MPPT Vs. PWM Cost Comparison -- Am I doing this right??

[FALCON]

So I was just about to order four 100w Renogy solar panels last night, but I when I looked at the volts and amps they would produce, I didn't feel sure enough about what type of charge controller I would be buying and whether I should hold off on buying panels until I figure that out... With the big price differences between PWM and MPPT (at least for Tristars) I wanted to have a good idea which way to go.

So today I did a little more reading on MPPT and PWM differences. I try to compare things in simple ways and the simplest way to me seems to be the resulting amps going into the battery considering the loss you get with a PWM compared to the voltage/amps conversion you get with an MPPT.

For this comparison, I'm looking at Renogy 100w Mono Panels:

And these charge controllers:
PWM: Morningstar Tristar PWM 45am. (TS-45)
MPPT: Morningstar Tristar MPPT 30amp (TS MPPT 30)

My power use calculations based off max expected power use (100Ah) and using 3.5 winter peak sun hours, with an MPPT CC, gave a required array wattage of 330 watts.

I'm just doing this cost calculation to see if there is a significant difference between using a PWM and more array wattage vs using an MPPT and less array wattage. I don't care whether is it dead on correct - just whether I'm using the right type of values and formulas for these calculations. Did I do the calculations the right way?

With my calculations, it doesn't look like the cost savings of using a PWM are worth the extra effort of having another panel on the roof (or, if I end up filling the roof space with panels for some reason - I believe an MPPT would allow me to get more energy from them into the batteries)

 

[FALCON]

I'm not really sure whether I should've used 14.5v or 12v for the charging current. I used 14.5 above. If I use 12, then the MPPT becomes cheaper per amp by 8-15%

 

[highdesertranger]

falcon where did you get those comparison charts? did you do that? if you did I am truly impressed. those are great. we need more of this stuff. by just glancing at your numbers(without getting a calculator out) they appear to be accurate. highdesertranger

 

[FALCON]

Yeah I made the comparison charts. Thanks

I've spent more hours in my life than I'd like to admit using Excel (and Mac Numbers, and even Google spreadsheet), so I'm very quick at making tables and charts and all that. I also studied engineering (and did extremely well in my classes), so I should be competent at this kind of comparison!

 

[highdesertranger]

very good my hat is off to you. highdesertranger

 

[MikeRuth]

X2 on your charts, and as TS-45 Owner I love my PWM CC, but I only went that way because I needed to keep my upfront cost down.



go MPPT I say

Mike R

 

[GotSmart]

My hats off to you.  MPPT is the way to go.  No power wasted and a cleaner power produced for you to use.  

What you have is a short term cost, the long term payout is much greater.

BTW, Renogy is having a 4th of july 10% off sale.  

Just go to the factory site.  I make nothing on this, just letting people know.

 

[johnny b]

:idea: You should do 2 - 200W or 2 - 250W panels in 24V along with the MPPT for more efficiency and lower panel cost to offset the higher MPPT price.

 

[FALCON]

I ordered 4 of the Renogy 100w Mono panels tonight. The sale didn't actually do me any good because it appears that when the sale started, the price reduction on these panels went away, so the price ended up being the same as it has been for months.

I probably could have done some more shopping around to find cheaper panels, but I'm in a bit of a hurry, as I want to sell me house soon. Once I sell the house, I think my spending will go down by $800 per month - so I have motivation to just get things done quickly and not draw out the timeline in order to optimize everything about the build. In a different situation I probably would've found a way to get the panels significantly cheaper..

 

[sushidog]

The biggest advantage of using an MPPT controller is being able to go to a high voltage (grid-tie) panel to save $/watt. For instance a single Sharp 300 watt panel from sun electric is only $216 or 72 cents/watt (but you must buy 4 of this panel from this vendor). Or you can buy a single 290 watt Solar Fennel panel for $238 or 82 cents/watt. If you have the room, you could get 2 of these for 580 total watts for $476 ($64 less for another 180 watts of power!) Of course you'd need a 40+ amp controller to handle the extra power if wired parallel but a 30 amp controller would do if wired in series. http://sunelec.com/solar-panels/high-voltage-solar-panels/solar-fennel-290w.html

This savings would pay the difference in price for the MPPT controller. Of course Morningstar is the best, but if you want to save some money the 30A SolarEpic Tracer 3215BN (you want the more expensive BN model which allows user defined voltage settings) is only $200 (their 40 amp model is only $239). Here: http://www.amazon.com/SolarEpic-Tra...&qid=1435001555&sr=1-2&keywords=tracer+3215bn

Chip

 

[AuricTech]

Unless space considerations or Voltage drop concerns make it not feasible, I would use the Renogy 100W polycrystalline panels for a PWM-based system.  Not only are they slightly less expensive, their rated Imp is 5.62 Amps (their rated Vmp is 17.8V).  That would boost your 500W PWM-based system to one providing 28.1 Amps, with a total cost of $792 and a cost-per-Amp of $28.19.  Meanwhile, your 400W PWM-based system would provide 22.5 Amps, with a total cost of $662 and a cost-per-Amp of $29.45.

