More Solar for "Us"

[MrWizard]

Ok, I went off.the deep end?
Bought 3 used 37watt uni-solar panels and a blue sky 2000e controller for the hefty price of $400,
They were being listed as 75w panels, they are not

The tested open circuit voltage was 21+, short circuit amps 2.9
I tested each panel multiple times tilted into the sun and laying flat on the ground , got the same readings each way, that was the deciding factor, they appear to put out full power while laying flat, I will try to mount them tomorrow before it gets hot, right now 104°F
I will use the existing wiring until I buy & install the new wire

[red31]

Abs = PWM. beside shunt how else ya gonna do it.

Small solar utilizes a bigger piece of potential power than those that float by 9am.

[BFL13]

pianotuna wrote:
Hi BFL13,

The line loss in the MPPT example is voltage. 2% is a good number. Does that change your wattage figures?

MPPT does not just buck. However, as Salvo used to point out MPPT controllers do switch over to PWM at some point--depending on their design.

PT, No. According to my calculator--YMMV--:) it comes out the same.

The MPPT does stop tracking once Bulk is done, but even in absorb or float doing "PWM" it is still a buck converter at say 24/12 so it is not the same action internally as a "PWM" doing 12/12.

Almot, I am happy for you that your solar controller can do your thinking for you, while you zone out. :)_ It watches the weatherman on TV and makes plans, and even looks out the window to see it getting more cloudy than forecast and adjusts itself. Pretty neat!

[pianotuna]

Hi BFL13,

The line loss in the MPPT example is voltage. 2% is a good number. Does that change your wattage figures?

MPPT does not just buck. However, as Salvo used to point out MPPT controllers do switch over to PWM at some point--depending on their design.

[Almot]

BFL13 wrote:
It is amps that charge the battery, not power.

NEED MORE POWER 🙂

More panels, more total wattage, whatever. Once you've got over 700-800W hump - life is good, no worries about "lost" amps or volts, heat losses in MPPT come in summer when there is a lot more sun=more output, and MS MPPT will do the thinking for you. I wish I had installed 3*250W, not 2*250. Works for me though.

[Almot]

BFL13 wrote:
It is amps that charge the battery, not power.

NEED MORE POWER 🙂

More panels, more total wattage, whatever. Once you've got over 700-750W hump - life is good, no worries about "lost" amps or volts, and MS MPPT does the thinking for you.

[BFL13]

There still seems to be some confusion in how to calculate the amps to the battery for MPPT and PWM comparisons.

For MPPT with its buck converter it is all about watts, where with PWM, watts don't matter.( Also with PWM there is no "working voltage"--just battery voltage)

EG, using 260w of panels in each case ( a 260 for the MPPT and a 160 and 100 in parallel for the PWM) Full high sun for each case at 25C ambient, 50C panel temp (the usual situation for panel heating then--as I have measured it too in the summer as reported in the past)

A. MPPT 260w. Heat loss =10% power at 50C so 260-26= 234w. Line loss panel to controller 2% (1-3 is decent) 234 x2/100 = 4.7 so 234-4.7 = 229.3w controller input. Controller efficiency 97% so controller output is 222.4w

Assuming no line loss controller to battery--Amps to battery is output watts /battery voltage. Using 13.5v = 16.48 amps

B. PWM 260w. You get Isc amps to about 13.5v (see IV curves)

Isc 100w panel = 6.2a and 160w = 10.1 so total Isc = 16.3 amps

So they are nearly equal.

You can get better results for MPPT with a lower battery voltage and less panel temperature, but as you all know, battery voltage in the morning with solar on, is over 13v on the way to 14.x, so picking a battery voltage in the 12s is just for "spin" as used by MPPT controller salesmen.

Their other spin fav is to claim that PWM fails to use all the panel power, while MPPT does, so PWM "wastes" all that power. Note the actual power from the MPPT in the above.(222 out of 260) Neither MPPT nor PWM gets all the power to the battery. But who cares? It is amps that charge the battery, not power.

[MrWizard]

Yes 17.5 to 18 VMP...working voltage
But 21+ VOC open circuit voltage no load
And this is the same series or parallel

So 5 series panels would be 85~90 volts working 105v open circuit

Computing power gain with mppt should be done with the Working Volts
Not the open circuit volts

90v*8 amps 720w solar input versus 18*40 amps 720 solar input
But the mppt will give more amps output to batteries

The pwm will lose approx 3v * 40 amps 120w
With a peak output of 600w
While the mppt peak will be closer to 700w

This of course is ideal conditions
Low batteries clear sky high sun angle

[CA_Traveler]

BTW How common are 21V 12V panels? On Solar Blvd they have 160W @ 18.7V and a flexible 120W @ 20.0V

AFAIK most 12V panels are 36 cells @ 0.5V/cell or about 18 Vmp.

