Solar Costs - MPPT vs PWM

[CA_Traveler]

The continued lower price of the larger panels has now dropped the MPPT costs just below PWM for a larger RV solar setup.

SolarBlvd Cynergy 3x$200 250W panels 750W $600
SolarHome Morningstar MPPT 60 controller $472
SolarHome Moriningstar TS-RM-2 remote meter $102
Total $1174

SolarBlvd Cynergy 5x$159 140W panels 700W $795
SolarHome Morningstar PWM 60 controller $206
SolarHome Moriningstar TS-RM-2 remote meter $102
Total $1103

Both sets of panels use about 52 sq feel of roof space and SolarBlvd has 250W panels for $160.

Add 5%+ more MPPT power and likely higher PWM wiring costs and MPPT is taking the lead in lower costs.

Other panels, controllers, etc are also available for both PWM and MPPT.

[BFL13]

I am not on solar until next April so all I can do is look at what I got, when I got it, on solar to the end of September this year, and in previous seasons. We only camp doing solar mid-April to end Sep. Winter camping Oct to mid-April is all gen /charger 50-90s and get back to 100% at home routine.

I always had the inverter doing high amps draw after solar wake-up so never saw what Jim is reporting. All I did see what that lousy 13.5 amps in July/August instead of the 15.5 I got in April/May, compared with the 14.5 I would have got with PWM the whole time. You could call it a wash for amps I suppose, sort of.

Turns out that was to do with panel temps when I deployed my IR temp gun at the under-side of the panel and did the math. Fact is I would do better for amps if I had the same 230w of solar panels but in 12v form and PWM so I would get 14.5 most of the "season" instead of 15.5 a short part of the time and 13.5 the longer part of the season. I sure would not get so annoyed seeing that 13.5 instead of 14.5 and would soon forget about any 15.5 magic stuff. Depends if you are a half empty or half full kind of guy I suppose.

However the money doesn't work out for that, so it is cheaper to lose that 1 amp most of the season than to pay the extra for the 12w panels instead of the cheaper 24w panel--but only thanks to the Eco-W being $102 instead of $300+ like the other MPPTs.

Which is the whole point of this thread's topic. You have to juggle a bunch of things that apply to you. Some other guy may have a different answer for how it all applies to him.

[Golden_HVAC]

I am not so sure that the MPPT controller will be worth it. This is because solar panels can put out more amperage at a lower voltage than when at a higher voltage.

So a 120 watt solar panel putting out 13.5 volts directly to a battery with a switch the only thing in between should be putting out more amperage than when the same solar panel is connected to a MPPT controller and putting out say 18 volts to the controller, then that controller is putting amps into the 13.5 volt battery.

It would take a bypass switch to find out, and accurate amperage readings though a shunt, with accurate voltage drop across the shunt readings. Then switch the power leaving the panel to the MPPT controller or directly to the battery, and see what one puts out the most apperage into the panel.

I found that if I changed my PWM controller (1994 - that is all they had back then) to a switch, I could get an additional 0.5 amps into the battery. Then when close to dark, I could switch it back to automatic (through the controller). This would give me an additional 3 - 4 amp hours per day from my original 90 watt solar system.

My MPPT controller (SP50) is not putting out nearly as much amperage as I expected, based on what I could get from the pair of 45 watt panels on a cold day in NOvember at 4,500' elevation in CA.

For my money, I will be buying more panels and PWM controllers. Not spending $300 on a MPPT controller anymore.

Fred.

[JiminDenver]

Salvo

I have three Eco-w's, well actually four but that's another story. All three have done it, High battery voltage = lower charging rate. Start charging when the battery is lower than any of us what to see and you will see max amps right off.

Maybe BFL can make his coffee on the stove tomorrow and see what I am talking about. Remember this has to do with the Eco-w controller only.

[wintersun]

To a degree you have it backwards. In general it is better to put more money into panels with greater output and let the excess be clipped during peak sun hours as you have more reserve capacity when the sun is not bright or is lower in the sky or there is partial shading.

MPPT provide less clipping (when panel output voltage exceeds the charging voltage to the batteries of 14.5 volts. In this scenario the MPPT controller works much better than a PWM charge controller.