 

[jimindenver]

Don't lock yourself into anything while planning. Check out as many possibilities and see what fits your needs/ wants as well as possible. That includes different gear as I can run a 300w 24v system off of a $100 MPPT controller.

 

[Canine]

Looks to me like:If you have room for more panels go with a pwm controller .If space is at a premium,go with mppt.$142 sounds awfully high for a pwm controller.

 

[Canine]

Anybody know where we can find actual INDEPENDENT lab tests on controllers?I've seen plenty of mfg. claims and plenty of theory.Has Consumer Reports ever tested solar controllers under actual conditions.Everybody has opinions and aholes,and most of both stink.

 

[Canine]

Just looking at the charts again.If the max amps per panel is 5.6,how do you get 28 amps out of four panels with an mppt controller?How does your controller boost the maximum amp production?

 

[GotSmart]

PWM only allows so much power per panel in. The rest is thrown away. The MPPT adds the extra power back in.

This is a generic answer.

 

[Canine]

What extra power?Where I went to school,4x5.6 =22.4.I would think if the panels would produce 7 amps each,Renogy would advertise them as 7 amp panels.Like I said,I would like to see some actual test results by an unbiased lab.

 

[Canine]

What extra power?Where I went to school,4x5.6 =22.4.I would think if the panels would produce 7 amps each,Renogy would advertise them as 7 amp panels.Like I said,I would like to see some actual test results by an unbiased lab.Damn Hughesnet is locking up,hope I don't double post.

 

[akrvbob]

The key difference between MPPT and PWM is that MPPT will step down the voltage and equally INCREASE the amps. A PWM can only step down the voltage, it can NOT increase the amps.

If your batteries can only take 13 volts at that moment, but your panels are producing 19 volts, a PWM will just throw away the other 6 volts, it can't raise the amps. 1/3 of the power the panel is producing is NOT going into the batteries.

A MPPT controller will reduce the voltage to 13 volts and raise the vamps equally. All the power the panels are producing goes into the battery.

Take note that because more amps are being carried from the controller to the battery than was carried from the panel to the controller, you need a heavier gauge wire from the controller to the battery.

Bob

 

[AuricTech]

To answer your question, Bob Dickerson, consider a single 100W panel, of the type the original poster specified.  I will, for illustration purposes, assume that the PV panel is performing as rated.

That PV panel has an Imp of 5.29 Amps and a Vmp of 18.9 Volts.  5.29 Amps * 18.9 Volts = 100 Watts.  Once that power reaches the charge controller, what happens next depends on the type of charge controller (CC):

A PWM CC will reduce the Voltage to match whatever Voltage it's programmed to provide the battery bank (typically 14.5 Volts or thereabouts).  Having reduced the Voltage, a PWM CC connected to the PV panel under discussion provides 5.29 Amps of current at 14.5 Volts.  Because the Renogy 100W monocrystalline PV panel has an unusually high Vmp of 18.9 Volts (and a correspondingly low Imp of 5.29 Amps), it's not as well-suited for a PWM-based PV system as is the Renogy 100W polycrystalline panel (Vmp of17.8 Volts; Imp of 5.62 Amps) to which I linked in my earlier reply on this thread.

However, an MPPT CC will provide the full 100 Watts (minus some conversion losses) to the battery bank at 14.5 Volts.  100 Watts / 14.5 Volts = 6.9 Amps of current to the battery bank.  This is also why some of the commenters on this thread recommend using "grid-tie" PV panels (unsurprisingly, these are PV panels intended for installation on buildings that already have grid power).  An example of a grid-tie PV panel is this Renogy 250W monocrystalline PV panel.  Though they refer to it as a 24V panel, its Vmp of 30.1 Volts isn't really high enough reliably to charge a 24V battery bank (since a 24V battery bank needs a charging Voltage of around 29 Volts, this PV panel doesn't have much leeway to account for Voltage losses due to wiring or panel heat).  With an MPPT CC, instead of the panel's rated Imp of 8.32 Amps (again, at 30.1 Volts), you can use this panel to provide 17.24 Amps to a 12V battery bank:

250 Watts / 14.5 Volts = 17.24 Amps


Does that make things clearer?

Of course, given both the price premium of an MPPT CC over a PWM CC and the low cost of PV panels, it can be more cost-effective to use a PWM CC and add another PV panel.  The smaller the PV system, the more pronounced the cost-effectiveness.  The original poster's charts demonstrate this principle.  The generally-accepted breakeven point is a PV system with around 400W of PV panels.

ETA: While I was giving you the math behind the difference between PWM CCs and MPPT CCs, I see our host gave you the "Reader's Digest version." :blush: