybe find them for $100 who can say?) so four is $600 plus $34 for the controller =$634

Two 200w panels say find a great deal on them at $180 each is $360 plus two $102 Eco Ws for $204 comes to $564.

So here we have crossed some sort of line on price where now the MPPT is a bit cheaper at 400w. If you insist on high priced USA controllers instead of Chinese, then obviously the cross-over point will be different "depending" but then you also need to know what the panel prices are on sale at the time whether 12s are that much more than 24s.

When I got my panels here last year, the 12s and 24s were the same price at $1.08/w. Prices are all over the map then and now. Some quite outrageous, picking on the unwary at hardware stores for "kits" at eg, 60w for $399 and all that kind of thievery. :(

EDIT here is that thread showing my actual PWM vs MPPT results in the OP

http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/27667574.cfm

EDIT some more: What is the size of battery bank and daily AH draw? What is worth it can depend on how far up in SOC you want to get daily or whether you just want to stay above 50% and then recharge with gen and solar as required.

Also with that whether you do your say, monthly overcharge to "True-Full" on solar or on shore power

Also what kind of batteries? 12s take a lot of work to get their SG up compared with 6s. My 6s can get by with high 14s and the odd 15.5, but the 12v Trojan 1275s (true deep cycles) need frequent 15.5 to keep their SG up and 14.4 all day is just a joke to them.