Most people do not have a clue as to their specific demand profile. How many start with a Trimetric or other accurate digital charge monitoring device? If you need 100 Watt hours during the night and are going to replace that discharge the next day during 8 hours of daylight and have two 6 amp output panels that is going to be enough to bring the battery bank back to 100%.

If instead of 12 amps per hour you add 18 amps of charging capability you gain nothing. So what if the bank is recharged in 5 hours instead of 8. What is gained? Absolutely nothing.

Currently with commercial power production this same approach is being taken. If an inverter is rated at 105% of its nominal input then the solar arrays are built that provide this 105% to the inverter. Over the course of the year the total output is higher with the same cost for the inverter, racking, wiring, etc.

[Salvo]

It doesn't know a slower, gentler rate. It only knows one rate and that's full speed when in boost mode.

What you're seeing in boost when running the coffee pot is that the pot wattage is greater than the solar wattage. Battery voltage goes down. MPPT charging current is a function of the maximum power point AND battery voltage.

Icharge = Psolar / Vbat

If Psolar = 200W and Vbat is 12V then Icharge = 16.7A.
When Vbat = 14V then Icharge = 14.3A

JiminDenver wrote:
AND just because you look at the output of a MPPT controller and see x amps, don't think that that is the max for the system. The controller also tries to determine what is the best way to charge the battery. Left alone the Eco-w's will take a slower, gentler rate but if I hit the system with running the coffee pot early on, I'll see a 2-3 amp increase and it will stay that way until the you hit around 14v or so. Then they go into boost until you reach the set voltage before dropping to float.

[BFL13]

Jim, I have not seen that at all with my Eco-W and I do use the big inverter at times early in the day after solar wake-up. (That leaves open that I may never have seen the slow way by not using the inverter)

I set mine to 14.8 Vabs, and 14.5 Float(highest float it will do)

I have watched the Eco-W's display several times when battery voltage is approaching the 14.8v set-point and it suddenly says "Absorb" for the blink of an eye and then says "Float" At the same time, wattage and amps readings collapse to the lower voltage and amps product.

The controller is now in PWM. In fact, that controller "Float" is actually while the battery is in its Absorption Stage at a Vabs of 14.5 my "Float" set-point

However, after it drops to "Float" I can now run a load and amps will jump back up but the display still says "Float" (Voltage stays at my Float set point 14.5 but as reduced by the load) ISTR the battery voltage has to drop below 13.2 before it will return to Bulk and MPPT, so there is plenty of scope there for loaded voltage to remain above 13.2 in Float but do extra loads above what is needed to continue with the battery recharge.

I would be happy to be able to kick the amps up in the morning by running the inverter, but need to know more about that trick I guess.
Do you also have to chant something in Navaho? 🙂

[JiminDenver]

AND just because you look at the output of a MPPT controller and see x amps, don't think that that is the max for the system. The controller also tries to determine what is the best way to charge the battery. Left alone the Eco-w's will take a slower, gentler rate but if I hit the system with running the coffee pot early on, I'll see a 2-3 amp increase and it will stay that way until the you hit around 14v or so. Then they go into boost until you reach the set voltage before dropping to float.

BFL

The Eco-w's go into absorb when they cross 14v or so and then float when they hit the set limit. The problem is at the 14-16a a 230w panel produces, it would take a huge bank to require any time to gain that voltage. So either we need to use a load to absorb some of the current or turn the panels so that they will not have the power to bring the voltage up so fast. Set it to 15v and derate the panel at 14.8v and you should be able to hold it there.

[BFL13]

CA Traveler wrote:
MPPT has no way to detect panel wattage. Power is the product of volts and amps and the controller searches for the point on the IR curve where it can maximize that product and convert the power to battery voltage and amps. That's how you can get 40A to charge the 12V batteries from 8A panels for example.

Voc is not a factor since the panel is producing amps so it's not operating open circuit and the panel voltage will be less than Voc. Voc or any input voltage to the controller is a factor due to electronic limitations.