[CA_Traveler]

pianotuna wrote:
Hi CA,

MPPT benefits from larger wire size. Of course series with by pass diodes may be the best solution for MPPT.

I've never accepted that PWM requires large wire size--especially with 21 volt panels.

My series panels have 61' of 10 gauge wire with a voltage drop of 0.5 percent. No practical gain from larger gauge wire.

Parallel panels with PWM should have larger wire from the combiner box to the controller. Let's say 5x panels at 8A each. That's 40A and I would not use 10 gauge wire to the controller. Sure with 12V panels that are 21V smaller wire can be used than with 18V panels.

But look at the 21V PWM limitation. You get 8A into the battery at 14.8V. With MPPT you get 21*8 / 14.8 = 11.3A. With a 5 percent MPPT conversion loss you have 10.8A and a little less with hot panels.

PWM does not use all of the panel power (watts) just the amps. MPPT does use all of the panel watts and converts it to battery watts or amps at a given battery voltage per the above example.

Try a lower battery voltage example say 13.6V float. PWM is still 8A. MPPT is now 21*8 / 13.6*.95 = 11.7A. While the charged battery won’t accept the amps the house loads will which translates to less battery drain for MPPT when 8A is exceeded.

PWM wins for smaller systems while MPPT wins in performance for larger systems.

[pianotuna]

Hi CA,

MPPT benefits from larger wire size. Of course series with by pass diodes may be the best solution for MPPT.

I've never accepted that PWM requires large wire size--especially with 21 volt panels.

[CA_Traveler]

pianotuna wrote:
CA Traveler,

What is the additional cost on the PWM system over the MPPT?

Additional mounting brackets and wiring. Combiner box, larger wires and disconnect to the controller. But there are of course many variations.

For my 3 panels the serial wiring is minimized, bypass diodes allow power generation with shade. Other factors would favor MPPT for me even with less than my $100 additional PWM cost.

Another big cost factor is panel shipping costs which are very high due to panel size. I pre planned that and picked mine up driving an additional 30 miles.

[pianotuna]

CA Traveler,

What is the additional cost on the PWM system over the MPPT?

CA Traveler wrote:
It's been awhile since I compared larger RV solar systems. So here is todays comparison:

Solar PWM vs MPPT Cost comparison 1-23-17

PWM
Cynergy 160W $115, 5 panels 800W $575
TriStar TS-60 $183
Installation cost over MPPT $100
Total $858

MPPT
Amerisolar 250W $163, 3 panels 750W $489
TriStar MPPT 60 $483
Total $972

Wow my 750W system cost $1400 without the remote display in 2014!

Based on this I'd make the same decision favoring MPPT today.
.
There are of course many other factors to consider. It seems clear to me from numerous posts that many start with a smaller solar system and PWM is much more cost effective for smaller sizes. And then with upgrades to more solar there is just no practical starting over.

[Crabbypatty]

I had 100 watt panels that I took off the house that I moved from. From what I have been reading here on the net is that there seems to be two schools predominately. Trickle charge and re charge. So I have noticed that most who dive in for recharge go with 400+. I have a Morning Star controller, Bogart Engineering Tri Metric Battery Minder and they charge up two Trojan t-125 flooded wet cell batteries. The Tri Metric tells you your battery state, watts in, amps in and what kind of charge. The panels lie flat on the roof in full sun they deliver 8+/- amps with the sun over head. The meter indicates that we drawn down to 70-75%. It gets up to 88-90% quickly then levels off. The last bit to 100% takes the longest. This was with 3 college kids charging everything possible, phones ipads laptops, plus ours and we even got lazy and brought the electric drip coffee pot. The only other use and biggest draw is the water pump. I do have two more panels which will bring me to over 600 watts. I noticed that a fair amount for person here where they go to 400 increase to 600. I would like to add two more Trojans but Im getting close on the tongue weight.

[MrWizard]

red31 wrote:
wiz, what did ya harvest today :B

off/on rain all day

32.58 amphrs
466.6 watt hrs

13.82 peak amps
196.8 peak amps

this was fringe effect from a hole in the clouds

[Almot]

CA Traveler wrote:
... many start with a smaller solar system and PWM is much more cost effective for smaller sizes. And then with upgrades to more solar there is just no practical starting over.

Small systems favor PWM, yes. The question is - do you need a small system.

If the intended living is offgrid and longer than a few days, - oversize the solar, cover the whole roof with panels. MPPT will be most logical choice then.

There can be exceptions to the rule when PWM would work better, ex. particular weather pattern or roof plan. Or - scenarios when solar won't work well, no matter what size or what controller - like rainy PNW/BC. Then just get whatever is cheaper, you won't be able to live off this power anyway.