At 60A the MS-MPPT-60 will start derating at 140V input down to 0A at 150V. 150V is the stated maximum voltage.

ISTR one way MPPT works is by fiddling with its R to keep panel voltage at where the MPPT experiences highest wattage using feedback on the wattage changes. EDIT--one way but there are others--see below link.

I think the buck converter 24 to 12 then takes that max wattage in and puts it out at battery voltage and whatever amps that comes to

AFAIK with PWM, the controller passes through the actual panel amps (same amps as its Isc at the time, which is a variable) The amps the panel does depends on panel voltage per the IV curve but that is in the panel, and the controller could care less about that, it just gets those amps in and starts controlling when battery voltage reaches the high set-point.

So it seems to me that PWM uses actual panel amps while MPPT is independent from panel amps and makes its own amps regardless of panel amps.

So MPPT on a 12v panel does not take the panel amps and then add some amps. It starts from scratch and gets it own amps, which might be higher or lower than panel amps (depending on panel temperature mostly)

http://en.wikipedia.org/wiki/Maximum_power_point_tracking

[JiminDenver]

You guys are right, there is no where to hook up a second array to the 60 and I'm not about to mix up the panels. Looks like the 45 will be the best I can do. Thank you

[JiminDenver]

It's not perfectly equal but darn close. Each has its own leads to the battery, all the same size, length etc. Believe me I would prefer 3 matching 250w polys and a single controller on a single bank.

[CA_Traveler]

MPPT has no way to detect panel wattage. Power is the product of volts and amps and the controller searches for the point on the IR curve where it can maximize that product and convert the power to battery voltage and amps. That's how you can get 40A to charge the 12V batteries from 8A panels for example.

Voc is not a factor since the panel is producing amps so it's not operating open circuit and the panel voltage will be less than Voc. Voc or any input voltage to the controller is a factor due to electronic limitations.

At 60A the MS-MPPT-60 will start derating at 140V input down to 0A at 150V. 150V is the stated maximum voltage.

[Salvo]

Thanks for the extra testing. I think the controller operates in constant voltage (CV) mode when in absorption. The controller's output voltage is fixed. The 3 controllers may all be set to 14.4V, but they are not exactly equal. One controller has the highest voltage and another the lowest. The highest controller should supply all the current.

There is an exception though. The cable resistance between controller and battery is a factor. There should be no sharing if the controllers are tied together and just one cable goes to the battery. It's best to have 3 cables.

If you have 3 cables and the cable voltage drop (I * Rcable) is greater than the difference in controller output voltage then there will be some sharing. It won't be equal though.

JiminDenver wrote:

When in absorb/float the controllers split the charging, each dropping equally until as I said, it's gets well into float. I'll have it all up this weekend so Ill pay attention to it more.

[BFL13]

Please clear this up---Does MPPT with buck converter even care about what amps the panel is doing? I have the notion it only cares about the voltage and the wattage and it makes its own amps output from that

PWM with 12s is completely different where it passes through the actual panel amps ( the Isc at the time) and could care less about panel voltage.

AFAIK this is why some panels have specs with higher Voc and lower Isc in order to work better with MPPT controllers.

Anyway, what is the real story on that whole business? Thanks.

[CA_Traveler]

JiminDenver wrote:
What attracts me to the 60 is that as I understand it is that it can track multiple arrays. That means I could run the two 245w monos AND the 230w poly and not worry about confusing the controller with different voltages. Should I have the wrong idea about this, someone tell me now. Otherwise a 45 for the monos and a Eco-W for the poly will do just fine.

Are the panels going to be connected in series? The controller just sees the voltage and amps.

However what would it see if the monos are under producing amps compared to the poly? If bypass diodes kick in then that might help.

I'd be concerned about amps produced for a given low light situation. What if the poly produced 3A and the monos 2A? ie For a given uniform light situation could cell technology limit overall power?

But perhaps you're considering parallel? Perhaps then the amps would just add with no degradation for cell technology?

I'm hoping that sometime next year I can do some shadow testing on my panels. ie Take the laptop and/or remote and shading materials on the roof. But if the panels are producing as expected I may just have another glass of wine and let the electrons flow. :B